The short version
- UPF tells you how much UV a fabric lets through: UPF 50 lets through about 1/50 of the sun's rays, blocking roughly 98%.
- On the water you get hit twice, from the sky and from light bouncing off the surface, and a breeze hides the burn until it's too late.
- A snug, long-sleeve rash guard rated UPF 50+ protects your torso and arms better than any sunscreen. It doesn't wash off, and you never have to reapply it.
- Fit, stretch and wear all change real-world protection. A shirt that's too tight, faded or worn thin protects less than its label says.
- Clothing covers the big areas. Use sunscreen, a hat and sunglasses for everything else.
Ask anyone who has spent a full day paddling, tubing or snorkeling where they burned, and you'll hear the same list: shoulders, the back of the neck, the tops of the thighs, the backs of the hands. Those are the spots that face the sun and get splashed the most, so sunscreen wears off them first. A good rash guard covers most of that list at once. Here's what UPF means, why water days are harder on skin, and how to pick a shirt that keeps protecting for seasons.
What a UPF rating actually measures
UPF stands for ultraviolet protection factor. It's the clothing version of a sunscreen rating, but it measures something more direct. SPF is based on how long sunscreen-covered skin takes to redden. UPF is measured in a lab, where instruments record how much UVA and UVB radiation actually passes through the fabric. The Skin Cancer Foundation puts it simply: a UPF 50 garment lets about 1/50th of the sun's rays through and blocks about 98%.
In the US, reputable UV-protective clothing is labeled to an ASTM standard (D6603). Fabric is tested new, and also after a simulated laundering and UV-exposure routine meant to stand in for roughly two years of normal use. The label shows the lower of the two results.
| UPF rating | Protection category | Approx. UV that gets through | Our take for water use |
|---|---|---|---|
| 15–24 | Good | About 1/15 to 1/24 | Fine for short, early or late sessions |
| 25–39 | Very good | About 1/25 to 1/39 | A solid everyday swim shirt |
| 40–50+ | Excellent | 1/40 or less (UPF 50 is about 2%) | Our pick for full days on the water |
Category ranges as summarized by UCSF Health. The Skin Cancer Foundation recently raised the bar for its own Seal of Recommendation on clothing and shade products: fabrics now need at least UPF 50, up from a previous minimum of 30. If a product carries that seal, it has been tested both wet and dry.
Why the water is harder on your skin
A day on the water hits you with several things a day in the park doesn't.
- Reflected UV. The CDC notes that UV rays bounce off surfaces like water, sand, cement and snow. On a board or a boat, light reaches you from below and the sides as well as from above. That's why the underside of your chin and the backs of your knees burn on a paddle day.
- Cooling hides the damage. Wind and splash keep you comfortable, so you don't feel heat building on your skin. The burn shows up that evening.
- Sunscreen washes off. The FDA is blunt about it: there's no such thing as waterproof sunscreen. "Water resistant" labels must state whether protection lasts 40 or 80 minutes while you're swimming or sweating. You're supposed to reapply at least every two hours, and more often in the water. On a paddleboard with wet hands, that rarely happens on schedule.
- Long exposure in peak hours. The CDC says UV is strongest from 10 a.m. to 4 p.m. daylight saving time in the continental US.
Fabric solves the last two problems outright. A rash guard doesn't sweat off, doesn't need reapplying, and works the same at 4 p.m. as it did at 9 a.m.
Check the UV Index before you go. The CDC advises taking sun protection steps whenever the index in your area is 3 or higher, and notes that UV still reaches you on cloudy and cool days. An overcast lake day is not a day off from sun protection.
Rash guard, swim shirt or wetsuit top?
| Garment | Fit | Typical fabric | Best for |
|---|---|---|---|
| Rash guard | Snug, second-skin | Nylon or polyester blended with spandex | Surfing, bodyboarding, snorkeling, layering under a wetsuit |
| Swim shirt / loose-fit sun shirt | Relaxed, like a T-shirt | Polyester knit, often quick-dry | Lake days, kids, boating, fishing, anyone who dislikes compression |
| Hooded sun shirt | Relaxed, with hood and often thumb loops | Lightweight polyester | Long paddles, fishing, tropical sun, open decks |
| Neoprene top / wetsuit jacket | Snug | Neoprene, 0.5–2 mm | Cooler water where you want warmth as well as sun cover |
If your water is cold enough that you want insulation, our wetsuit temperature guide and wetsuit thickness chart will tell you when to move past a rash guard.
How to choose a rash guard that protects
Start with the label number. These details decide how it performs on the water.
Sleeve length and coverage
Go long-sleeve. Forearms and the backs of the hands get constant sun on a paddle or tow rope, and short sleeves leave the top of the shoulder exposed whenever you reach forward. If you burn easily, look for thumb loops, a high or mock neck, and a hood. For kids, a long-sleeve top with full-length swim leggings or board shorts covers most of the body with nothing to reapply.
Fit: snug isn't the same as stretched
UCSF Health lists stretch among the factors that reduce a fabric's UV protection. A rash guard should feel close and stay put when you paddle or duck-dive. It shouldn't be stretched so hard that the fabric goes see-through over your shoulders. If you're between sizes and you won't be layering it under a wetsuit, size up.
Weave, color and weight
Tightly knit, heavier fabrics generally block more UV, and darker colors usually beat lighter ones. A quick test from UCSF Health: if you can see light through the fabric when you hold it up, UV can get through too. That said, a light-colored shirt with a UPF 50+ label has been tested and will outperform a dark shirt with no rating. A light color is also cooler in strong tropical sun.
Wet performance
UCSF Health notes that dry fabric generally protects better than wet fabric. That's why a labeled synthetic swim shirt beats a cotton tee in the water, and why the Skin Cancer Foundation's seal now requires testing in both states.
Chafe and comfort
Rash guards got their name from preventing rashes caused by wax, sand and board rails. Look for flatlock seams, which lie flat against the skin. If you'll be lying on a board, pick a top without a chest zipper or chunky logo prints.
Pair your rash guard with a wide-brim or legionnaire-style hat that has a chin strap, and with polarized sunglasses on a retainer. The CDC recommends wraparound glasses that block both UVA and UVB. Glare off the water is as hard on your eyes as it is on your skin.
Shop the gear: Rash guards, sun shirts and wetsuits · Kids' sun-safe swimwear
Where sunscreen still matters
Even the most covered-up paddler has exposed skin. Plan on sunscreen for the face, ears, neck, backs of the hands, and legs below the shorts line. The FDA recommends a broad-spectrum product with SPF 15 or higher, applied about 15 minutes before you head out. It says an average adult needs about an ounce, roughly a shot glass, to cover the full body. Once your torso and arms are in fabric, the amount you need drops a lot, and so do the odds of missing a spot.
Spots people miss most on the water:
- Tops of the ears and the part line in your hair
- Back of the neck where a collar rides down while you paddle
- Tops of the feet (on a SUP they point straight at the sky all day)
- Backs of the knees and the strip between shorts and rash guard
- Lips. Use a lip balm with SPF.
If you're snorkeling over coral, read our guide to reef-safe sunscreen and reef etiquette. Covering up with fabric is the most reef-friendly approach there is, because less lotion ends up in the water.
The FDA recommends keeping infants out of the sun between 10 a.m. and 2 p.m. and using protective clothing when they must be outside. It does not recommend sunscreen for babies under six months. Shade, a sun hat and a UPF suit are the tools for the littlest beach-goers.
Care: keeping your rating for years
Rated fabric is built to hold its UPF through normal use, but rough treatment shortens its life. Chlorine, heat and abrasion break down spandex fibers, and once a rash guard loses its stretch, it bags out or gets stretched thin. Both cost you protection.
- Rinse after every session. Fresh water flushes out salt, chlorine and sunscreen residue, all of which degrade elastic fibers.
- Wash cold and gentle, with mild detergent. Skip fabric softener, which can coat fibers and trap odor.
- Dry in the shade, flat or on a wide hanger. Skip the dryer, and don't leave it baking in the sun for days.
- Don't wring it. Press the water out instead of twisting.
- Retire it when it's see-through. If you can see light through it, or the shoulders have gone thin and shiny, it's time for a replacement.
For storing apparel over the off-season, see our guide to winter storage for watersports gear.
A quick sun plan for common water days
| Activity | Clothing | Also bring |
|---|---|---|
| SUP or kayak, 2+ hours | Long-sleeve UPF 50+ hooded sun shirt, quick-dry shorts or leggings | Brimmed hat with strap, polarized sunglasses, SPF for face, hands and feet |
| Surfing or bodyboarding | Snug long-sleeve rash guard | Water-resistant SPF for face; reapply between sessions |
| Snorkeling | Long-sleeve rash guard plus leggings (your back faces the sun the whole time) | Reef-conscious sunscreen for the face and neck |
| Boat, tubing or wake day | Loose UPF shirt for the boat, rash guard for riding | Shade on board, plenty of water, a hat for between runs |
| Kids at the beach or lake | Long-sleeve rash guard and leggings or long trunks, UPF bucket hat | Shade structure, sunscreen for face, hands and feet |
Put sun gear on your pre-trip safety checklist and our lake day packing list so it goes in the bag every time.
FAQ
Is UPF 30 enough, or do I need UPF 50+?
UPF 30 falls in the "very good" range and blocks the large majority of UV. For full days on the water, especially in summer or the tropics, we recommend UPF 50+. That's also the current minimum for the Skin Cancer Foundation's Seal of Recommendation on clothing.
Does a regular cotton T-shirt work as sun protection in the water?
Not well. Thin, light cotton is often modest protection even when dry, and UCSF Health notes that wet fabric generally protects less than dry. A synthetic rash guard or swim shirt with a tested UPF label is a much better choice.
Do I need sunscreen under a rash guard?
Not under a UPF 50+ garment that fits properly and is in good condition. Put sunscreen on everything the fabric doesn't cover: face, ears, neck, hands, legs and feet.
Does a rash guard lose its UPF over time?
Rated garments are tested after simulated washing and UV exposure, so they're designed to hold up. Chlorine, heat, over-stretching and abrasion still wear fabric down. If it has gone thin, saggy or see-through, replace it.
Should a rash guard be tight?
Snug enough not to ride up or balloon in the water, but not stretched so tight that the fabric thins over your shoulders and back. Stretch reduces UV protection.
Sources
- The Skin Cancer Foundation — The Science Behind the Seal: What Safe, Effective Sun Protection Means
- UCSF Health — Protective Clothing for Skin Cancer Prevention
- CDC — Preventing Skin Cancer
- US FDA — Sunscreen: How to Help Protect Your Skin from the Sun
- ASTM International — D6603 Standard Guide for Labeling of UV-Protective Textiles