Sunscreen Calculator
How long does SPF 30 or 50 actually protect you? Pick the UV index, your skin type, and the SPF on the bottle — get a Diffey-model burn-time estimate, reapply schedule, and per-body-zone dose.
Your skin type
Fitzpatrick scale. Pick by burn behavior — color swatches are approximate references.
SPF
Sweating or swimming?
Time to burn — unprotected
1h 14min
Bare skin in this UV.
Protected with SPF 50
8h+
Capped at 8h — reapply long before this.
Reapply every 120 minutes
Standard outdoor reapplication. Sooner if you towel off. Lab numbers above assume you applied the full dose (most people use 25-50% of the needed amount).
How much to apply
Most people apply 25-50% of the dose tested in SPF labs. Use these reference amounts.
Read the UV index guide
What each level means and when sun is most dangerous.
Burn-time estimates use a Diffey-style 1 MED model and assume full-dose application. Real-world protection is usually 30-50% of label SPF. This tool is educational — when in doubt, reapply early and seek shade.
About this tool
Burn-time uses the Diffey 1 MED model with skin-type adjustment (fair = 1.5, medium = 3, dark = 8). Real-world SPF performance is usually 30-50% of the label because most people apply too little. When in doubt, reapply within 2 hours regardless. Read the UV index guide for context, and the sunscreen guide for product selection. This is not a medical device.
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