Quick answer: US baby shoe sizes run from 1 to 10 before kids' sizes begin, and each number is about a third of an inch longer than the last. Size 1 fits a newborn, size 2 fits around 3 to 6 months, size 3 around 6 to 9 months, size 4 around 9 to 12 months, and size 5 around 12 to 18 months. Those ages are averages, though. The number is really a foot length, and the chart below shows which length each size fits.
This is the question we get asked most after "what size does my baby wear": a relative mentions a size 4, a hand-me-down box says size 2, and nobody knows what age that corresponds to. Here's the whole scale in one place, with what to do when your baby doesn't fit the average.
Baby shoe sizes by number
| US size | Foot length | Typical age | Stage |
|---|---|---|---|
| 1 | 3.5 in / 8.9 cm | 0 to 3 months | Newborn |
| 2 | 3.75 in / 9.5 cm | 3 to 6 months | Kicking, not yet crawling |
| 3 | 4.1 in / 10.5 cm | 6 to 9 months | Sitting and crawling |
| 4 | 4.5 in / 11.4 cm | 9 to 12 months | Crawling and cruising |
| 5 | 4.75 in / 12.1 cm | 12 to 18 months | First steps |
| 6 | 5.1 in / 13 cm | 18 to 24 months | Walking |
| 7 | 5.5 in / 14 cm | About 2 years | Running |
| 8 | 5.75 in / 14.6 cm | 2 to 2.5 years | Running and climbing |
| 9 | 6.1 in / 15.6 cm | About 3 years | Preschool |
| 10 | 6.5 in / 16.5 cm | 3 to 4 years | Preschool |
Sizes 1 through 10 are sometimes written with a letter after them, like 4C or 4M, depending on the brand. The C or the M means "child" or "medium width," not a different size. After 13, sizing starts over at 1 in youth sizes, which is why a 4-year-old and a 9-month-old can both "wear a 4" and mean completely different shoes.
Why the ages are only a guide
Foot length at a given age spreads out more than height does. At 12 months, a perfectly healthy foot can be anywhere from 4.3 to 5.2 inches, which spans three sizes. Birth weight, whether a baby walked early, and plain genetics all play a part. So when a chart says size 4 is for 9 to 12 months, read it as "the middle of the range," and measure before you buy.
Measuring is quick. Stand your baby on a sheet of paper, mark the heel and the longest toe, and measure between the marks. Our 1-year-old sizing guide walks through it step by step, and the free size finder turns the measurement into a Grookz size.
How Grookz sizes map to the number sizes
Our sock shoes are sized by age range instead of a number, because the knit stretches and one size covers what would be two or three number sizes in a hard shoe. If you know your baby's US size, this is the match:
| Grookz size | Age range | Fits foot up to | Roughly US sizes |
|---|---|---|---|
| Size 1 | 0 to 6 months | 4.5 in / 11 cm | 1 to 3 |
| Size 2 | 7 to 12 months | 5 in / 12.5 cm | 4 to 5 |
| Size 3 | 13 to 18 months | 5.5 in / 14 cm | 6 to 7 |
| Size 4 | 19 to 24 months | 6.5 in / 16.5 cm | 8 to 10 |
| Size 5 | 2 to 3 years | 7 in / 17.8 cm | 10 to 11 |
When a foot lands right at the top of a range, go up. A roomy sock shoe stays on because the knit holds the ankle, and the foot grows into it within weeks. A tight one makes toes curl.
What age is a size 2 baby shoe?
Usually 3 to 6 months, for a foot around 3.75 inches. Babies this age aren't walking, so a size 2 "shoe" is really for warmth, photos, and keeping socks on. A soft sock shoe or a Size 1 Grookz does the same job with less fuss.
What age is a size 3 baby shoe?
About 6 to 9 months, foot length around 4.1 inches. This is crawling age, when the sole starts to matter: a non-slip bottom keeps knees and feet from sliding on hardwood. Still Size 1 in Grookz sizing for most babies, moving to Size 2 near the 7-month mark or at 4.5 inches.
What age is a size 4 baby shoe?
About 9 to 12 months, foot length around 4.5 inches. Cruising along the couch starts here, and many babies take first steps before the end of this range. A size 4 corresponds to a Grookz Size 2.
What age is a size 5 baby shoe?
About 12 to 18 months, foot length around 4.75 inches. This is the first-steps size, and it's where parents tend to buy the most pairs because growth is fast. A US 5 sits at the top of Grookz Size 2 and the bottom of Size 3; measure to pick between them.
Common sizing mistakes
- Buying by age alone. The age column is an average. The foot length column is the real size.
- Measuring a sitting baby. Feet spread under weight. Measure standing when you can.
- Sizing to the smaller foot. Most people have one foot slightly longer. Size to that one.
- Buying two sizes up. Half an inch of room is right. More than that trips a new walker.
- Trusting an old measurement. Feet grow about half a size every two to three months in the first two years. Re-check every two months.
For a month-by-month version of the chart, see our infant shoe size chart by age, and for the full Grookz range by age, the size chart page.
Frequently asked questions
What age is size 1 baby shoes?
Newborn to about 3 months, for a foot around 3.5 inches. Most newborns don't need shoes at all; socks or a soft sock shoe for warmth is enough.
Is baby size 4 the same as toddler size 4?
Yes. US sizes 1 to 13 run as one continuous scale from infant through toddler. Some stores label the small end "infant" and the larger end "toddler," but a 4 is a 4.
What does the C mean in baby shoe sizes?
C stands for child. A 5C is the same length as a plain 5. Some brands use M for medium width instead.
How many sizes do babies go through in the first year?
Typically four or five, from a 1 or 2 at birth to a 4 or 5 by the first birthday.
Do sock shoes run by number sizes?
No. Sock shoes stretch, so they're sized by age range and foot length. Each Grookz size covers roughly two to three number sizes.
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