☀️ SPF Calculator
Choose your skin type and the SPF you're wearing to see the theoretical protection time, how much UVB is blocked, and when to reapply.
🔧 Estimate Your Sun-Protection Time
What is an SPF Calculator?
An SPF calculator turns the abstract number on your sunscreen bottle into something intuitive: roughly how long it could extend the time before your skin starts to burn, and what share of UVB rays it filters out. It uses a baseline burn time tied to your Fitzpatrick skin type, then scales it by the SPF.
Treat the result as an illustration, not a timer. Real protection depends heavily on how much you apply and how often you reapply, so daily broad-spectrum SPF, shade, and reapplication every two hours matter far more than chasing the highest SPF on the shelf.
❓ Frequently Asked Questions
How is the protection time calculated?
The estimate multiplies a baseline burn time for your Fitzpatrick skin type by your sunscreen's SPF number. For example, if your unprotected skin would start to burn in 20 minutes and you apply SPF 30, the theoretical maximum is roughly 600 minutes. It's a useful illustration of how SPF scales, not a guarantee of safe time in the sun.
Why is real-world protection much lower than the theoretical maximum?
These numbers come from lab tests that apply 2 mg of sunscreen per square centimeter — far more than most people use. In practice we apply a quarter to half that amount, and sweat, water, towel-drying, and rubbing remove it over time. That's why real protection is far lower than the theoretical maximum, and why reapplication matters more than the headline SPF number.
How often should I reapply sunscreen?
Reapply at least every two hours, and immediately after swimming, sweating heavily, or towelling off, regardless of the SPF or the protection time this tool shows. A high SPF does not let you skip reapplication; it only raises the ceiling on a single, perfectly applied coat that real life quickly degrades.
Is this medical advice?
No. This calculator is an educational estimate only and is not medical advice. Sun sensitivity varies with medications, altitude, reflective surfaces, time of day, and individual skin, and the safest approach is daily broad-spectrum SPF 30 or higher, shade, and protective clothing. See a dermatologist for skin concerns or a personalised sun-protection plan.