When evaluating UPF fabric, the most critical metric is the UPF rating (15 to 50+), which indicates how much UV radiation reaches the skin. You should prioritize fabrics with a UPF 50+ rating, as they block 98% of UV rays. Additionally, consider the fabric construction (density), fiber type (polyester and nylon are naturally better at reflecting UV), and the presence of UV-absorbing treatments like titanium dioxide. For performance wear, ensure the fabric maintains its rating after 40+ simulated laundry cycles.