Baby clothing size calculator
Enter your baby's length and weight and this tool gives the size to buy in UK age bands, European centimetres and US months at once. It sizes on the measurement rather than the birthday, because two healthy babies of the same age can be two sizes apart.
Baby clothing size calculator
No sign-up · PrivateLying down for babies who cannot stand, standing after about 2 years. This is the measurement all three systems are built on, so it matters more than the age on the label.
Optional, but worth adding — a chunky baby often needs the next size up before their length says so, because babygros fasten at the crotch and a nappy takes up room.
Optional. Only used to tell you how your baby’s size compares with the age on the label — never to choose the size.
Measure your baby’s length and enter it to get a size. Age is the worst predictor there is — two healthy babies of the same age can be two sizes apart.
Nothing you type leaves your device. The whole calculation runs in your browser.
How this is calculated
FormulaHow the size is chosen
From the measurement, not the birthday. All three systems are defined on body measurements underneath the label; only the label is in months.
- Convert the length you enter to centimetres.
- Round up to the next step on the European ladder: 44, 50, 56, 62, 68, 74, 80, 86, 92, 98, 104. A baby measuring 69 cm goes into 74, not 68.
- Read the UK age band and US month label attached to that step.
- If the weight is above the figure UK size guides give for that band, size up once more regardless of length.
The three ladders, side by side
Europe sizes by body height in centimetres. A size 68 is for a baby who is 68 cm long. There is no age on the label at all.
The UK sizes by age band over the same centimetre ladder. Mamas & Papas publishes the mapping and is explicit that the sizes “refer to the baby's body measurement, rather than the measurement of the garments”: Tiny baby 44 cm and 2.3 kg, Newborn 50 cm and 3.2 kg, up to 1 month 56 cm and 4.5 kg, 0–3 months 62 cm and 6.5 kg, 3–6 months 68 cm and 8 kg, 6–9 months 74 cm and 9.5 kg, 9–12 months 80 cm and 10 kg, 12–18 months 86 cm and 11 kg, 18–24 months 90 cm and 12.5 kg.
The US sizes by a single month with published ranges. Carter's gives Newborn up to 21.5 in and 5–8 lb, 3M 21.5–24 in and 8–12.5 lb, 6M 24–26.5 in and 12.5–16.5 lb, 9M 26.5–28.5 in and 16.5–20.5 lb, 12M 28.5–30.5 in and 20.5–24.5 lb, 18M 30.5–32.5 in and 24.5–27.5 lb, 24M 32.5–34 in and 27.5–30 lb.
The label trap, proved twice
Carter's own guidance says a garment marked 3M “typically compares to the 0-3M range in other brands”. Burt's Bees Baby proves it independently: its 0–3M carries exactly Carter's 3M height range of 21.5–24 in, its 3–6M carries Carter's 6M range of 24–26.5 in, and so on down the ladder. A US 6M is roughly a UK 3–6 months.
Three countries, three ways of labelling the same babygro
Buy baby clothes in more than one country and you quickly notice that the number on the tag is not describing the same thing each time.
In Europe the label is a measurement. A size 68 is made for a baby who is 68 centimetres long, the ladder climbs six centimetres a step, and no age appears anywhere on the garment. It is the clearest of the three systems, and once you know your baby's length you never have to think about it again.
In the UK the label is an age band — Tiny baby, Newborn, 0–3 months, 3–6 months — laid over exactly the same centimetre ladder underneath. UK retailers publish the mapping, and the good ones say plainly that the size refers to the baby's body measurement rather than to the garment.
In the US the label is a single month: 3M, 6M, 9M. That is the one that catches people out, because a single month reads like the top of a range when it is closer to the middle of one.
Why a US 6M is not a UK 6 months
Carter's, the largest childrenswear brand in the United States, says it directly: a garment labelled 3M “typically compares to the 0-3M range in other brands”. Line the charts up and you can watch it happen. Carter's 3M covers 21.5 to 24 inches; Burt's Bees Baby labels exactly that height range 0–3M. Carter's 6M covers 24 to 26.5 inches; Burt's Bees calls the same range 3–6M.
So a US 6M is roughly a UK 3–6 months, not a UK 6–9 months. Buying across markets by the number on the tag will reliably put you a size out, and in the wrong direction — you will buy something too small for a baby who is already growing out of it.
Age is the worst predictor available
Two entirely healthy babies at the same birthday can be ten centimetres apart, which is nearly two sizes. That is not a problem with either baby; it is what a growth chart looks like. Roughly half of all babies sit below the middle line at any moment, and clothing labels are built around the middle.
So a five-month-old in 3–6 months clothing and a five-month-old in 9–12 months clothing are both ordinary, and neither fact tells you anything about how they are doing. If you want to know where your baby sits on a growth chart, that is a different question with a different tool: our baby growth percentile calculator answers it properly, against named reference data.
Buy for the body. A too-big sleepsuit rides up under the arms and the poppers land in the wrong place, which is uncomfortable and makes nappy changes worse at three in the morning.
Why weight can override length
Sleepsuits and vests fasten at the crotch and around the thigh, so a heavier baby runs out of room across before they run out of room lengthways. If your baby's weight is above the figure UK guides give for the band their length points to, this tool sizes up, and says why. Nappy bulk is real, and it is worse overnight.
Buy fewer of the small sizes than you think
Newborn clothes fit for weeks, sometimes days. Every parent ends up with unworn 0–3 months with the tags still on, and a good proportion of babies skip a size entirely. The sizes that earn their keep start at 3–6 months, when growth slows and a garment lasts a season rather than a fortnight.
If people are buying you presents, ask for larger sizes and for the season your baby will actually be that size in. A 12–18 month summer dress given in November is a real present; a newborn one is a photograph.
Two things that do not follow the size chart
Sleeping bags are sized by length and weight for safety rather than for fit. A bag that is too big lets a baby slip down inside it, which is exactly what the design is meant to prevent. Check the manufacturer's minimum weight, follow it, and never size up on those the way you would with a sleepsuit.
Shoes are measured in a shop, not calculated from a chart, and babies who are not yet walking do not need them at all. Soft socks and bare feet are better for developing feet than anything with a sole.
Measuring at home
For a baby who cannot stand, lie them on their back on a firm surface, mark the top of the head and the bottom of the heel with the leg gently straightened, and measure between the marks. It is easier with two people. From about two years, measure standing with the heels against a wall. Do it once a month rather than once a week — a centimetre here or there will not change the size, and it is one fewer thing to worry about.
Sources
- Baby clothing size guide — Mamas & Papas, accessed
- Baby and children's size guide — JoJo Maman Bébé, accessed
- Baby and toddler clothing size chart — Carter's, accessed
- Baby clothes size chart — Burt's Bees Baby, accessed