How long do babies and toddlers stay in each clothing size?

Sizes are measurement bands, not birthdays

The months printed on baby clothes look like ages, but the standards behind them say otherwise. ASTM International publishes the body measurement tables that clothing makers use as a starting point for infant clothing, and the scope of ASTM D4910 states plainly that infant sizes 0 to 24 designate sizes and do not refer to age. Toddler sizes 2, 3 and 4 and kids' sizes 5 and 6 come from a separate table, ASTM D6192, which lists them for boys and girls alike.

Those tables are body measurements, not finished garments. ASTM describes them as a baseline that makers adapt for fabric type, room for movement, styling and fit, which is why two shirts with the same label can fit the same child differently.

Children of the same age also differ in size. On the WHO length chart, boys at 12 months run from about 71.8 centimeters at the 5th percentile to 79.7 at the 95th, a spread of almost 8 centimeters. On CDC's chart for 2-year-old boys the same two lines sit at 80.7 and 92.2 centimeters. A child near either line will move through labeled sizes earlier or later than the age on the tag suggests.

This guide uses population growth charts to explain clothing sizes. It is not a way to judge your own child's growth, which is something your pediatrician follows with measurements over time.

The first year: sizes go by in weeks

CDC recommends the WHO Child Growth Standards for children from birth to age 2. WHO built them from children aged 0 to 59 months in six countries, including the US, living in environments its researchers considered supportive of optimal growth. The median lines show how fast the first months move.

For boys, median length goes from 49.9 centimeters at birth to 54.7 at one month, 61.4 at three months, 67.6 at six months and 75.7 at twelve months. For girls the same points are 49.1, 53.7, 59.8, 65.7 and 74.0 centimeters. That is close to 5 centimeters in the first month alone.

The pace then slows quickly. Between 9 and 12 months the median boy adds 3.7 centimeters and the median girl 3.9, or roughly 1.2 to 1.3 centimeters a month, about a quarter of the first month's rate. Whatever the exact band printed on a label, a newborn crosses it far faster than a one-year-old does. This is why newborn and early month sizes can feel outgrown almost as soon as they are washed.

Year two and the move to toddler sizes

In the second year growth settles to about a centimeter a month. On the WHO standards the median boy goes from 75.7 centimeters at 12 months to 87.8 at 24 months, and the median girl from 74.0 to 86.4, a gain of a little over 12 centimeters for both.

Around the second birthday two things change at once. Clothing moves from infant month sizes to toddler sizes, and CDC switches from the WHO standards to its own 2000 growth charts for children 2 and older. The WHO chart for under-2s uses length measured lying down, while CDC's chart uses standing height, so the two do not join exactly: the WHO median for boys at 24 months is 87.8 centimeters, the CDC median at 24 months is 86.5. For choosing clothes, the gap is small.

From 3 to 10: each size lasts longer

After age 2 the yearly gain keeps shrinking, but slowly. CDC's median stature for boys is about 95.3 centimeters just after the third birthday, 102.5 at four, 109.2 at five, 115.7 at six, 122.0 at seven, 128.1 at eight, 133.7 at nine and 138.8 at ten. For girls the same points are 94.2, 101.0, 108.0, 115.0, 121.8, 127.8, 133.1 and 138.2 centimeters.

Put as yearly gains, the median boy adds about 7 centimeters between three and four, close to 6.5 a year from five to seven, about 6 between seven and eight, and about 5 between nine and ten. Girls follow a similar curve. Compared with the five centimeters a newborn can add in a month, these are slow changes, so a size that fits now has far more time left in it than a size did in the first year.

The practical result is a change in rhythm. In the first year it makes sense to have the next size washed and within reach. From about age 3, a bin per size can stay closed for most of a year, and the bigger question becomes which season the next size will arrive in.

Common mistakes when stocking the next size

Reading the label as an age. ASTM is explicit that infant sizes are not ages. A long, lean baby may need 6-9 months clothes well before six months, and a smaller one may wear 3-6 months long after. Go by how the current size fits, not by the birthday.

Stocking far ahead in the first months. Growth is fastest at the start, so a pile of newborn and 0-3 month clothes can go mostly unworn. The slower pace after the first year is where planning a size or two ahead pays off.

Forgetting the season. A child who reaches 3T in December needs a 3T coat, not just 3T T-shirts. When you estimate when the next size will arrive, look at the month as well as the size.

Assuming every brand runs the same. ASTM's tables are a starting point that makers adjust for fabric, ease and styling, so the same label can fit differently. Keep the outgrown size around until the new one really fits.

Comparing with a sibling's timeline. The spread between the 5th and 95th percentile lines covers many centimeters at every age, so a younger child may need 3T earlier or later than an older one did. A record of what is in each bin helps here. SizeShelf, for example, estimates roughly when the next size is likely to be needed from published growth charts, shows it as a range next to a note that every child grows differently, and counts the pieces you already have in that size.

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