UV Index Exposure Calculator
This UV index exposure calculator estimates how long you can stay in the sun before your skin reddens, based on the UV index, your skin's baseline sensitivity and the sun protection factor of any sunscreen you apply. The UV index is a standardized measure of the strength of ultraviolet radiation at the surface, running from 0 on a dim day to 11 or more under tropical midday sun. Because burn time is inversely proportional to UV intensity, doubling the index roughly halves the safe exposure. The calculator divides a baseline burn time, the minutes an unprotected person of your skin type can tolerate at UV index 1, by the current index, then multiplies by your SPF to reflect the longer protection a sunscreen provides when applied correctly. Enter the UV index, a baseline time in minutes for your skin type, and an SPF (use 1 for no sunscreen). The result is an estimate of unprotected and protected exposure time before erythema. Use it to plan outdoor work or recreation safely. Every figure is computed deterministically from the inverse-UV formula shown in full below, with a worked example that reconciles exactly to the calculator so you can follow each step yourself.
Burn time is the baseline divided by the UV index, scaled by SPF: at UV index 7 with a 250 minute baseline, unprotected skin lasts about 35.71 minutes, and with SPF 30 roughly 1,071.43 minutes. Higher UV means less time before burning.
UV exposure formula
unprotected minutes = baseline / UV index
protected minutes = (baseline / UV index) x SPF
baseline = burn time in minutes at UV index 1
SPF = sun protection factor of sunscreen (1 if none)
Burn time falls in inverse proportion to the UV index because the index measures the rate of skin-damaging radiation. A correctly applied sunscreen of factor SPF extends the safe time by roughly that factor, though real-world coverage is usually less than the laboratory ideal.
Worked example
Estimate safe time at UV index 7 for skin with a 250 minute baseline, with and without SPF 30 sunscreen.
- unprotected = 250 / 7 = 35.71 minutes
- SPF multiplier = 30
- protected = 35.71 x 30 = 1,071.43 minutes
- in hours, protected = 1,071.43 / 60 = 17.86 hours (capped in practice by reapplication and daylight)
Unprotected skin reddens after about 35.71 minutes, while SPF 30 extends that to roughly 1,071.43 minutes in ideal conditions. These are the calculator's default inputs, so the result above matches the widget exactly.
UV index exposure categories
Public-health guidance groups the UV index into bands that signal how quickly skin can burn.
| UV index | Exposure category |
|---|---|
| 0 to 2 | Low |
| 3 to 5 | Moderate |
| 6 to 7 | High |
| 8 to 10 | Very high |
| 11 and above | Extreme |
Occupational sun-exposure and outdoor-work context: US Bureau of Labor Statistics (BLS).
UV Index Exposure Calculator: frequently asked questions
How do I pick a baseline burn time?
The baseline is roughly how many minutes your unprotected skin tolerates at UV index 1 before it starts to redden. Very fair skin that always burns might use a low baseline near 150 minutes, while darker skin that rarely burns might use 400 minutes or more. The exact number is individual, so treat it as an editable estimate and err on the cautious side, especially for children.
Does SPF really multiply my safe time?
In laboratory conditions an SPF multiplies the time to reach the same dose of burning radiation. In practice most people apply far less sunscreen than the test amount and miss spots, so real protection is often a fraction of the label figure. Treat the protected time as an optimistic ceiling, reapply every two hours, and reapply after swimming or sweating.
Why does burn time halve as UV doubles?
The UV index is proportional to the rate at which damaging ultraviolet radiation reaches the skin. Reddening depends on the total dose, which is rate multiplied by time. To reach the same dose, doubling the rate means halving the time, so safe exposure is inversely proportional to the index, which is exactly what the formula encodes.
Is the protected time a hard limit?
No. It is an estimate of when reddening may begin under ideal sunscreen application. Skin damage accumulates over a lifetime even below the burn threshold, so shade, clothing and a hat matter even when the clock says you have time left. The calculator does not account for reflection off water, snow or sand, which raises effective exposure.
What is a high UV index?
An index of 6 or 7 is high, 8 to 10 is very high, and 11 or more is extreme. At those levels even short unprotected exposure can burn fair skin. The index typically peaks within a couple of hours of solar noon and is stronger at high altitude and low latitude.
Official sources
- Outdoor occupations and sun-exposure workforce data: US Bureau of Labor Statistics (BLS). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.