Sunscreen Calculator — SPF Protection Time
Find out how long your sunscreen should protect you: pick your skin type, enter the SPF of your sunscreen and today's UV index — and get an estimated protection window plus the percentage of UVB rays blocked.
Skin type (Fitzpatrick scale)
Estimated time before sunburn risk — reapply every 2 hours regardless
- 1
UV adjustment factor
7 ÷ 3 = 2.333UV index 3 is the moderate-exposure reference; higher UV shortens the safe window proportionally. - 2
SPF × base burn time
50 × 15 min = 750 min - 3
Protection time
750 ÷ 2.333 = 321 min
How does this calculator work?
Protection time ≈ (SPF × base burn time) ÷ (UV index ÷ 3). SPF 50 blocks ~98% of UVB rays. Pick your skin type (Fitzpatrick scale I–VI), enter your SPF and today's UV index to estimate your safe exposure window. Reapply every 2 hours regardless of SPF.
Formula
How this is calculated
SPF (Sun Protection Factor) measures how much longer it takes UVB radiation to redden protected skin compared with unprotected skin. An SPF of 30 means it takes 30 times as long — so if your bare skin starts to burn in 15 minutes, SPF 30 theoretically extends that to 450 minutes. The key word is "theoretically": the SPF number is measured in a lab at a fixed UV intensity.
Real outdoor UV intensity varies with time of day, latitude, cloud cover and altitude, roughly captured by the UV index. A UV index of 3 is used as the reference for "moderate" exposure in this calculator; higher UV shortens the safe window proportionally. The formula used here is: protection minutes = (SPF × base burn time) ÷ (UV index ÷ 3). Base burn times are derived from the Fitzpatrick skin type scale, ranging from ~10 minutes for very fair (Type I) skin to ~60 minutes for dark (Type VI) skin.
Important limitations: SPF only measures UVB protection; no standard rating covers UVA, which causes ageing and deeper skin damage. Most dermatologists recommend reapplying every 2 hours regardless of SPF — sweat, towelling and swimming reduce effective SPF dramatically. Choose a broad-spectrum sunscreen for UVA+UVB coverage.
Frequently asked questions
SPF (Sun Protection Factor) is the ratio of UV energy needed to cause a minimal erythema (slight redness) on protected skin versus unprotected skin. SPF 30 blocks about 97% of UVB; SPF 50 blocks about 98%; SPF 100 blocks 99%. The difference between SPF 30 and SPF 50 is smaller than many people think.
UV filters in sunscreen degrade on exposure to sunlight (photo-degradation). Sweat, water and physical rubbing also reduce coverage. Most sun-safety guidelines recommend reapplying every 2 hours and immediately after swimming or heavy sweating, regardless of the SPF rating.
Only marginally — the protection increase from SPF 30 to SPF 50 is about 1% more UVB blocked (97% vs 98%). Higher SPF buys you a small extra buffer, not license to stay out all day without reapplying. It also does not protect against UVA.
Also known as
TG we-Calculate Editorial Team. (2026). Sunscreen Calculator — SPF Protection Time [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/sunscreen-calculator
TG we-Calculate Editorial Team. "Sunscreen Calculator — SPF Protection Time." TG we-Calculate. 2026. https://we-calculate.com/calculator/sunscreen-calculator.
TG we-Calculate Editorial Team, "Sunscreen Calculator — SPF Protection Time," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/sunscreen-calculator
@misc{wecalculate_sunscreen_calculator, title = {Sunscreen Calculator — SPF Protection Time}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/sunscreen-calculator}}, year = {2026}, note = {TG we-Calculate} }
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