UV Index Calculator

PHP

Inputs

Calculation mode

Skin type (Fitzpatrick)

SPF factor

Shade / clothing transmittance (0.05-1)

1.00 means no shade; 0.50 halves UV; 0.10 is deep shade.

Quick preset

Notes

Direct erythemal irradiance

Use measured erythemal-weighted irradiance from a sensor or model.

Erythemal irradiance value

Units

Tip

Clear page Results appear above this form after submit.

Typical values linking erythemal irradiance to UVI and risk. Times assume skin type II, no protection.

Eery (W/m^2)	UVI	Risk	Approx. time to MED (min)
0.025	1.0	Low	167
0.075	3.0	Moderate	56
0.150	6.0	High	28
0.225	9.0	Very High	19
0.300	12.0	Extreme	14

UV Index definition: UVI = Eery / 0.025 = 40 * Eery, where Eery is erythemal irradiance in W/m^2.
Band approximation: Eery ~ (UVA * kuva) + (UVB * kuvb), with effectiveness factors kuva and kuvb (dimensionless).
Clear-sky parameterization: UVI ~ 12 * sin(elevation)^1.2 * exp(-(ozone-300)/180) * (1+0.08*alt_km) * (1+0.25*albedo) * exp(-0.4*AOD) * cloudFactor.
Minimal erythema time: time(min) = MED / Eery / 60. With SPF and shade transmittance t: timeProtected ~ time * SPF / t.

All models are approximations; local aerosols, altitude, and clouds can change UV rapidly.

If you have a UV sensor reporting UVI directly, use the direct mode and enter Eery = UVI * 0.025 W/m^2.