☀️ 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?
An SPF calculator turns the abstract number on your sunscreen bottle into something 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. 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
Can SPF tell me how long I can stay in the sun?
No, and this calculator no longer tries. The FDA puts it plainly: "Some people incorrectly believe that SPF relates to time of solar exposure. For example, some people believe that, if they normally get sunburned in one hour, then an SPF 15 sunscreen allows them to stay in the sun for 15 hours (e.g., 15 times longer) without getting sunburn. 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.
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.
How often should I reapply sunscreen?
Reapply at least every two hours, and immediately after swimming, sweating heavily, or towelling off, whatever the SPF. The FDA says: "Reapply sunscreen at least every two hours, and more often if you're sweating or swimming." 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.