UPF is to clothing what SPF is to sunscreen — except it does not wash off. Here is what the number means, how it is tested, and how to pick a shirt that actually protects you on the water.
UPF vs SPF
SPF (sun protection factor) measures how well a sunscreen delays UVB burning, and only while it stays on your skin — about 40 to 80 minutes in the water. UPF (ultraviolet protection factor) measures how much UVA and UVB a fabric blocks, all the time you wear it. UPF 50+ is the top of the scale: the fabric lets through 1/50th of the UV that hits it, so it blocks about 98%.
What makes a fabric UPF 50+
Three things: how tightly it is knit, the fibre (polyester blocks more UV than cotton), and any treatment or dye. A white cotton T-shirt is roughly UPF 5 dry and UPF 3 wet. A UPF 50+ performance knit keeps its rating wet, which is the whole point on a boat or over a reef. Our shirts are a lightweight polyester knit rated UPF 50+, rated under the AATCC 183 / ASTM D6603 UPF test methods.
Does a UPF shirt make you hot?
Not if it is built for it. Bare skin in direct sun absorbs heat; a light, moisture-wicking knit shades the skin and moves sweat off it, and dries in minutes after a swim. Most people find a long-sleeve sun shirt cooler than bare arms once the sun is up.
Where UPF does not help
Anywhere the fabric is not: face, neck, ears, hands. Pair the shirt with a hat, sunglasses and sunscreen on the exposed bits. A sun hoodie covers the neck and ears; a hat handles the face.
How to choose
- Long sleeve for fishing, boating, snorkeling and beach days — full arm coverage.
- Sun hoodie for long runs offshore, the flats, paddling — neck and ears too.
- Short sleeve for the hottest still afternoons with sunscreen on the arms.
Size: our cut is relaxed; size down for a trimmer fit. Care: rinse after salt, wash cold, no fabric softener (it coats the wicking fibres).
Off the water, too
UPF does not care whether you are wet. The same shirt is the right thing for a pickleball session at noon, a hike in July, the yard on a Saturday, or a long day of travel — anywhere you would otherwise be reapplying sunscreen to your arms every hour. A long sleeve in a light colour is cooler than bare arms once the sun is up; that surprises everyone the first time.