Published 17 September 2026
Age labels are not measurements. A tape measure and two minutes will tell you more than the number on the hanger — and for swimwear specifically, the instinct to size up is usually the wrong one.
“Age 5–6” describes a rough average of a five-year-old, drawn from whatever fit block that brand happens to use. It is not a measurement, it is not standardised between brands, and it is not standardised between garments within a brand.
Two shorts both labelled 5–6 can differ by several centimetres at the waist. That is why a child can be one size in one brand and another size elsewhere, and why nothing is wrong with your child when that happens.
The useful number is the garment measurement, not the age. Any brand worth buying from publishes one. If a listing gives you only an age, you are being asked to guess.
Two measurements do almost all the work. Measure over thin underwear, not over clothes, and keep the tape snug but not pulled tight.
1. Waist. Around the narrowest part, roughly level with the belly button. Ask them to breathe out normally rather than hold it in.
2. Hip. Around the widest part of the bottom. On many children this is the larger of the two numbers, and on a pull-on short it is the one that decides whether the garment goes on at all.
Worth adding for swim shorts: the upper thigh, about where the hem will sit. It is the measurement nobody takes and the one that decides whether the leg grips or digs.
Write them down somewhere you will find them again. Children change slowly enough that the numbers stay useful for months, and quickly enough that you will not remember them.
A t-shirt is cut larger than the body. Swimwear is usually cut smaller and relies on the fabric stretching to fit — which is why a size chart for swimwear can show numbers below your child's actual measurements and still be correct.
That difference matters because a stretch garment only works under tension. The fabric holds its shape, stays where it is put, and moves with the leg because it is slightly stretched. Take the tension away and it stops doing any of those things.
| Swim short too big | What happens |
|---|---|
| Waist has no grip | Slides down, gets hauled up all day |
| Leg is loose | Rides up and bunches at the top of the thigh — which is exactly where chafing starts |
| Fabric sits away from skin | Holds water and sand between layers instead of draining |
| Extra fabric in the water | Drags, and a child feels it |
Everything parents buy swimwear a size up to avoid, buying a size up tends to cause.
The instinct with children's clothes is to go up and get another season out of it. For outerwear and school uniform that is sound. For swimwear it usually backfires, for the reasons above.
Between sizes, take the smaller one — unless the larger measurement is the hip and the smaller size will not physically go on.
Check the hip, not the waist, for whether it fits at all. An elastic waist stretches a long way; the hip is what a pull-on short has to pass over.
Check the waist for whether it stays up. These are two different questions and they have two different answers.
A swim short that is slightly snug is doing its job. A swim short bought to grow into spends its first season being pulled up, and by the time it fits, it has had a year of chlorine.
Turn the garment inside out and look for a mesh liner — a second layer, shaped like briefs, sewn in at the waistband. It is fitted to the size on the label, so on a short bought a size up it is loose as well, and its cut edge sits directly against the thigh.
Enough parents cut them out that there are guides explaining how. If sizing has been a running battle, the liner may be a bigger part of it than the size was.
Why swim shorts have mesh liners — where they came from, and what to look for instead.
We are making swim shorts in five sizes, 2–4 through 11–12. Every figure we publish is a garment measurement, taken flat across one layer.
| Size | Waist, relaxed | Hip | Side length |
|---|---|---|---|
| 2–4 | 18.5 cm | 26.0 cm | 21.5 cm |
| 5–6 | 20.0 cm | 28.0 cm | 24.0 cm |
| 7–8 | 21.5 cm | 30.0 cm | 26.5 cm |
| 9–10 | 23.0 cm | 32.0 cm | 29.0 cm |
| 11–12 | 24.5 cm | 34.0 cm | 31.5 cm |
Half measurements, garment laid flat and relaxed. Double them for the way round a body. The 5–6 column was physically measured from a reference garment; the other four are graded from it and will be confirmed against real samples before anything is sold. We would rather say which is which than present all five as though they were equally certain.
There is nothing to buy yet. We are in sampling, and we are not launching until 2027. If you want to know when there is, there is a list.
Join the list — we write when there is something worth saying, and not otherwise.
What colour swimwear can you actually see under water? — we modelled eight prints at depth. Blue, the most common colour in children's swimwear, turned out to be close to invisible.