UPF stands for Ultraviolet Protection Factor. It’s a rating system for clothing and fabrics that tells you how much ultraviolet radiation can pass through the material to reach your skin. A UPF 50 shirt, for example, blocks 98 percent of UV rays and lets only 1/50th through. Think of it as the clothing equivalent of SPF for sunscreen, but with one important difference.
How UPF Differs From SPF
SPF only measures protection against UVB rays, the type most responsible for sunburn. UPF measures protection against both UVB and UVA rays, which means it accounts for the full spectrum of ultraviolet radiation that damages skin and contributes to aging and skin cancer. The other key difference is practical: sunscreen needs to be reapplied every two hours and rubs off with sweat and water. A UPF-rated shirt works as long as you’re wearing it.
What the Ratings Mean
UPF ratings fall into three protection categories:
- UPF 15 to 24: Good protection. Blocks 93 to 96 percent of UV radiation.
- UPF 25 to 39: Very good protection. Blocks 96 to 97.4 percent.
- UPF 40 to 50+: Excellent protection. Blocks 97.5 percent or more.
The Skin Cancer Foundation requires a UPF of 50 for a garment to earn its Seal of Recommendation. The jumps in percentage between categories look small, but they represent meaningful differences in how much radiation actually reaches your skin. A UPF 15 fabric lets through roughly three times more UV than a UPF 50 fabric.
How UPF Ratings Are Determined
Manufacturers can’t just slap a UPF number on a label. Garments are tested under a standardized protocol that measures UV transmittance through the fabric. What makes the testing rigorous is that the fabric is laundered 40 times and exposed to UV radiation before measurement, simulating about two years of normal wear. The UPF number on the label reflects the lowest protection level the garment is expected to provide during its usable life, not its performance fresh off the shelf.
What Makes One Fabric Better Than Another
Several factors determine how much UV protection a fabric offers, even before any special treatment is applied. Tighter weaves block more light simply because there are fewer gaps between threads. If you hold a shirt up to a lamp and can see light coming through, it’s letting UV through too. Darker and more saturated colors absorb more UV radiation than lighter shades. Research on dyed cotton fabrics has confirmed that color is one of the most influential variables in a fabric’s UV protection.
Fiber type matters as well. Polyester naturally blocks more UV than cotton because of its chemical structure. A standard white cotton T-shirt typically has a UPF of about 5 to 7, which is surprisingly low. That same shirt in a dark color with a tight weave might reach UPF 15 or 20 without any special treatment. Garments marketed as “UPF clothing” often use tightly woven synthetic fabrics, darker dyes, or chemical UV absorbers woven into the fibers to push protection into the 50+ range.
How Protection Changes With Use
A UPF rating isn’t permanent. Several real-world conditions can lower it.
Getting the fabric wet is the most immediate factor. When fibers absorb water from sweat or rain, they swell and shift, creating gaps that let UV through. Cotton and linen are especially vulnerable because they absorb so much moisture. Synthetic fabrics like polyester handle wetness better, and those treated with a water-repellent finish keep their UPF drop within about 5 percent.
Stretching also opens up gaps between yarns. A low-elasticity polyester fabric stretched just 10 percent (typical during normal movement) can see its yarn gaps widen by 20 to 30 percent, potentially dropping a UPF 50+ rating to around 40. Fabrics blended with spandex stretch more but face a different problem: after about 100 stretch-and-rebound cycles, the fibers fatigue and don’t fully snap back. Tiny permanent gaps form, and UPF can decay from 50+ down to the 35 to 40 range over time.
Washing causes the most consistent long-term decline. Machine agitation wears down fabric surfaces, and detergents gradually strip UV-protective finishes. How much protection you lose depends on how the UV protection was built in. Garments where a UV-blocking coating was applied to the surface can lose 30 to 50 percent of that coating within 10 washes, with UPF dropping by over 40 percent. Garments where UV absorbers are integrated directly into the fiber during manufacturing hold up far better, keeping loss rates under 10 percent even after 20 washes and maintaining UPF above 40.
How to Get the Most From UPF Clothing
If you’re buying UPF clothing specifically for sun protection, look for garments rated UPF 50+ with fiber-integrated UV protection rather than surface coatings. Darker colors in tightly woven synthetic fabrics will give you the most durable protection. Keep in mind that the fabric protects only what it covers. Your face, hands, and any exposed skin still need sunscreen or other protection.
For everyday clothes without a UPF label, you can still make smarter choices. A dark, tightly woven polyester shirt offers significantly more protection than a thin white cotton tee. Dry fabric protects better than wet fabric, and clothing that fits loosely protects better than stretched-tight garments, simply because the weave stays intact.