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
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.
Done with the sun math? Check a spot on your skin
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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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