UV Erythemal Dose CalculatorFree online fitness & health calculator — instant results, no sign-up
Estimate standard erythema dose from constant UV index and exposure time.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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About the UV Erythemal Dose Calculator
The UV Erythemal Dose Calculator estimates erythemally weighted UV energy and standard erythema doses (SED) from a constant UV index and an exposure duration in minutes. It converts UV index into irradiance weighted for its ability to cause skin redness, then multiplies by exposure time. This tool measures estimated exposure, not a safe exposure limit or a prediction of when you will burn.
- UV index
- The unitless UV index represents the erythemally weighted intensity of UV radiation, assumed constant throughout the exposure; the default is 6.
- Exposure duration (minutes)
- The exposure duration is the total time in minutes over which the constant UV index applies; the default is 20 minutes.
Formula and method
E = U / 40; t = 60m; D = E × t = (U / 40) × 60m; S = D / 100. For U = 6 and m = 20: D = (6 / 40) × (60 × 20) = 180 J/m²; S = 180 / 100 = 1.8 SED.
- UUV index, a unitless value.
- EErythemally weighted UV irradiance in W/m²; one UV index unit corresponds to 0.025 W/m².
- mExposure duration in minutes.
- tExposure duration in seconds, calculated as 60 times the duration in minutes.
- DErythemally weighted radiant dose in J/m².
- SNumber of standard erythema doses, using 1 SED = 100 J/m² of erythemally weighted dose.
Divide the UV index by 40 to obtain erythemally weighted irradiance in W/m². Convert exposure minutes to seconds, then multiply irradiance by seconds to obtain dose in J/m². Divide that dose by 100 J/m² per SED to express the same exposure in standard erythema doses.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- UV index6
- Exposure duration20 minutes
Results
- Erythemally weighted dose180 J/m²
- Standard erythema doses1.8 SED
- NoteNot a safe exposure or burn-time prediction; UV varies and skin sensitivity differs.
How to use the UV Erythemal Dose Calculator
- 1Enter the UV index for the exposure period, using a nonnegative value; the default is 6.
- 2Enter the exposure duration in minutes, using a nonnegative value; the default is 20.
- 3Check that a constant UV index is a reasonable approximation for the entire period.
- 4Run the calculation and read both dose outputs; the defaults produce 180 J/m² and 1.8 SED.
- 5Read the caution note and do not treat the result as a safe exposure allowance or a burn-time prediction.
Reading your result
The 'Erythemally weighted dose' row gives accumulated UV energy per square metre after weighting wavelengths for their skin-redness effect; the default result is 180 J/m². The 'Standard erythema doses' row expresses that same dose in units of 100 J/m²; the default result is 1.8 SED. The 'Note' row states: 'Not a safe exposure or burn-time prediction; UV varies and skin sensitivity differs.' SED is a standardized exposure unit, not a count of sunburns or a personalized redness threshold.
Limitations
The calculation assumes a constant UV index throughout the entered duration. Actual UV can change with time of day, cloud cover, ozone, altitude, and surroundings, while personal exposure also depends on body orientation, shade, clothing, and sunscreen use. A forecast daily maximum may not represent the UV index during your exposure. This tool does not model protection or individual sensitivity, including skin type, photosensitizing medicines, or medical conditions. It does not predict sunburn, vitamin D production, or long-term skin damage, and it cannot establish a safe exposure duration.
Common questions
Why do UV index 6 and 20 minutes produce 1.8 SED?+
UV index 6 corresponds to 0.15 W/m² of erythemally weighted irradiance. Twenty minutes is 1,200 seconds, so the dose is 0.15 × 1,200 = 180 J/m². Dividing by 100 J/m² per SED gives 1.8 SED.
Does 1.8 SED mean I will get sunburned?+
No. SED measures standardized erythemally weighted exposure, whereas the dose that produces visible redness varies between people and circumstances. The calculator does not estimate your individual burn threshold.
What if the UV index changes during my exposure?+
Use separate calculations for intervals with approximately constant UV index and add their doses, or use a reliable time-weighted average UV index for the full duration. A single peak UV index is not necessarily representative of the whole exposure.
Can I divide the calculated dose by my sunscreen SPF?+
This calculator does not support that adjustment. SPF is measured under standardized test conditions, and real-world protection depends on application amount, coverage, wear, and reapplication. Dividing the result by SPF would not provide a reliable personal dose or safe exposure time.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.