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UV Exposure Safe Time Calculator

Work out uv exposure safe time instantly with clear inputs, formula shown and shareable results.

About UV Exposure Safe Time Calculator

Converts the UV index into erythemal irradiance and divides the minimal erythema dose for your phototype by it to give the time to visible reddening.

Formula

UV dose and time

Erythemal irradiance (W/m2) = UV index x 0.025; time to one MED (s) = MED (J/m2) / irradiance; correctly applied sunscreen multiplies that time by its SPF

Worked example

With the default values below, UV Exposure Safe Time Calculator returns Time to one minimal erythema dose of 24m. Change any input above and every figure here is recalculated in your browser.

Inputs used

UV indexaccepts 0.5–16
7
Skin phototypeone of Type I (200 J/m2), Type II (250 J/m2), Type III (350 J/m2), Type IV (450 J/m2), Type V (600 J/m2), Type VI (1000 J/m2)
Type II (250 J/m2)
Sunscreen SPF appliedaccepts 1–100
30

Results

Time to one minimal erythema dose
24m
Time with the stated SPF applied correctly
714 min
Erythemal irradiance
0.1750 W/m2
Minimal erythema dose assumed
250 J/m2

Step by step

  1. 1Values used
  2. 2UV dose and time
  3. 3Time to one minimal erythema dose= 23.80
  4. 4Time with the stated SPF applied correctly= 714 min
  5. 5Erythemal irradiance= 0.1750 W/m2
  6. 6Minimal erythema dose assumed= 250 J/m2

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.
  • This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.

Frequently Asked Questions

Is the SPF multiple realistic?

Only with the full 2 mg/cm2 test dose; real-world application typically achieves a third to a half of the labelled protection.

Does the calculation account for reflection?

No. Snow, sand and water reflect ultraviolet and shorten the safe time considerably.

Related Calculators

UV Exposure Safe Time Calculator | Calcrivo