What is UPF? How to read a UPF 50+ sun shirt label

Almost every sun shirt online says "UPF 50+". But what is UPF, what does the number measure, and how do you know it is real? This short explainer helps you read a sun shirt label properly instead of taking the marketing at face value.

What is UPF?

UPF stands for Ultraviolet Protection Factor. It tells you how much ultraviolet radiation, both UVA and UVB, a fabric blocks before it reaches your skin. The simple way to read it: UPF 50 means only about 1/50 of UV passes through the fabric, so roughly 98% is blocked. UPF 15 means about 1/15 gets through.

UPF is measured in a lab under standards such as AS/NZS 4399 (Australia and New Zealand), ASTM D6603 and AATCC 183 (United States) or EN 13758 (Europe). The Australian and US standards use the same three bands: UPF 15 to 24 is good, 25 to 39 is very good, and 40 to 50+ is excellent. Fabric below UPF 15 is not considered sun protective.

Related: Sun protection for scooter riders: cover up and stay cool

UPF versus SPF

The two are not interchangeable. Clothing protects the skin it covers; sunscreen is for whatever is still exposed, such as your face, neck and the backs of your hands.

What makes a fabric block more sun

• Weave density and thickness: the tighter the weave, the fewer gaps for UV to slip through.
• Colour: darker shades usually absorb more UV than light ones in the same fabric.
• Fibre: synthetics such as polyester tend to block more than thin cotton, and some fabrics are treated with UV absorbers.
• Stretch and moisture: fabric that is pulled tight or wet usually blocks less than when it is dry and relaxed.
• Age: fabric that has been washed thin or worn through blocks less.

So a thin white T-shirt may protect less than you think, especially once it is damp with sweat.

How to check a UPF claim on a marketplace

On online marketplaces, "UPF 50+" is often just part of the product title. It is the seller's claim. aochongnang.com does not test or assign UPF ratings to any product. Before you rely on it, you can:

• Check whether the description names a test standard (AS/NZS 4399, ASTM D6603, EN 13758) or shows a test report.
• Read the fabric content, weight and weave.
• Look at buyer photos: if the fabric looks paper-thin and you can see light through it, be cautious.
• Prefer sellers who give a clear answer when you ask where the number comes from.

Without test information, treat a UPF figure as marketing and choose on coverage, fabric weight, colour and how it feels to wear.

Choose by label and by need

The label is only part of the picture. A high-UPF shirt with short sleeves and a low neckline still leaves a lot of skin exposed. Look at the whole garment: long sleeves, a high collar, a hood and a long enough hem. Riders need a longer cut, beachgoers need quick-dry fabric, office workers need something thin that packs small.

Browse women's sun shirts and men's sun hoodies, or use the sun protection finder for a shortlist based on how you spend time outdoors.

The short version

UPF measures how well fabric blocks UV; UPF 50 means about 1/50 of UV gets through. The number is trustworthy when a test standard backs it. A sun shirt does not replace sunscreen on exposed skin, and any skin concern, such as a changing mole or a bad burn, should be checked by a dermatologist.