Baby and Toddler Clothing Size Calculator
Enter your child's weight and optional age to find the clothing size that fits best right now, and learn when to trust weight over the age label.
Enter a weight to find the size.
Add your child’s weight in pounds, and optionally their age, to see the clothing size that fits best right now.
Clothing sizing is not standardised, so ranges and labels differ between brands and even between styles, and this uses common US sizing as a guide. When in doubt, go by your child's actual measurements and build rather than the printed month, and remember a size up buys longer wear far more often than a size down buys a fit.
The formula
Weight (lb) → the highest size band whose upper limit is not exceeded; age is only a cross-check, since build varies more than the calendar
- Weight band — The size range your child's weight falls into, which is the primary recommendation.
- Age label — The month-based size most charts assume, used here only to cross-check.
- Build — A child's height and proportions, which is why two same-age babies need different sizes.
- Sizing up — Choosing the larger size when between bands, for comfort and longer wear.
Common baby and toddler clothing sizes
| Size | Typical weight | Typical age |
|---|---|---|
| Newborn (NB) | Up to 10 lb | 0 to 1 month |
| 3 months | 10 to 13 lb | 0 to 3 months |
| 6 months | 13 to 17 lb | 3 to 6 months |
| 9 months | 17 to 21 lb | 6 to 9 months |
| 12 months | 21 to 25 lb | 9 to 12 months |
| 18 months | 25 to 28 lb | 12 to 18 months |
| 2T | 28 to 32 lb | 18 to 24 months |
| 3T | 31 to 36 lb | 2 to 3 years |
Approximate common US sizing. Ranges and labels vary between brands and styles, so always cross-check a specific brand's measurement chart, and favour weight over the age label when the two differ.
Buying clothes for a fast-growing baby is an exercise in forecasting, and the size printed on a shop label is only a guess built around an average child who may look nothing like yours. The good news is that one measurement cuts through most of the uncertainty far better than age alone.
This tool takes your child's weight, optionally with their age, and returns the baby or toddler size most likely to fit now, using common sizing bands. If age and weight disagree, which happens constantly, it tells you to trust the weight and to size up when you are caught between the two.
The whole design leans on a simple truth of infant clothing: babies the same age can differ enormously in size, while a weight tells you a great deal about what will actually fit today. Get that one number and the endless debate over month labels largely dissolves.
Why weight beats the month label
Clothing brands put an age on a size for shelf simplicity, but that age is really shorthand for a weight and height range, and it is the range that determines the fit. A robust four-month-old and a slender four-month-old routinely need different sizes despite being identical in age.
Weight is the more stable, more informative single figure because it tracks overall body mass, which is what a bodice, nappy area and sleeve length have to accommodate. Two children at the same weight will usually fit the same size far more reliably than two children at the same age.
That is why this calculator leads with weight and treats age as a cross-check only. Age is genuinely useful for judging length in a tall or petite child, and for gift-giving when you do not know a weight, but for your own child the scales tell you more than the birthday does.
How the common size bands line up
In typical US sizing the ladder runs from Newborn, roughly up to ten pounds, through three, six, nine and twelve-month infant sizes, then into toddler sizing labelled by years such as 2T, 3T and 4T. Each band covers a weight range, and moving between them is gradual rather than a cliff edge.
The handover from month-based infant sizes to year-based toddler sizes usually happens around the second birthday, near the 28 to 32 pound area. It can be confusing because a 2T size and an 18 to 24 month size overlap heavily, and different shops use one or the other for the same child.
The bands this tool uses are pragmatic averages that most high-street brands roughly follow. Treat them as a starting point, then let a brand's own measurement chart, which you can usually find online, confirm the call on anything you are ordering rather than guessing.
Reading a gap between age and weight
When you enter both figures and they point to different sizes, that is not an error, it is the single most useful thing the tool shows you. A baby who is five months old but weighs like a typical nine-month-old will be uncomfortable and short-lived in a five-month size, and happier in the larger one.
The general rule is to follow the weight for fit and the age only as a tiebreak. If weight suggests a bigger size than age, trust weight, because the garment has to accommodate the body that is actually there, and taller or chubbier babies need the room the larger size provides.
Where you land right on a boundary between two bands, sizing up is almost always the safer buy. A slightly roomy garment is simply worn sooner and lasts longer, while a snug one is outgrown within weeks and can be uncomfortable around the thighs, neck and nappy area.
A worked example
Enter thirty pounds and twenty-two months. By weight your child sits in the 2T band, and the age cross-check of 18 to 24 months agrees, so 2T is a confident, comfortable choice right now rather than a stretch of the label.
Now try a fifteen-pound, three-month-old. Weight points to the six-month band while the month label says three months. The tool nudges you to favour weight and size up, because a chunky three-month-old simply needs the body room of a larger size to fit over the nappy and allow for growth.
The same logic explains why gift-buying by age is so hit-or-miss and why parents joke about outgrowing presents before opening them. If you are buying ahead for a new baby, a slightly larger size you are unsure about is nearly always the better bet than a newborn size that may never get worn.
Fit, safety and sensible buying
Beyond size, a couple of fit details matter for comfort and safety. Sleepwear should be snug-fitting rather than baggy, since loose fabric near a heat source is a fire risk and standards label sleep garments accordingly, and drawstrings or long ties around the neck are best avoided on young children.
Think in seasons and growth spurts rather than single outfits: babies grow fast, so buying a few items in the next size up to live into is often wiser than filling a wardrobe in a size they will outgrow in a month. A well-fitting layer that has room to grow earns its place far longer.
Finally, use the calculator as a starting point and your eyes as the final judge. If a neckline is tight, a fastener strains, or the crotch sits high over a nappy, that is the garment telling you to go up a size regardless of what any label or month chart would have predicted.
Key terms
- Infant sizing
- Sizes named by months, such as 3, 6, 9 and 12 months, used for babies.
- Toddler sizing
- Sizes named by years with a T, such as 2T or 3T, used once a child walks.
- Snug fit
- Close-fitting fabric required on children's sleepwear for fire-safety reasons.
- Size up
- Choosing the larger of two sizes, usual when between bands or gifting ahead.
Pro tips
- Use your child's current weight as the primary guide.
- Size up when you are between two bands or buying ahead.
- Check each brand's own measurement chart online.
- Choose snug-fitting sleepwear and avoid neck drawstrings.
Common mistakes
- Buying strictly by the month label for a bigger or smaller baby.
- Assuming every brand sizes the same way.
- Filling a wardrobe in a size that will be outgrown in weeks.
- Choosing baggy sleepwear when a snug fit is the safe option.
Baby and toddler clothing sizes — your questions
Should I buy baby clothes by age or by weight?
Buy primarily by weight, and use age only as a rough cross-check. The month printed on a size is really shorthand for a weight and height range, and because babies of the same age vary hugely in build, weight is the more reliable single figure for what will actually fit. Age is still helpful for judging length in a tall or petite baby, and for gift-buying when you have no weight to work from.
What is the difference between 12 months and 12-18 months?
A plain twelve-month size is cut for a smaller baby than a 12 to 18 month or eighteen-month size, which allows for a few extra pounds of length and girth. Brands are inconsistent about which they use, so treat the ranges as guides and check the specific measurement chart. If your child sits near the top of a size or you want longer wear, go for the larger of the two labels rather than the smaller.
When do you move from baby sizes to toddler 2T?
The shift from month-based infant sizes to year-based toddler sizes typically happens around the second birthday, roughly in the 28 to 32 pound area. The overlap can be confusing, because a 2T and an 18 to 24 month garment are often cut very similarly. Rather than a fixed rule, follow the fit: when your child is walking and a toddler cut gives comfortable room over the nappy and thighs, that is the cue to move to the T sizes.
My baby is between two sizes. Which should I pick?
In almost every case size up. A slightly roomy garment is simply worn sooner and lasts longer, whereas a snug one gets outgrown within weeks and can dig in around the thighs, neck or nappy. The main exception is sleepwear, which should be snug-fitting rather than baggy for fire-safety reasons. For everyday clothes, choosing the larger of the two sizes you are between is the safe and economical call.
Are baby clothing sizes standard across brands?
No, and that is the source of most frustration. There is no universal standard, so two brands can both print six months yet cut noticeably different garments, and styles vary too, with some running slim and others roomy. Use a size calculator and the weight bands as a starting point, then rely on each brand's own measurement chart and, once you find a label that fits your child well, stick with it where you can.