SPF stand for Sun Protection Factor and it is something common for sunscreens. Clothing manufacturers generally use the term UPF because it’s a bit different. SPF only measures UVB protection, and doesn’t show UVA parameters. On the other hand, UPF measures protection against both UVA and UVB. Chemical UV absorbers and colorless dyes are used to increase this number.
Like we discussed, you should look for a higher UPF rating for better protection. The highest specified UPF is 50+, and clothing with this rating will absorb over 98% of the harmful rays. The lowest UPF for a piece of clothing to be considered sun-protective is 15. If the value is lower than this, the manufacturer can’t state that it’s going to protect you from the sun.