How Long Does Your SPF Actually Last?
The SPF number on your sunscreen isn’t a time — it’s a multiplier. SPF 30 doesn’t mean 30 minutes; it means your skin takes about 30× longer to burn than unprotected. The real number depends on your skin type, today’s UV index, and how much sunscreen you actually applied — most people use about half the tested amount. This calculator gives you the real answer.
How we calculate your SPF duration
The estimate is built from published, citable sources rather than a brand’s marketing — measured, not guessed:
- Baseline burn time comes from US National Weather Service / EPA UV index burn-time tables, indexed by your Fitzpatrick skin type.
- UV index conversion follows the WHO/EPA standard; values between whole numbers are interpolated.
- Real-world application factor — research in the Journal of the American Academy of Dermatology shows typical application delivers roughly 50% of the labeled SPF, so we halve it.
- Activity modifier accounts for sweat and water exposure reducing effective protection.
- Two-hour reapplication cap follows the American Academy of Dermatology guideline (80 minutes around water).
v1 does not fetch live UV, or model altitude, cloud cover, or UVA (PA+++) protection. For UVA ratings, see the SPF & PA+++ Decoder.
Frequently asked questions
Does SPF 50 last longer than SPF 30?
Yes — but only because it delivers higher effective protection, not because the number is a time. SPF is a multiplier on your burn time, so a higher SPF stretches the same unprotected window further. You still reapply at least every two hours.
How often should I reapply sunscreen?
The American Academy of Dermatology recommends reapplying every two hours regardless of SPF, and immediately after swimming or heavy sweating. This tool caps its recommendation at two hours (80 minutes around water) for that reason.
Does sunscreen expire?
Yes — and degraded sunscreen protects less. Check the period-after-opening date and active ingredients to see when yours stops working. See the Product Expiry Tracker.
What is the difference between SPF and PA+++?
SPF rates protection against UVB (burning). PA+++ rates UVA protection (ageing and deeper damage) and is common on sunscreens sold in India and East Asia. See the SPF & PA+++ Decoder.
Does the UV index change throughout the day?
Yes. UV peaks between roughly 10am and 4pm, when the sun is highest. Enter the UV index for the part of the day you will be outdoors — and re-check it, since a morning estimate understates midday exposure.