☀️ SPF Calculator
Enter the SPF you're wearing to see how much UVB it filters in the lab test, and how many coats your time outdoors takes at the every-2-hours reapply rule.
🧴 What Does Your SPF Mean?
What is an SPF Calculator?
It makes an SPF number concrete: the share of UVB rays it filters, and the share it lets through, in the lab test. SPF 50 lets through 2% against SPF 30's 3.4%. It does not say how long you can stay out: SPF measures an amount of sun, not a time.
Treat the result as an illustration, never a timer. Reapply at least every two hours, and right after swimming, sweating or towel drying, whatever the SPF.
❓ Frequently Asked Questions
Can SPF tell me how long I can stay in the sun?
No, and this calculator no longer tries. The FDA says: "Some people incorrectly believe that SPF relates to time of solar exposure." And: "This is not true because SPF is not directly related to time of solar exposure but to amount of solar exposure." The tool shows the share of UVB an SPF filters in the lab test and how many coats your time outdoors takes.
How often should I reapply?
At least every two hours, whatever the SPF, and right after swimming, sweating or towel drying. The FDA says: "Reapply sunscreen at least every two hours, and more often if you're sweating or swimming." The American Academy of Dermatology says: "To remain protected when outdoors, reapply sunscreen every two hours, and immediately after swimming or sweating."
Why is real-world protection lower than the lab figure?
The SPF number comes from a lab test that applies a set, generous amount of sunscreen. Applying less, and losing it to sweat, water, towel drying and rubbing, leaves less on the skin, which is why reapplication matters more than the headline number.
Does a higher SPF let me reapply less often?
No. SPF 50 lets through 2% of UVB in the lab test against SPF 30's 3.4%, but the two-hour reapply rule is the same for both.