Baby clothes are sold by labels such as Newborn, 3M, 6M, 12M and 2T that look like ages but are really body sizes. Parents often guess from the age on the label, then find the sleeves too long or the bodysuit too tight. This calculator removes the guesswork: you enter your child’s height (or length) and weight, and it matches both against a published brand size chart and tells you which size to buy.
How to use the baby clothes size calculator
- Measure length or height. Lay a baby on a flat surface and measure from the top of the head to the heel with a soft tape, with the legs gently straightened. For a toddler who can stand, measure against a wall.
- Weigh the child. A recent weight from a clinic visit is fine. A baby’s weight changes fast, so use the latest figure.
- Pick the unit for each box (centimetres or inches, kilograms or pounds).
- Read the results. The calculator shows the size for height, the size for weight, and the recommended size, which is the larger of the two. It also prints the chart range for that size.
The sizing formula: how the size is chosen
size by weight = first size whose upper weight limit ≥ your weight
recommended size = the larger of the two
The ranges come from the Carter’s size chart (retrieved 2026-10-15) listed below. A child exactly on the boundary between two sizes goes in the smaller size, because the upper limit belongs to it. A child who falls in the small gap between two printed ranges (for example 87 cm, between 24M and 2T) is treated as belonging to the next size up.
| Size | Height (in) | Height (cm) | Weight (lb) | Weight (kg) |
|---|---|---|---|---|
| Newborn (NB) | 18 – 21.5 | 46 – 55 | 6 – 9 | 2.7 – 4 |
| 3M | 21.5 – 24 | 55 – 61 | 9 – 12.5 | 4 – 5.7 |
| 6M | 24 – 27 | 61 – 69 | 12.5 – 17 | 5.7 – 7.5 |
| 9M | 27 – 28.5 | 69 – 72 | 17 – 21 | 7.5 – 9.3 |
| 12M | 28.5 – 30 | 72 – 76 | 21 – 25 | 9.3 – 11.1 |
| 18M | 30 – 32 | 76 – 81 | 25 – 28 | 11.1 – 12.5 |
| 24M | 32 – 34 | 81 – 86 | 28 – 30 | 12.5 – 13.6 |
| 2T | 35 – 36.5 | 88 – 93 | 29 – 31 | 13.2 – 14.1 |
| 3T | 36.5 – 39 | 93 – 98 | 31 – 33 | 14.1 – 15 |
| 4T | 39 – 41.5 | 99 – 105 | 34 – 37.5 | 15.4 – 17 |
| 5T | 41.5 – 44 | 105 – 112 | 37.5 – 42 | 17 – 19.1 |
The chart is a general guide from the brand; individual garments, such as footed sleepers, can run differently. Always check the label of a specific item.
Worked example
A baby is 72 cm long and weighs 8.4 kg.
- Size by height: 72 cm is not above the 9M limit of 72 cm, so the height says 9M.
- Size by weight: 8.4 kg is above the 6M limit (7.5 kg) and within the 9M limit (9.3 kg), so the weight says 9M.
- Both agree, so the recommended size is 9M.
If the same baby grew to 73 cm without gaining weight, the height would move to 12M and the recommendation would follow it.
Newborn to 24 months and toddler sizes
- Newborn to 24M sizes are cut for babies who are not yet walking, with snap closures and extra room for a nappy. They step up by roughly 5 to 7 cm in length at a time.
- 2T to 5T are toddler sizes. The cut is for a child who walks and runs, with a slimmer nappy allowance, and the labels count years rather than months.
- The two systems overlap at the junction, which is why 24M and 2T are listed with a short gap between them. Choose by measurement rather than by whether the child has turned two.
Tips for buying baby clothes
- Newborn is short-lived. Many babies move past Newborn within weeks, so buy a few pieces and rely on 3M for the rest.
- Sleepwear and outerwear: if between two sizes, take the larger so the garment lasts longer.
- Wash first: cotton baby clothes can shrink, so a size that fits exactly before washing may feel snug afterwards.
- Safety: choose sleepwear that fits snugly, as loose fabric near a sleeping baby is a hazard; follow the labelling and local sleepwear rules.
For feet see the Kids Shoe Size Calculator, and for older children the Letter Size to Number Converter (S/M/L) converter explains S, M and L against numeric sizes. This page gives general guidance only and does not replace the size information printed on a specific garment.
Sources
Every figure above comes from these pages, read directly on the dates shown.
- Carter’s — size chart (newborn, 3M–24M, 2T–5T) — checked 2026-10-15
Frequently asked questions
What size baby clothes should I buy for my baby?
Measure your baby’s length and weigh them, then match both to the size chart. The calculator picks the size for height and the size for weight and recommends the larger of the two, because a garment that is too short or too tight is more of a nuisance than one that is slightly roomy.
Do baby clothes sizes mean age?
Not exactly. A label such as 6M is a body size, not a promise about age. The Carter’s chart ties each label to a height and weight range, and babies of the same age can sit in different sizes. Use measurements and treat the age on the label as a rough guide.
Should I size up for growth?
Babies grow quickly, so many parents buy one size above for sleepwear and outerwear they want to last. The calculator shows both the size for height and the size for weight so you can see how close your baby is to the next size. See also Kids Shoe Size Calculator for feet.
Why do brands run differently?
There is no legal standard for baby clothing sizes. This page uses one brand’s published chart, so another brand’s 6M can be longer or shorter. When a brand publishes its own chart, trust that chart for that brand.
What if my baby is between two sizes?
Take the larger size. A small amount of extra length can be rolled or grown into, whereas tight sleeves and leg openings are uncomfortable. If the height and weight point to different sizes, the larger one is shown as the recommendation.