Kids Unicorn Costume Onesie Pyjamas

Picking the right onesie size comes down to one simple measurement: height. Unlike regular clothing, onesies are designed to be roomy and comfortable, so height (not chest or waist) is the most reliable way to choose your size.

Below you'll find our full sizing charts for kids and adults, plus tips on measuring correctly and what to do if you're stuck between two sizes.

Quick Answer

Measure height from head to toe without shoes, then match it to the size charts below. If your height falls between two sizes, choose the larger one, onesies are made to be worn loose and cosy, not tight.

Kids Onesie Size Chart

Kids onesies are sized by height in centimetres, so you can match your child's measurement directly to a size.

Tip: If your child is between two sizes, or you'd like room to grow, size up. Kids onesies are made with a relaxed fit for comfort and layering over pyjamas on cold nights.

Adult Onesie Size Chart

Adult onesies follow the same height-first logic. If you're taller or shorter than average for your size, or carry more weight around the middle, sizing up gives extra room through the body without dragging on the length.

How to Measure Yourself (or Your Child) for a Onesie

1. Stand flat against a wall with shoes off and heels touching the skirting board.

2. Use a tape measure or a marked pencil line to measure straight up from the floor to the top of the head.

3. Record the height in centimetres and match it against the charts above.

4. If the measurement falls exactly between two sizes, choose the larger size for comfort.

Onesie Fit Tips

• Between sizes? Always round up. A slightly loose onesie is far more comfortable than a snug one.

• Buying for a teenager? Many teens fit better into adult small (S) than the largest kids size, check both charts before deciding.

• Washing note: Onesies can shrink slightly after the first few washes. If you're right on the size boundary, sizing up gives a buffer.

• Gift buying? If you don't know the recipient's exact height, choose based on their usual clothing size and round up rather than down.