SPF (Sun Protection Factor) measures how much longer skin can be exposed before burning compared to unprotected skin — but the relationship between the number and actual UV protection isn't as linear as most people assume.
What the number technically measures
SPF 30 means, in theory, skin can tolerate 30 times longer UV exposure before burning compared to no protection at all — under laboratory conditions with a specific, generous application amount rarely matched in real-world use.
The diminishing-returns curve
SPF 15 blocks about 93% of UVB rays, SPF 30 blocks about 97%, and SPF 50 blocks about 98% — the jump from 30 to 50 is much smaller in actual protection than the numbers suggest, since the scale isn't linear.
Why real-world protection is usually lower
Most people apply far less sunscreen than the lab-tested amount (roughly a shot glass' worth for full body) and don't reapply as often as needed — actual real-world protection is typically meaningfully lower than the labeled SPF implies.
Is SPF 100 meaningfully better than SPF 50?
Only marginally in percentage terms (about 99% vs 98% UVB blocked) — the bigger practical factors are application amount and reapplication frequency, not chasing an ever-higher SPF number.