Zerdly

UV Exposure Time Calculator

Estimate how long in the sun it takes to reach an erythemal (skin-reddening) UV dose from the UV Index. Enter the UV Index, a target dose in Standard Erythema Doses, and an optional skin-type factor to see the erythemal irradiance and the time to the dose.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the current UV Index (UVI) for your location and time of day.
  2. Optionally set the target dose in SED (default 1 SED roughly reddens fair skin) and a skin-type factor above 1.0 for skin that reddens more slowly.
  3. Read the time to reach the dose, and plan to apply sun protection well before it.

How it works

Erythemal irradiance in W/m² is UV Index × 0.025. One Standard Erythema Dose (SED) equals 100 J/m², so the target dose in J/m² is SED × 100 × the skin-type factor. Dividing that dose by the irradiance gives the time in seconds, which is converted to minutes.

Worked example

Worked example. At a UV Index of 10, the erythemal irradiance is 10 × 0.025 = 0.25 W/m². For a 1 SED target the dose is 100 J/m², so the time is 100 / 0.25 = 400 s = 6.667 min.

Common mistakes

  • Treating the result as a safe sun-exposure limit — it is the time to skin reddening, so protect well before it.
  • Ignoring reflected UV from water, sand, snow or concrete, and altitude, which all raise the real dose above this clear-sky estimate.
  • Assuming a fixed skin-type factor for everyone — thresholds vary widely between individuals and with recent sun history.

Frequently asked questions

What is a Standard Erythema Dose (SED)?

1 SED is a standard erythemal UV dose of 100 J/m². Around 1 SED is enough to start reddening fair, sun-sensitive (type II) skin, though sensitive individuals redden with less.

Does this account for shade or sunscreen?

No. It is an unshaded, midday, clear-sky estimate. Shade, clothing and sunscreen all extend the time, while reflective surfaces and altitude shorten it, so use it as a planning guide only.

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Tip: Enter any known values to calculate the remaining results.

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