How to do the shirt test at home

What you need

You need a UV card, a bright day and the shirt you want to check. Every Rezzella order comes with a UV card, and cards like it are easy to find. The card holds a dye that darkens in ultraviolet light and fades again away from it, so it shows you something your eyes cannot.

Choose a time when the sun is high and the sky is clear. Work outside, in direct sun, on a flat and pale surface such as a table or a sheet of white paper, which makes the colour of the card easier to read.

Start with a pale card

Keep the card indoors until it is fully pale. These cards darken faster than they fade: a 2024 study of UV indicators found that they change colour almost at once in simulated sunlight, but need a minute or more to return to their starting colour. A card that is still a little dark from the last try will spoil the comparison.

The card in the sun

Take the pale card outside and lay it face up in full sun. It darkens almost straight away. Note the colour, or take a photograph of it beside the white paper, because that is the colour of full sun on that day, at that hour.

Then bring the card back inside, and wait until it is pale again before the next part of the test.

The card under the shirt

Lay the shirt flat in the same sun and slide the pale card inside it, between the front and the back, so that only the front layer sits above the card. Leave it for as long as the card took to darken in the open. Then lift the fabric and look straight away, before the card starts to fade.

If you have two cards, you can run both parts at once: one card in the open, and one under the shirt, side by side. The sun and the heat are then the same for both, which makes the comparison fairer and quicker.

Reading the result

If the card under the shirt is as dark as the card in the open, a lot of ultraviolet is getting through. If it is a little lighter, some is getting through. If it stays pale, the fabric is stopping most of what the card can detect.

It is worth trying the same shirt twice more: gently stretched, as it would be across your shoulders, and again when it is damp. ARPANSA notes that stretching opens the spaces in a knit, and SunSmart notes that most fabrics protect less when wet, so these are often the moments a shirt lets more through.

What it cannot tell you

The shirt test is a comparison, not a rating. The same 2024 study found that these cards reach their darkest colour at low levels of ultraviolet, so a card cannot separate a good fabric from an excellent one. An earlier study of UV indicator cards found that heat changes their colour too, so keep the cards off hot stone or metal, and compare cards that have sat in the same place.

For a rating, fabric goes to a laboratory, which measures it to a standard such as AS/NZS 4399:2020. We explain that process in what UPF is and how clothing is tested, and what the card shows about everyday tees in does a T shirt block the sun.

Why we ask you to do it

Most shirts let the sun through. Rezzella is made not to, and the UV card in every order lets you check that with your own eyes, on your own day. Try it with the Sun Tee, then with a tee you already own, and keep the card for the next piece.

Our shirt test page shows the same test as a demonstration. Our fabric goes to ARPANSA, Australia’s radiation protection agency, for testing to AS/NZS 4399 before the first run. The full report goes on the shirt test page.