SPF Effective Protection Calculator

Current Effective SPF is built entirely from SPF Rating, Application Amount, Minutes Since Applied, and Swimming/Sweating (lines 12-18) -- UV Index is never part of that formula. It only enters Protected Exposure, a separate output that estimates relative UV exposure as UV Index times elapsed time divided by Current Effective SPF (lines 23-24), so a higher UV Index raises your estimated exposure without ever changing the SPF figure itself.

Application Amount doesn't scale Current Effective SPF linearly -- it is raised to the power of 1.7 before being used as the exponent on SPF Rating (line 13), modeling the documented finding that thin sunscreen application loses far more protection than the percentage-of-recommended-dose would suggest. Cutting Application Amount from 100% to 50% can drop a label SPF of 30 down to a single-digit effective SPF.

Reapply In is derived from a half-life -- 80 minutes normally, 40 minutes if Swimming/Sweating is on -- and because exponential decay always takes exactly one half-life to cut any starting value in half, the underlying calculation reduces to that fixed half-life minus Minutes Since Applied, floored at zero (lines 27-28, 34). SPF Rating and Application Amount only matter as a threshold check: if your freshly-applied effective SPF starts too low, Reapply In reports 0 immediately instead of counting down.

Current Effective SPF is floored with Math.max(1, ...) so it never reports below a minimum of 1, which represents essentially no protection (line 18). Once enough time has passed, or the application was thin enough, or the SPF rating and thickness together produce a low enough freshly-applied figure, the decayed value drops beneath 1 and the output simply holds at that floor rather than continuing to decrease.