Sunscreen SPF Protection Time Calculator
Calculate sunscreen time from SPF, Fitzpatrick skin type (I-VI), and UV index.
Returns exposure window and reapplication schedule for outdoor activities.
How SPF works
SPF (Sun Protection Factor) tells you how much longer you can stay in the sun before burning, compared to wearing nothing at all. SPF 30 does not mean 30 minutes. It means thirty times your own burn time, whatever that happens to be at today’s UV level.
That multiplier only holds if you apply the product the way the lab did. Almost nobody does, which is the entire reason this page exists.
Where the burn time comes from
Rather than a table of memorized minutes, this calculator works from the minimum erythemal dose (MED): the amount of UV energy a given skin type absorbs before it reddens. One UV Index unit is 25 milliwatts per square meter of erythemally weighted ultraviolet, which delivers 1.5 joules per square meter every minute. So:
burn minutes = MED ÷ (UV Index × 1.5)
| Fitzpatrick type | Description | MED | Burn time at UV 6 |
|---|---|---|---|
| I | Very fair, always burns, never tans | 200 J/m² | 22 min |
| II | Fair, usually burns, tans minimally | 250 J/m² | 28 min |
| III | Medium, sometimes burns, tans gradually | 350 J/m² | 39 min |
| IV | Olive, rarely burns, tans well | 450 J/m² | 50 min |
| V | Brown, very rarely burns, tans deeply | 600 J/m² | 67 min |
| VI | Dark, almost never burns | 1,000 J/m² | 111 min |
Halve the UV Index and every figure doubles. At UV 12 they all halve.
The part most SPF calculators get wrong
The honest formula is not base time × label SPF. Faurschou and Wulf measured it in 2007 and found protection falls off as a power of how much product you used:
effective SPF = label SPF ^ (applied dose ÷ 2 mg/cm²)
The 2 mg/cm² figure is the laboratory standard. Repeated field studies put actual application between 0.5 and 1.0 mg/cm². Take the generous end, 1.0, and the exponent is ½:
| Label SPF | Effective SPF at real-world application |
|---|---|
| 15 | 3.9 |
| 30 | 5.5 |
| 50 | 7.1 |
| 70 | 8.4 |
| 100 | 10.0 |
This is why “I wore SPF 50 and still burned” is such a common complaint, and why the answer is almost never a bad product. Note what the table does to the marketing case for high numbers: tripling the label from 30 to 100 not quite doubles what you actually get. Buy SPF 30 and use twice as much of it.
UV Index guidance
| UV Index | Risk | What to do |
|---|---|---|
| 1-2 | Low | SPF 15 is enough for most people |
| 3-5 | Moderate | SPF 30, hat, seek shade at noon |
| 6-7 | High | SPF 30-50, sunglasses, shade 10am to 4pm |
| 8-10 | Very high | SPF 50 plus protective clothing |
| 11+ | Extreme | Minimize exposure entirely |
What SPF blocks
SPF 15 blocks 93.3% of UVB, SPF 30 blocks 96.7%, SPF 50 blocks 98.0% and SPF 100 blocks 99.0%. The formula is simply 1 − 1/SPF. Nothing blocks 100%, and the gap between 50 and 100 is one percentage point of UVB for roughly double the price.
Always reapply
No matter how high the SPF, reapply every 2 hours, and straight away after swimming or heavy sweating. Water-resistant formulas hold their rating for 40 or 80 minutes of immersion, and the label has to say which. Past that they are assumed gone.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.