How SPF works

SPF (Sun Protection Factor) represents the ratio of UV exposure required to cause sunburn with sunscreen versus without it. SPF 30 means you can be exposed to 30 times more UV before burning compared to unprotected skin. In practical terms:

If your unprotected skin burns after 10 minutes in the sun, SPF 30 theoretically extends that to 300 minutes. However, this assumes correct application amount and no sweating or water exposure - real-world protection is shorter.

SPF number guide

SPF	UVB blocked	UVB transmitted
SPF 15	93.3%	6.7%
SPF 30	96.7%	3.3%
SPF 50	98.0%	2.0%
SPF 100	99.0%	1.0%

The difference between SPF 30 and SPF 50 is relatively small (1.3%). SPF 30 is generally considered sufficient for most outdoor activities when applied correctly. Higher SPF provides additional margin for inadequate application.

Correct application amount

Most people apply only 25–50% of the recommended sunscreen amount, which dramatically reduces effective protection:

Standard amount: 2 mg per cm² of skin - the amount used in SPF lab testing.
Face: approximately ⅓ teaspoon (1.5 mL) for the face alone.
Full body: approximately 30 mL (1 oz, or a shot glass full) for complete body coverage.

Applying half the recommended amount roughly reduces effective SPF to its square root: SPF 50 applied at half-dose provides approximately SPF 7 protection.

Reapplication rules

Reapply every 2 hours during outdoor activity.
Reapply immediately after swimming, toweling off, or heavy sweating - even if less than 2 hours have passed. "Water resistant" sunscreens maintain effectiveness for 40 or 80 minutes of water exposure (as specified on the label).
Apply sunscreen 15–30 minutes before going outdoors to allow it to bind to the skin.