November 7, 2026 · Little Zoo
Baby Onesie Size Guide: How to Choose the Right Size by Age
A onesie that's too small bothers baby; one that's a little big leaves room to grow. Here's how to pick the right size, from 2 months to 3 years.
Size chart by age
| Size | Age |
|---|---|
| 70 | 2 to 5 months |
| 80 | 6 to 12 months |
| 90 | 13 to 18 months |
| 100 | 18 to 24 months |
| 110 | 2 to 2½ years |
| 120 | 2½ to 3 years |
On every product page, the size is shown with the age, for example "6-12 months (80)".
5 tips to get it right
- Between two sizes, go for the larger one. Our onesies have a roomy fit, and babies grow fast.
- Think about the season. Made of warm flannel, they're mostly worn in fall and winter: pick the size your baby will be then.
- For a gift, go one size up. If you don't know the baby's exact size, the next size up will be worn longer.
- Check freedom of movement. Baby should be able to bend their legs and raise their arms without pulling on the zipper.
- Look at build as much as age. Ages are a guide: a tall or chubby baby will often need the next size.
What if the size isn't right?
You have 14 days after delivery to send the item back, unworn and in its original packaging. To get another size, return it and order again in the right size: everything is explained in our refund policy.
Frequently asked questions
Do the onesies run small?
No, they have a rather roomy fit. Follow the age shown, and size up if in doubt.
What size for a 12-month-old?
Size 80 (6 to 12 months) for a petite baby, size 90 (13 to 18 months) for a tall baby or to last all winter.
How do I wash the onesie?
Machine wash at 30 °C (86 °F), inside out, and skip the dryer to keep the flannel soft.