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UV Exposure Time Calculator

Estimates the time to skin reddening (erythema) from the UV index, the Fitzpatrick skin type (minimal erythema dose 200–1000 J/m²) and sunscreen SPF, and returns the WHO UV index category with its protection advice.

UV indexForecast or measured UV index (1 unit = 25 mW/m² of erythemally weighted irradiance). Fitzpatrick skin typeFitzpatrick sun-reactive skin type; sets the minimal erythema dose (MED) used for the estimate. Sunscreen SPF (optional)Sun protection factor of the sunscreen applied (1 = no sunscreen). Used as a theoretical multiplier of the burn time. Shade / cloud reduction (%) (optional)Percentage of UV blocked by shade, clouds or clothing (open shade ≈ 50 %, heavy overcast ≈ 70 %).

When to use

You want a theoretical sunburn-time estimate for a UV index forecast and a skin type, with or without sunscreen, shade or cloud cover.

Do not use when: You need a safety guarantee: individual MED varies about two-fold within a skin type, sunscreen is usually under-applied, and snow, sand and water reflection add exposure; the result is informational only, not medical advice.

Formula

uv_dose_rate_j_m2_per_min = uv_index × 0.025 W/m² × 60 s × (1 − shade_factor_percent / 100) = uv_index × 1.5 × (1 − shade_factor_percent / 100); minutes_to_burn_unprotected = med_j_m2 / uv_dose_rate_j_m2_per_min; minutes_to_burn_with_spf = minutes_to_burn_unprotected × spf

UV index 1 equals 25 mW/m² of CIE erythemally weighted irradiance (WHO), i.e. 1.5 J/m² per minute. The MED values 200, 250, 300, 450, 600 and 1000 J/m² for Fitzpatrick types I–VI are typical literature values; measured protection from sunscreen is usually a third to a half of the label SPF because people apply 0.5–1 mg/cm² instead of the 2 mg/cm² used in SPF testing. Informational only, not medical advice.

Inputs

Parameter	Type	Unit	Required	Description
`uv_index`	number		yes	Forecast or measured UV index (1 unit = 25 mW/m² of erythemally weighted irradiance). Range: > 0, ≤ 16
`skin_type`	enum: type_1 \| type_2 \| type_3 \| type_4 \| type_5 \| type_6		yes	Fitzpatrick sun-reactive skin type; sets the minimal erythema dose (MED) used for the estimate.
`spf`	number		default 1	Sun protection factor of the sunscreen applied (1 = no sunscreen). Used as a theoretical multiplier of the burn time. Range: ≥ 1, ≤ 100
`shade_factor_percent`	number	%	default 0	Percentage of UV blocked by shade, clouds or clothing (open shade ≈ 50 %, heavy overcast ≈ 70 %). Range: ≥ 0, ≤ 90

Outputs

Output	Type	Unit	Description
`minutes_to_burn_unprotected`	number	min	MED / dose rate, with the shade factor applied but no sunscreen.
`minutes_to_burn_with_spf`	number	min	Unprotected time × SPF (theoretical, full-label protection).
`hours_with_spf`	number	h	minutes_to_burn_with_spf / 60.
`med_j_m2`	number	J/m²	Erythemally weighted UV dose that produces just-visible reddening for the chosen skin type.
`uv_dose_rate_j_m2_per_min`	number	J/m²/min	uv_index × 1.5 J/m² per minute, reduced by the shade factor.
`uv_index_category`	string		WHO exposure category: low 0–2, moderate 3–5, high 6–7, very high 8–10, extreme 11+ (index rounded to the nearest whole number).
`advice`	string		WHO sun-protection message for the category.

Example

UV index 8, skin type II, SPF 30: {"uv_index":8,"skin_type":"type_2","spf":30} → {"minutes_to_burn_unprotected":20.8,"minutes_to_burn_with_spf":625,"hours_with_spf":10.42,"med_j_m2":250,"uv_dose_rate_j_m2_per_min":12,"uv_index_category":"Very high (8–10)"}

UV index 5, skin type IV, 50 % shade, SPF 15: {"uv_index":5,"skin_type":"type_4","spf":15,"shade_factor_percent":50} → {"minutes_to_burn_unprotected":120,"minutes_to_burn_with_spf":1800,"uv_dose_rate_j_m2_per_min":3.75,"uv_index_category":"Moderate (3–5)"}

GET https://tttkmbb.com/api/v1/calculate/uv-exposure-time?uv_index=8&skin_type=type_2&spf=30

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/uv-exposure-time (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/uv-exposure-time · Markdown: https://tttkmbb.com/health/uv-exposure-time.md · JSON definition: https://tttkmbb.com/health/uv-exposure-time.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="uv-exposure-time"
OpenAPI operationId: estimate_uv_burn_time
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

WHO (2002) Global Solar UV Index: A Practical Guide (standard)
Wikipedia – Ultraviolet index (reference)
Wikipedia – Fitzpatrick scale (reference)

FAQ

Why is the SPF time unrealistic?

SPF is measured with 2 mg/cm² of product; typical use is a quarter to half of that, and sweat, water and rubbing remove it. Reapply every 2 hours and treat the SPF time as an upper bound, not a plan.

Is the UV index constant during the day?

No. The forecast value is the daily maximum around solar noon; the dose rate is much lower in the morning and late afternoon, so the burn time computed from the peak index is conservative for those hours.

What is a minimal erythema dose?

The smallest UV dose that produces just-perceptible reddening 24 hours later. It rises from about 200 J/m² (type I) to 1000 J/m² (type VI) but varies about two-fold between people of the same type.

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