UPF (Ultraviolet Protection Factor) tells you how effectively a fabric blocks UV radiation — basically SPF for your clothing. For most riders, UPF 30+ provides solid sun protection for typical rides. If you're fair-skinned or regularly spend long, sun-drenched days in the saddle, stepping up to UPF 50+ gives you the highest level of tested protection available.
One thing many cyclists overlook: when your pants stretch over working muscles, the fabric thins out and lets more UV through than its rating suggests. So a UPF 50+ short might perform more like a lower rating when stretched tight on the bike. This is why a dedicated UPF rating matters — it confirms the fabric was actually tested, rather than just assuming any dark material will shield you.
Also remember that cycling pants only cover part of your legs. You'll still want sunscreen on exposed skin above and below the hem, especially on long summer rides.