UPF stands for Ultraviolet Protection Factor, a rating system that measures how much UV radiation a fabric allows to reach your skin. It works like SPF for sunscreen, but applies to clothing. A garment rated UPF 50, for example, allows only 1/50th of the sun’s UV rays to pass through the fabric, blocking about 98% of radiation.
How UPF Ratings Work
UPF ratings fall into three tiers based on how much UV radiation they block:
- UPF 15 (Fair protection): blocks 93% of UV radiation
- UPF 30 (Good protection): blocks 96.7% of UV radiation
- UPF 50+ (Excellent protection): blocks 98% of UV radiation
The jump from UPF 15 to UPF 50+ sounds dramatic, but the actual difference in blocked radiation is about 5 percentage points. The practical difference matters most for people who spend extended time outdoors, since that remaining 2% to 7% of transmitted UV accumulates over hours. The Skin Cancer Foundation updated its Seal of Recommendation criteria in 2025 to require a minimum UPF of 50 for fabric products, up from previous thresholds.
UPF vs. SPF
UPF and SPF are related but not equivalent. SPF is tested on human skin: researchers apply sunscreen and measure how much longer it takes for skin to burn compared to unprotected skin. It only accounts for UVB rays, the primary cause of sunburn. UPF, by contrast, is measured in a lab with a device called a spectrophotometer that shines simulated sunlight through fabric and measures how much gets through across both UVA and UVB wavelengths.
This is a meaningful distinction. UVA rays penetrate deeper into the skin and contribute to premature aging and skin cancer risk, but they don’t cause the redness that SPF testing relies on. UPF captures a broader picture of how much total UV radiation a garment lets through, though the rating still weights UVB wavelengths more heavily because those cause the most immediate skin damage.
What Makes One Fabric More Protective Than Another
Not all clothing provides the same UV protection, even without a UPF label. Several physical properties of fabric determine how much radiation passes through.
Fiber type plays a significant role. Synthetic fibers like polyester absorb considerably more UV radiation than natural fibers like cotton, silk, or wool. Among synthetics, polyester outperforms acrylic and nylon. Unbleached natural fibers also offer more protection than bleached ones, since the natural pigments in raw cotton or linen absorb some UV on their own.
Weave tightness is one of the biggest factors. A tightly woven fabric with minimal gaps between threads physically blocks more light from passing through. You can do a quick test yourself: hold a garment up to a light source. If you can see light coming through the weave, UV radiation is getting through too. Two fabrics of identical weight per square meter will have very different UPF values if one has a tighter weave than the other.
Color matters more than most people realize. Darker shades of the same fabric consistently block more UV radiation than lighter shades because the dye molecules absorb additional UV energy. A navy blue cotton shirt provides noticeably more protection than a white one with the same weave. This runs counter to the common instinct to wear light colors in the sun for comfort, which works for staying cool but not for UV protection.
Stretch reduces protection. When fabric stretches across your shoulders or back, the gaps between threads widen and let more UV through. A garment that fits loosely will generally protect better than a tight-fitting version of the same material.
How Wetness and Washing Affect Protection
Getting your clothes wet can lower their UPF rating. Research on cotton fabrics and certain polyester blends found that UPF decreased significantly when the fabric was saturated with water. This matters for swimming, sweating, or any activity where your clothing gets damp. Not all fabrics respond the same way, though. Some synthetic materials maintain their rating when wet, which is why dedicated swim shirts and rash guards are typically made from polyester or nylon blends rather than cotton.
Washing also affects UPF over time, but the story is more nuanced than you might expect. The industry standard for testing (ASTM 6544) requires garments to be washed 40 times before their UPF rating is assigned, simulating roughly two years of regular use. In a study that tracked seven commercial UPF brands through 50 wash cycles, five maintained relatively stable protection throughout. Two brands, however, lost 70% to 78% of their UPF value by the 50th wash.
The key difference was how the protection was built into the fabric. Brands that relied on chemical UV finishes, like nano-zinc coatings, saw dramatic drops as those coatings washed away. Brands that achieved their UPF through fiber composition and tight construction held steady or even improved slightly, since washing can cause minor shrinkage that tightens the weave further. One brand with no chemical finishes maintained its maximum UPF value through all 50 cycles. If long-term durability matters to you, look for garments that protect through construction rather than applied finishes, though manufacturers rarely make this distinction clear on the label.
When UPF Clothing Makes the Most Sense
Any shirt provides some UV protection, but dedicated UPF clothing fills specific gaps that sunscreen alone doesn’t cover well. Sunscreen needs to be applied thickly (about a shot glass worth for your whole body), reapplied every two hours, and reapplied again after swimming or sweating. In practice, most people apply far less than the recommended amount and reapply inconsistently. A UPF 50+ shirt, by contrast, provides consistent protection without any user error as long as the fabric is intact and not stretched thin.
UPF-rated clothing is most practical for extended outdoor activities: hiking, gardening, fishing, beach days, outdoor work. For everyday errands or short periods outside, a regular dark-colored shirt with a tight weave already blocks a substantial amount of UV. The people who benefit most from purpose-built UPF gear are those with high sun exposure, a history of skin cancer, fair skin that burns easily, or conditions that increase sun sensitivity.