What is UPF? UPF stands for Ultraviolet Protection Factor and indicates how much of the sun's UV radiation is absorbed. A fabric with a rating of 50 will allow only 1/50th of the sun's UV rays to pass through. This means the fabric will reduce your skin's UV radiation exposure significantly, because only 2 percent of the UV rays will get through.  Which Fabrics are Best? As a rule, light-colored, lightweight and loosely-woven fabrics do not offer much protection from the sun. That white T-shirt you slip on at the beach when you feel your skin burning provides only moderate protection from sunburn, with an average ultraviolet protection factor (UPF) of 7. At the other end of the spectrum, a long-sleeved dark denim shirt offers an estimated UPF of 1,700 – which amounts to a complete sun block. In general, clothing made of tightly-woven fabric best protects skin from the sun. The easiest way to test if a fabric can protect your skin is to hold it up to the light. If you can see through it, then UV radiation can penetrate it – and your skin. What's the Difference between UPF and SPF? SPF stands for Sun Protection Factor and is the rating you're familiar with for sunscreens and other sun-protective products. It measures the amount of time it takes for sun-exposed skin to redden, while UPF measures the amount of UV radiation that penetrates a fabric and reaches the skin. Source: Skin Cancer Foundation