SPF, explained: what sunscreen really does

By Trung Hieu Lam, developer of UV Tracker · Published 1 September 2026

What SPF means

SPF (Sun Protection Factor) is measured under standardized test conditions — testers apply exactly 2 mg of product per square centimeter of skin and measure how much longer it takes to reach the sunburn threshold compared to bare skin. SPF 30, tested that way, multiplies your baseline sunburn threshold roughly 30-fold. The number is a laboratory result, not a promise about how you'll actually apply it.

The under-application problem

A pilot study using ultraviolet photography — which makes sunscreen visible as a metallic coating on the skin under UV light — found that people typically apply well under half the 2 mg/cm² thickness sunscreen is tested at, whether they were using a physical or chemical formula. Applying half the tested thickness does not give half the SPF; protection falls off faster than that, so a "SPF 30" applied thinly can perform far closer to SPF 10 in practice. Seeing their own coverage under UV light measurably increased how much people applied afterward.

Broad spectrum, UVA and UVB

SPF on its own measures protection against UVB, the primary cause of sunburn. "Broad spectrum" labeling means the product also filters UVA, which contributes to longer-term skin damage without necessarily causing visible sunburn. Look for both the SPF number and a broad-spectrum claim.

How much and how often

The tested amount works out to roughly a shot-glass-sized amount (about 30 mL) for an adult body in a swimsuit, or about a quarter-teaspoon for the face and neck alone. Reapply at least every two hours, and immediately after swimming, sweating heavily, or towel-drying — "water resistant" on a label means it survives a defined amount of water exposure, not that it lasts indefinitely.

Mineral vs chemical

Mineral (physical) sunscreens sit on the skin and reflect UV; chemical sunscreens absorb it and convert it to heat. Both are effective when applied correctly and reapplied on schedule — the best sunscreen is the one thick enough, and reapplied often enough, that you'll actually keep using.

Sunscreen plus timing

Sunscreen extends your safe sun time, but the safest plan combines it with shade and timing rather than relying on it alone. Try the calculator for a sunscreen-adjusted estimate, or see the full tables for the baseline numbers.

UV Tracker measures the UV where you stand — sky scan for local conditions, one photo for your skin type, and your safe sun time on one card. Free forever.