Guide
How SPF actually works — and why most people are using it wrong
The SPF number tells you less than you think. Coverage and reapplication matter more. Here's what the number actually means in practice.
What the SPF number actually measures
SPF stands for Sun Protection Factor. It measures how much longer protected skin takes to redden compared to unprotected skin. SPF 30 means it takes 30 times longer. In practice, this is calculated against your skin's baseline burn time — which varies significantly by skin type. SPF 30 blocks about 97% of UVB radiation; SPF 50 blocks about 98%. The marginal difference between SPF 30 and SPF 50 is small. The difference between SPF 15 and SPF 30 is more meaningful.
Why real-world protection is much lower than the label
SPF ratings are calculated in laboratories using 2mg of product per square centimetre of skin — about twice as much as most people actually apply. Using half the amount doesn't give you half the protection; the relationship is non-linear and the reduction in protection is typically much larger than expected. Sweat, contact, and time degrade the protection further. After two hours of activity, most sunscreens have lost a significant portion of their protective effect.
Reapplication is the variable most people underestimate
The two-hour reapplication guideline exists because sunscreen chemicals degrade under UV exposure. After two hours — or after swimming or heavy sweating — a significant portion of the active protection has broken down. Reapplying restores it. A single application of SPF 50 that isn't reapplied provides much less real protection over a full day than SPF 30 reapplied properly every two hours.