Published 17 September 2026
It is a rating for a piece of fabric, measured new, dry and unstretched in a laboratory. A swimsuit on a child in the sea is none of those three things.
UPF is Ultraviolet Protection Factor. It describes how much ultraviolet radiation a fabric lets through.
| Rating | Roughly what it means |
|---|---|
| UPF 15 | About 1/15 of UV gets through — roughly 93% blocked |
| UPF 30 | About 1/30 — roughly 97% blocked |
| UPF 50+ | About 1/50 or better — roughly 98% blocked |
50+ is the top of the scale. There is no meaningful difference between a fabric that tests at 50 and one that tests at 300, because both are already stopping almost everything. A brand advertising a bigger number is not offering you more protection.
The gap between 30 and 50+ is also smaller than it sounds: 97% against 98%. Useful, but not the difference the labels imply.
The rating is measured on a fabric swatch in controlled conditions. Four things change in real use.
Water fills the gaps between fibres and changes how the fabric scatters light. How much protection is lost depends heavily on the fibre — cotton loses a great deal, technical swim fabrics much less — but the tested number is a dry number, and a swimsuit's whole job is to be wet.
Stretching a knitted fabric opens the spaces between the yarns and lets more light through. A garment stretched tight over a child who has grown into and then past it is protecting less than the same garment on the day it fitted.
This is the least known of the four and, for children's clothing, probably the most relevant — because children grow and garments get handed down.
Chlorine, salt, sunlight and washing all degrade fibres over time. A third-season swimsuit does not perform like a new one, and nothing on the label tells you where it is in that decline.
The most obvious and the most overlooked. A UPF 50+ swim short protects the part of the child it covers, which is the thighs. Shoulders, back of the neck, ears, nose and the tops of the feet are not covered by shorts at any rating.
If sun protection is the goal, a long-sleeved rash vest and a hat do vastly more than any rating printed on a pair of shorts. The shorts are the least important garment in that argument.
Ask which standard it was tested to. Ratings are issued under different test standards, and they do not all test the same way. Some assess fabric dry, new and flat. UV Standard 801 is the demanding one, because it assesses fabric wet, stretched and after wear — conditions closer to a real swimsuit. A garment rated under it has cleared a harder test.
Look at the fabric, not the claim. Dark, dense, tightly knitted fabric blocks more than pale, thin, open fabric. Hold it up to a window: if you can see light through the weave, UV is going through too.
Check whether the claim covers the garment you are buying. A brand may have tested one fabric and print the claim across a range. The printed version of a fabric is not necessarily the tested version.
Our fabric is a dense recycled knit, and it would very likely test well. We could probably print the claim and never be challenged on it.
We have not tested it, so we are not going to print it.
A UPF claim is a claim about a child's safety. Standing behind it means holding a test report for the finished, printed garment — not for the fabric before it was printed, and not for a similar fabric from the same supplier. Testing costs a few hundred pounds and we would rather spend it than borrow the number.
If and when we hold that report, we will say so and say which standard it was tested to. Until then the label stays quiet, and this page is the reason.
None of this is medical advice, and no garment is a substitute for shade, timing and supervision. UV is strongest in the middle of the day and reflects off water and sand. Clothing is the most reliable of the protections available, which is exactly why the claims on it are worth reading carefully.
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.
What size swim shorts does my child need? — and why sizing up, the usual instinct, tends to backfire on swimwear.