Sunscreen Calculator
Estimate how long skin can take the sun before burning. Enter the UV index (0–11+), your Fitzpatrick skin type (I–VI), and an SPF to see unprotected burn time, protected time, and the WHO risk band.
Example: with UV index 6 · Skin type (Fitzpatrick) III — sometimes burns, tans · Sunscreen SPF 30 → Time to burn: 39 minutes unprotected.
- With sunscreen19 h 27 min with SPF 30
- UV risk bandHigh (6-7)
- ProtectionSPF 30+, hat, shade 10am-4pm, reapply every 2 hours.
Computed by the calculator below using its default values. Change any input to see your own numbers.
Burn time ≈ MED ÷ (UV index × 1.5) minutes, where MED (minimal erythemal dose) rises with darker skin. Real exposure adds up all day, so reapply and seek shade.
How burn time is estimated
The UV index is built so that erythemal (sunburning) irradiance equals the index times 0.025 watts per square meter. Each skin type has a minimal erythemal dose — the energy needed to redden it — that ranges from roughly 200 J/m² for the palest skin to about 1,000 J/m² for the darkest. Dividing dose by irradiance gives time, which simplifies to MED ÷ (UV index × 1.5) minutes. At UV 6, type III skin reddens in about 39 minutes; at UV 10 that drops to around 23.
SPF multiplies that time in theory, but only if you apply enough (about a shot glass for the body) and reapply every two hours. Most people apply a quarter to a half of the tested amount, so treat the protected number as a best case, not a countdown to keep tanning — a tan is itself UV damage.
How it’s calculated
Time to burn (minutes) = MED ÷ (UV index × 1.5), from erythemal irradiance = UV index × 0.025 W/m² and 60 s/min. MED by Fitzpatrick type (J/m²): I 200, II 250, III 350, IV 450, V 600, VI 1000. Protected time = unprotected × SPF. Risk bands follow WHO/WMO UV index categories.
An idealized clear-sky, sea-level estimate; clouds, altitude, and reflection off snow, sand, or water change real exposure, and medications or conditions can raise photosensitivity. Not medical advice.
Approx. burn time at UV 6
| Skin type | Description | Time to burn |
|---|---|---|
| I | Always burns | ~22 min |
| II | Burns easily | ~28 min |
| III | Sometimes burns | ~39 min |
| IV | Rarely burns | ~50 min |
| V | Very rarely burns | ~1 h 7 min |
| VI | Never burns | ~1 h 51 min |
Computed with time = MED ÷ (UV index × 1.5); MED per Fitzpatrick type (CIE standard erythema dose).
Common mistakes
- Treating SPF time as permission to bake — under-applying cuts real protection far below the label number.
- Ignoring reflection: snow, water, and sand bounce UV and can nearly double exposure.
- Forgetting UV peaks near solar noon; the same index earlier or later still adds up over the day.
- Assuming a base tan protects you — it is roughly SPF 3 at best and is skin damage itself.
Frequently asked questions
How long does it take to burn at UV 6?
For fair type II skin, roughly 28 minutes unprotected; for medium type III, about 39 minutes. The estimate is MED ÷ (UV index × 1.5), so higher UV or lighter skin shortens it.
Does SPF 30 really give 30 times longer?
Only in lab conditions with a full, even application reapplied every two hours. In practice people apply a fraction of that, so use the protected time as an upper bound, not a target.
Is tanning safer than burning?
Not really. A tan is the skin's response to UV damage. You can tan without a visible burn and still raise your long-term skin-cancer and aging risk.
Should I rely on this for a skin condition or medication?
No. Some drugs and conditions make skin burn far faster, and personal risk varies. If you have a history of skin cancer or photosensitivity, follow a dermatologist's guidance.