Onesie size guide: how to pick the right one

Most onesie returns are not quality problems. They are sizing problems, and nearly all of them come from the same assumption: that a onesie is sized like a jumper. It is not. It is sized like a set of overalls, because that is structurally what it is.

Get the logic right and you will almost certainly order the correct size first time.

Onesies are sized by height

A one-piece garment has a fixed distance between the shoulder seam and the crotch seam. Nothing about that can be adjusted after the fact. If it is too short you feel it immediately, every time you raise your arms or sit down, and no amount of extra width will fix it.

So the primary measurement is your height, and the secondary one is your hips. Your usual dress size, chest size or waist size tells you very little here. A 5ft 2in person and a 6ft person can wear the same size shirt and will never share a onesie.

How to measure your height properly

It sounds patronising until you find out you have been rounding up for a decade.

1. Take your shoes off and stand with your back against a wall, heels touching it.
2. Stand up straight, feet together, look forwards rather than up.
3. Get someone to lay a book flat on your head, level against the wall, and mark the underside.
4. Measure from the floor to the mark, in centimetres.

Do it in the evening if you want the honest number. Everyone is a few millimetres shorter by the end of the day.

And your hips

Measure around the fullest part of your hips and seat, tape level, not pulled tight. This is the number that decides whether the suit sits comfortably when you sit down, which is the position you will be in for most of its life.

Reading the size guide on the product page

Every product page has a size guide with height ranges for each size, and the bands are the same across our adult onesies. Find the row your height falls into and start there. Then apply the two rules below.

Rule one: if you are between sizes, go up

This is the single most useful sentence in this article. A kigurumi is supposed to be loose. Extra length in the sleeve or leg looks intentional and feels lovely. A suit that is a fraction too short pulls at the shoulders whenever you move, and you will stop wearing it within a month.

Rule two: hips beat height

If your height says one size and your hip measurement says the next one up, take the larger. Length can be turned back at the cuff. Width cannot be conjured.

The three mistakes that cause most swaps

Ordering your clothing size out of habit

If you usually wear a medium, a medium onesie feels like the obvious answer. It is only the right answer if the height band happens to agree. Check the band, not the letter.

Buying a gift by eye

Guessing someone's height from memory is surprisingly hard, and people are reliably wrong about their partners by about two inches. If you are buying a present, size up rather than down. A slightly roomy onesie reads as cosy. A slightly small one reads as an awkward conversation. There is more on this in our onesie gift guide.

Sizing a child by age

Children's onesies are sized by height for the same structural reason as the adult ones, and children of the same age vary enormously. Measure them. If they are mid-growth-spurt, size up: a kids' onesie with turned-back cuffs lasts two winters instead of one.

Special cases

Tall and slim

Go by height and accept the extra room through the body. It will look relaxed, which is how the garment is meant to look anyway.

Short and broad

This is the awkward one. Go up for the hip measurement and turn back the cuffs on the sleeves and legs. Fleece cuffs hold a fold well and it is barely visible.

Closed feet or open ankles

Designs with built-in feet need your height to be right, because the foot lands where the foot lands. Designs that finish at the ankle are far more forgiving, so if you are on the edge of a band and there is an open-ankle version, that is the safer buy. It also means you can wear your own slippers, which most people do at home regardless.

Sharing one

Plenty of households buy one onesie and then quietly fight over it. If two people of different heights will realistically wear it, size for the taller one. The shorter person can roll the cuffs. The taller person cannot grow the sleeves.

If you still get it wrong

You might, and that is fine. We offer 30-day free returns, so a swap costs you an envelope and a walk to the post box rather than money. Keep the packaging until you have tried it on, that is the only thing we would ask.

Ready to pick one? Browse the collection. If you are still deciding whether a onesie is the right thing at all, we compared it against a dressing gown and a hoodie blanket, and once yours arrives, wash it properly and it will stay soft for years.