Every baby clothing label is a guess wearing the costume of a fact. A tag that reads 0-3M sounds like a measurement. It is not. It is shorthand for a body shape, and babies arrive at that shape on wildly different schedules. One baby leaves 0-3M at seven weeks. Another is still in it at four months. Neither parent has done anything wrong.
Most sizing guides stop there, with a shrug and a line about every baby being different. This one goes further, because we can do the arithmetic. We publish the exact height and weight band every PatPat size is cut to, and those bands can be laid against published growth standards to answer the question parents are actually asking: not just which size fits today, but how long it will keep fitting.
If you only need the numbers, the full PatPat size chart has every measurement we publish, in inches and centimetres, for every size we cut. This page is for the decision that comes before the chart.
The short answer, if you are deciding right now
| Size | Height | Weight | Typical time in this size |
|---|---|---|---|
| Newborn | 45.7 to 55.9 cm | 6 to 9 lb | 0 to 3 weeks |
| 0-3M | 55.9 to 61.0 cm | 9 to 13 lb | about 6 to 10 weeks |
| 3-6M | 61.0 to 68.6 cm | 13 to 17 lb | about 2 to 3 months |
| 6-9M | 68.6 to 73.7 cm | 17 to 21 lb | about 3 months |
| 9-12M | 73.7 to 78.7 cm | 21 to 25 lb | about 3 to 4 months |
| 12-18M | 78.7 to 83.8 cm | 25 to 28 lb | about 4 to 6 months |
| 18-24M | 83.8 to 88.9 cm | 28 to 30 lb | about 4 to 6 months |
| 2Y / 2T | 83.8 to 88.9 cm | 28 to 30 lb | about 6 to 9 months |
| 3Y / 3T | 88.9 to 99.1 cm | 30 to 34 lb | about 9 to 12 months |
| 4T | 99.1 to 104.1 cm | 34 to 38 lb | about 12 months |
| 5T | 104.1 to 111.8 cm | 38 to 42 lb | 12 months or more |
Height and weight bands in this table are PatPat pattern data, current as of September 2026. The wear windows are calculated from those bands against WHO median growth, and describe a child growing along the middle of the curve. Method and sources at the end.
Three rules carry most of the weight:
- Under two years, order to weight, not to the month on the label.
- Do not size up in pajamas. This is the one place where the standard advice is wrong, and the reason is a safety rule rather than a fit preference.
- If anyone in the order needs an end size, a women's XS, a 4XL, a Newborn, start the whole order from that size rather than from the print you liked.
The rest of this guide is why.
Why the age on the label is the least useful number
PatPat's baby patterns are not drafted to months. They are drafted to height and weight, and the month name is applied afterwards as shorthand. PatPat's Newborn size is cut for 45.7 to 55.9 cm and 6 to 9 lb. The 0-3M size is cut for 55.9 to 61.0 cm and 9 to 13 lb. Nothing in either pattern knows how old the baby is.
That matters because age is the variable that spreads fastest. Take a three-month-old boy. On the WHO Child Growth Standards weight-for-age table, the healthy range at that age, third to ninety-seventh percentile, runs from 5.1 kg to 7.9 kg. In pounds that is 11.2 to 17.4.
Now lay that against the PatPat bands. A three-month-old at the bottom of the healthy range is in 0-3M. One in the middle is in 3-6M. One at the top has already reached the bottom of 6-9M. Three sizes, one age, every one of those babies growing normally.
It gets wider, not narrower. At twelve months the WHO healthy range for boys runs 7.8 to 11.8 kg, which is 17.2 to 26.0 lb, and that spans 6-9M, 9-12M and 12-18M. Girls sit slightly lighter throughout, which shifts the whole picture down by roughly one band at the edges without making it any narrower.
No label that names a month can express that. The month is a guess about a population. The weight is a fact about your baby.
What to use instead
Before about two years, use weight. Bodysuits, rompers and sleepers are cut with ease through the body, and what runs out first is room through the torso and thigh.
After about two years, use height. Once a child is walking and proportions settle, length becomes the limiting dimension. Our kids range is built on height bands from 83.8 cm upward for exactly that reason.
There is one place where the two systems overlap and parents reliably get stuck. Our 18-24M and our 2Y are cut for the same height band, 83.8 to 88.9 cm, and the same weight band, 28 to 30 lb. That is not a duplication. The difference is diaper allowance: the baby-range pattern carries more room through the seat. If the child is still in diapers, take 18-24M from the baby range. If they are out of them, take 2Y from the toddler range. No measurement will tell you which, because on every measurement the two are identical.
How long each size lasts, and what runs out first
For a baby growing along the middle of the WHO curve, a PatPat baby size lasts between three weeks and three months, and the window lengthens with every size. Newborn is the shortest at roughly three weeks. By 4T it is about a year.
The wear windows in the table above are not folklore. They come from putting the PatPat bands against the WHO median growth curves for weight and for length and asking, for each size, when a child of median growth enters the band and when they leave it.
Doing that turns up something we did not expect, and it changes how you should shop.
In the first six months, weight runs out before height does. A median baby passes 9 lb, the bottom of the PatPat 0-3M band, at around three weeks, and passes 13 lb, the top of it, at around ten weeks. Over the same stretch their length moves through the 55.9 to 61.0 cm band slightly more slowly. The garment stops fitting because the baby got rounder, not longer.
After about six months, it inverts. The PatPat 6-9M band is 17 to 21 lb and 68.6 to 73.7 cm. By weight alone, a median baby would stay inside that band for close to six months. By height, they pass through it in under four. Height wins, and from that point on it keeps winning: every subsequent band is exhausted by length before it is exhausted by weight.
Two practical consequences follow.
Before six months, a heavier-than-average baby will move through sizes faster than the label suggests, and a lighter one slower. Buy to weight and buy less far ahead than feels prudent, because the window is genuinely short. Newborn is the extreme case: for a median baby it is a matter of weeks.
After six months, a tall baby moves faster regardless of weight. This is the point where parents who have been sizing by weight start finding that sleeves and trouser legs are short while everything else still fits. Switch to height at that point, and stop cross-checking against the weight column.
It is also why the sizes get longer as they go. 0-3M is a six to ten week purchase. 4T is a year. The same twenty dollars buys very different amounts of wear depending on where in the range you are standing, and that is worth knowing before you decide how many of something to buy.
Should you size up? Yes, except in pajamas
The standard advice, PatPat's included, is that when a child falls between two sizes you take the larger one. The garments are cut with ease, children only grow in one direction, and the larger size buys another season. For everyday clothes that is right.
For pajamas it is wrong, and the reason is not fit. It is regulation.
In the United States, children's sleepwear is governed by the Consumer Product Safety Commission under 16 CFR part 1615, which covers sizes 0 through 6X, and 16 CFR part 1616, which covers sizes 7 through 14. The CPSC states that it issued these standards to protect young children from death and serious burn injuries caused by sleepwear igniting, specifically from small open-flame sources such as matches, lighters, candles and space heaters.
Sleepwear in those sizes has two routes to market. It can pass the flammability test, or it can qualify as a "tight-fitting garment" as defined at 16 CFR §§ 1615.1(o) and 1616.2(m), which means meeting maximum dimensions, tagging and labelling criteria written into the rule. Most cotton pajamas sold in the US take the second route. Loose cotton pajamas in those sizes are not a legal option.
Snug-fitting cotton pajamas are compliant precisely because they are snug. Ordering a size up removes the property the garment qualified on. A pair of cotton pajamas bought two sizes large is no longer the garment the rule was written about.
Two clarifications, because this one is widely half-remembered.
Infant garments are excepted, not unregulated. Sleepwear that meets the "infant garment" definition at 16 CFR § 1615.1(c), which covers sizes for nine months and under along with specific measurement and labelling criteria, is excepted from 1615 and 1616 testing on the reasoning that babies that age cannot reach an ignition source independently. Those garments are still subject to the general textile flammability standard at 16 CFR part 1610. That is why a newborn sleeper can be cut loose and a five-year-old's pajamas cannot. The CPSC sets this out in its children's sleepwear FAQ.
Snug fit is not the same as too small. The rule sets maximum measurements, not minimums. Correctly fitting pajamas should not restrict movement or leave marks. If a child has genuinely outgrown a size, move up. Just do not move up in advance.
Footed sleepers
Footed sleepers hit their limit in length before girth, which makes them the wrong garment to size up in for a second reason. Excess length pools at the foot, and a child pulling to stand or walking is doing it on a surface that slides. Size these to current height and check the length at the foot rather than the fit at the chest.
Swimwear and outerwear
Two categories where the usual logic inverts, in opposite directions. Swimwear ordered large fills with water and drags, so order to current measurements. Outerwear is the one category where sizing up is almost always right, because it has to close over whatever is underneath it in the coldest week of the year, not in the week you bought it.
Between two sizes: a rule for each kind of garment
| Garment | If you are between two sizes | Why |
|---|---|---|
| Bodysuit, romper | Take the larger | The crotch snaps set the limit, the body has ease |
| Footed sleeper | Order to current height | Length limits first, and snug fit is a compliance condition |
| Two-piece pajamas | Order to current size | Snug fit is a compliance condition |
| Dress | Take the larger | Length is forgiving, and it buys a second season |
| Trousers | Order to waist, hem the length | The waistband is the measurement you cannot adjust |
| Swimwear | Order to current size | Extra volume fills with water |
| Outerwear | Take the larger | It needs room for layers underneath |
| Adult pieces in a matching set | Order to chest, not to the letter you usually wear | Letters are not consistent between brands, centimetres are |
Our stated tolerance is 2 cm on baby and toddler products and 4 cm on regular adult sizes. Within that range, the table above matters more than the label does.
Seven ways a size gets named
We use more naming systems than most brands, for a structural reason. A single matching family product is cut for a woman, a man, a girl, a boy, a baby, a plus size and sometimes a dog, and those patterns do not share a naming convention. That is why one product page can show you sizes that look like they belong in different shops.
Two of them get misread constantly.
A youth M is not an adult M. On the small number of kids products that use youth letters, M(9-10Y) is our 9-10Y kids pattern: 134.6 to 139.7 cm tall, with a 27.5 to 28.5 in chest for girls. The age is printed inside the label for exactly this reason.
A plus-size 2XL is not a standard XXL. They come from two different charts. The standard adult chart runs XS through XXXL. The plus-size range runs 0XL through 4XL against US 14W to 22W, and it is a fuller cut rather than a scaled-up one. Match on chest and hip and the letters stop mattering, which is the point.
Every system we use is set out side by side, with the measurements that separate them and a height conversion for numeric labels like 90, 110 and 140, on our size chart page. What none of those systems can give you is a guarantee that two garments in one size are identical, and the reasons are set out in why “true to size” means nothing in children’s clothing.
Buying for the whole family at once
Most sizing advice assumes you are buying for one person. Matching family outfits are a different problem, and one we are in an unusual position to describe, because a single PatPat order routinely spans a newborn and an adult 4XL.
Here is the failure mode. You choose a print, you commit to it, and then you find the one size you need for one person is not offered on it. Now the whole set is off and you start again.
The fix is to choose from the end sizes first. Every apparel range is deep in the middle and thin at the edges, and ours is no exception. On a stock snapshot taken on 4 September 2026:
- Baby sizes: 6-9M appeared on 517 products, 0-3M on 258, and Newborn on 61.
- Women's sizes: XL appeared on 417 products, XS on 21, and 4XL on 25.
- Men's sizes: 2XL appeared on 468 products, S on 54, and 4XL on 5.
If everyone you are buying for lands in the middle of the range, choose on style and skip the rest of this section. If one person needs an end size, start from that person. Find which prints carry their size, then let everyone else follow. It inverts the usual shopping order, and it prevents most abandoned matching-set orders.
One of those numbers deserves an explanation rather than a defence. Men's S sits on far fewer products than men's M because most of our menswear exists inside matching family sets, where the adult men's cut starts higher. That is a real gap in the range, not a stock fluctuation.
The tall teenager problem
Our largest kids size, 13-14Y, is drafted for 157.5 to 163.8 cm. The women's chart opens at XS for 154.9 to 160.0 cm. They overlap by roughly nine centimetres, so a teenager in that band genuinely fits both and height will not break the tie.
Use chest instead. The 13-14Y is cut for a narrower chest than a women's XS. Above 163.8 cm there is no kids size and the adult chart is the only one that applies.
We should also say plainly that 13-14Y is currently sold only as XL(13-14Y), on the products that use youth letters. The pattern exists and is published in our charts, but no product carries the 13-14Y label right now. If you are shopping for a thirteen-year-old, XL(13-14Y) is what to look for.
If the set is Christmas pajamas
Seasonal collections behave differently from the main range. The patterns are the same, but a seasonal print does not carry every size, and sets sell down one size at a time rather than all at once, so the range narrows as the season goes on. The Christmas pajama size guide sets the same charts against live stock for that collection, which is the fastest way to see which sizes are already thin before you commit to a print.
Where we get it wrong
A sizing guide that only explains how to read our charts correctly is not much use if the charts and the garments sometimes disagree. Here is where ours do, drawn from what customers say in reviews of our own products.
Our adult sizes measure smaller than the same letter elsewhere
This is the most consistent fit complaint we receive, and it is the one worth acting on before you order. Customers buying adult pieces in matching sets report that our letters sit roughly a size below what they are used to from US high-street brands, and the effect is most frustrating for mothers buying a mommy-and-me set in the first year after birth, when the letter they wore before is no longer the letter they wear now.
The letter is the problem. Order adult pieces to your chest measurement and ignore the letter entirely. Our women's M is cut for a 35.5 to 37.5 in chest and our women's XL for 40.5 to 43 in. Those numbers are published, they do not move, and they are the only reliable thing on the page. If a tape measure says 38 in, take the L, whatever you wore last time.
Consistency between products is our hardest problem
We add new styles at a rate that few brands match, across a supply base of many factories. The upside is choice and price. The cost is variance: two garments carrying the same size label, bought in the same order, can fit differently. Customers tell us this, and they are right.
It does not run in one direction either, which makes it harder to plan around than a simple "runs small" would be. Adult pieces tend to measure small. Some baby items measure large, and reviews of individual rompers describe a 6-9M fitting closer to 9-12M.
Two things help, and both are more reliable than any general rule:
- Use the size chart on the specific product page, not a remembered one. It is generated from that garment's own pattern data. Where a product differs from the range, that chart is where it shows.
- Read the photo reviews on that product. Fit comments on the item you are actually buying beat fit comments about the brand.
Our larger men's sizes are genuinely thin
Customers have asked us for more men's 3XL and 4XL, and the stock figures above show they have a point: men's 4XL currently appears on five products. Men's S is thin at the other end for a different reason, because most of our menswear sits inside family sets where the adult men's cut starts higher. Both are real gaps in the range rather than temporary stock-outs, and if either is the size you need, shop from that size first.
Fabric weight is not on any size chart
A recurring disappointment has nothing to do with measurements: a set bought for winter arriving lighter than expected. A size chart cannot tell you how warm something is, and a seasonal name on a collection is not a weight specification. Check the fabric described on the product page rather than the season in the collection title.
Shrinkage is a fit problem that arrives late
Cotton knit shrinks, most of it in the first wash, and a garment that measured correctly on arrival can measure small afterwards. If something fitted and then stopped fitting after laundering, that is a finishing and care issue rather than a sizing one. Wash cold and dry low, particularly for anything you need to last a second season.
Why none of this is unique to us
There is no mandatory children's sizing standard in the United States. The ASTM body-measurement tables that most of the industry works from are voluntary consensus standards, developed by ASTM Subcommittee D13.55 on Body Measurement for Apparel Sizing. ASTM D4910/D4910M, covering infant sizes from preemie to 24 months, is active. ASTM D5826, which covered sizes 2 to 6x/7, was withdrawn in 2009 and has not been replaced.
That is worth stating plainly: the size range where most toddler clothing sits has had no active US consensus body-measurement standard for more than fifteen years. It is why a 3T from three brands can be three different garments, and why the instruction to measure rather than to guess is not a way of avoiding responsibility. It is the only method that works.
What we can do is publish everything we cut to, in both units, from the pattern data itself, including the sizes we are thin on and the one we draft but do not currently sell. That is on the size chart page, and it is there so you can check us rather than trust us.
Buying ahead, and buying as a gift
Both of these are the same calculation run backwards, and both go wrong the same way.
The mistake is buying for the size the child is now. The question is what size they will be in the season the garment will be worn. A 12-18M sweater bought in July for a baby who turns eighteen months in December is wrong twice, size and season, from one decision.
If you are buying a gift and do not know the size
Skip Newborn. For a median baby it is a three week window, new parents receive more of it than can possibly be worn, and a large share of it never leaves the drawer. Our own range reflects the same reality from the supply side: Newborn appears on 61 products against 517 for 6-9M, because that is where demand actually sits.
Buy larger than feels right. A gift that arrives too big becomes useful in a few months. A gift that arrives too small is already finished.
Favour categories that tolerate error. Sleepwear with room, bodysuits and rompers with snap adjustment, outerwear, anything without a fitted waist. A fitted dress will not forgive a size of error.
Buying ahead for the next season
Use the wear windows. In the first six months a size lasts six to ten weeks, so buying more than one size ahead is a bet on a body you cannot yet measure. From about two years the windows stretch to nine or twelve months, and buying a season ahead becomes reasonable. The rough rule: never buy further ahead than the wear window of the size you are buying.
How to measure
Children's clothing needs less measuring than adult clothing. Under two years, weight and height are enough, and weight matters more. After two, height carries the decision, with chest as the tiebreaker between adjacent sizes.
For a child who will not stand still, measure them lying down, crown of the head to heel with the leg gently straightened, and take the reading twice. If the two differ by more than a centimetre, take a third.
The five adult body measurements our charts read against, with photographs of each, are on the size chart page.
Frequently asked questions
How long will my baby actually wear 0-3M? Roughly six to ten weeks for a baby growing along the middle of the WHO curve. The band is 9 to 13 lb, one of the shortest windows in the whole range, which is worth knowing before you buy six of anything.
Should I size up in pajamas? No. Snug-fitting cotton pajamas qualify under 16 CFR 1615 and 1616 by fit rather than by flame testing, so ordering a size up removes the property the garment qualified on.
What size should I buy for a baby shower gift? Larger than newborn. Newborn fits a median baby for about three weeks and most parents already receive more of it than they can use, so count forward to the season you want the gift worn in.
My toddler is tall and thin — height or waist? Tops and dresses to height, trousers to waist. A hem can be adjusted after the fact and a waistband cannot, and if the gap is more than one size, separates beat sets.
When does a child move out of toddler sizes? At around 111.8 cm, where 5T ends and the kids range picks up at 5-6Y. It is a height threshold rather than a birthday, and children cross it anywhere between four and six.
Is 2T the same as 24 months? Not in fit, even where the measurements match. Both cover 83.8 to 88.9 cm, but the baby-range pattern carries more room through the seat for a diaper, so choose 18-24M if your child is still in them.
My child is between sizes in a matching set. Size everyone up, or down? Neither — size each person independently. There is no visual benefit to everyone being wrong in the same direction, and a set only reads as coordinated if each piece fits the person wearing it.
About this guide
Written by the PatPat Editorial Team. How we research, source and review what we publish, and who is on the team, is set out on our editorial standards page.
Pattern and stock data verified by Alex Deng, Merchandise Director. Sleepwear safety section reviewed by Melody Zhao, Legal Affairs Manager, on 14 September 2026.
Published 14 September 2026.
Where the numbers come from. Height bands, weight bands and body measurements are read from PatPat pattern data, the same source the size chart on every product page uses. Stock counts are a snapshot taken on 4 September 2026 and move daily.
The fit issues described in "Where we get it wrong" are drawn from customer reviews of PatPat products on our own site and on third-party retail and review platforms, not from a controlled study. They are the patterns that recur often enough to be worth telling you about in advance.
Wear windows are calculated, not surveyed. For each size we took the height band and the weight band the pattern is cut to, found when a child growing along the fiftieth percentile of the WHO Child Growth Standards enters and leaves each band, and took whichever of the two dimensions is exhausted first. Figures are given for boys, who sit slightly heavier and longer than girls at every age in the range. They describe a median child and are not a prediction about any individual one, and they are not medical guidance. Source: WHO Child Growth Standards, weight-for-age and length-for-age, birth to 2 years and birth to 5 years.
Sleepwear flammability requirements are cited from the US Consumer Product Safety Commission: 16 CFR part 1615 and part 1616, the CPSC Flammable Fabrics Act business guidance, and the CPSC children's sleepwear FAQ.
The status of US children's sizing standards is per ASTM International Subcommittee D13.55: D4910/D4910M covers infant sizes and remains active, and D5826, covering sizes 2 to 6x/7, was withdrawn in 2009.
Growth figures are from the WHO Child Growth Standards: weight-for-age, boys, birth to 5 years and length-for-age, boys, birth to 2 years. The full WHO length and height-for-age standards include girls' tables and z-score versions.
Last updated 14 September 2026 · Next review September 2027, or sooner if our size charts change.
