UV Index Converter: Calculate Irradiance and Burn Time
Turn the UV index into an action: Convert a UV index value to erythemal irradiance (mW/m²) and estimated time-to-burn by skin type, with the full WHO risk scale.
Erythemal irradiance
200 mW/m²
Risk category
Very high
| Skin type | Description | Est. time to burn |
|---|---|---|
| I | Very fair, always burns | ~17 min |
| II | Fair, burns easily | ~21 min |
| III | Medium, sometimes burns | ~25 min |
| IV | Olive, rarely burns | ~38 min |
| V | Brown, very rarely burns | ~50 min |
| VI | Dark brown or black | ~83 min |
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What to do at this UV level
Use SPF 50 and minimize midday exposureAt UV 8, unprotected skin can burn quickly. Seek shade from 10am to 4pm.
Burn-time estimates are not a safe exposure target. Reflection, altitude, medication and individual skin response can shorten them.
WHO UV index risk categories
| UV index | Category | Protection needed |
|---|---|---|
| 0-2 | Low | Sunglasses on bright days |
| 3-5 | Moderate | SPF 30, hat, midday shade |
| 6-7 | High | SPF 30+, cover up 10am-4pm |
| 8-10 | Very high | SPF 50, minimize midday sun |
| 11+ | Extreme | Avoid midday sun entirely |
What the UV index measures physically
The UV index compresses erythemal irradiance into a simple scale: one UV index point equals 25 mW/m² of sunburn-weighted ultraviolet power. Enter a UV index to see the physical irradiance and an estimated unprotected time-to-burn for each skin type.
For the evidence, limits and practical use behind this result, read UV Index Explained. The weather tools guide explains when each calculator is the right one.
1 UVI = 25 mW/m² erythemal irradianceburn time ≈ MED ÷ (1.5 × UVI) minutes
Protection decisions from the result
- Each UV point = 25 mW/m² of sunburn-weighted radiation reaching the ground.
- From UV 3 upward, protect: SPF 30, hat, and shade at midday.
- Burn times are ceilings, not targets - water, sand, snow and altitude all shorten them.
UV exposure and burn-time questions
What does one point of UV index equal physically?
25 milliwatts per square meter of erythemally-weighted (sunburn-effective) UV irradiance. UV index 8 means 200 mW/m² is reaching the ground.
How accurate are time-to-burn estimates?
They are rough guides based on the minimal erythemal dose (MED) for each skin type under the standard weighting. Reflection, altitude, medication and prior tanning all shift the real number - treat them as ceilings, not targets.
Does a higher UV index mean it feels hotter?
No. UV is invisible and carries little heat - a cool spring day can have a burning UV index of 8 while a scorching evening has UV 0.