Time in the sun, counted.
Your Fitzpatrick phototype, the day's UV index and your sunscreen SPF combine into one estimate: the minutes before the first sign of sunburn. Start the countdown and a reapply reminder tracks the real time as it runs.
Skin, sun and sunscreen, read together.
Pick your Fitzpatrick phototype, today's UV index and the SPF you are wearing. The estimate updates as you move each control, then Start countdown turns it into a running clock with a reapply reminder.
Skin phototype
UV index today
High Protection needed: shade, clothing and sunscreen.
Sunscreen SPF
Application
Most people apply about half the tested 2 mg per square centimetre, so a realistic layer roughly halves the labelled SPF. Full amount uses the SPF as printed.
Phototype III skin under a UV index of 7 with SPF 30 applied at a realistic thickness has about 7 hours 9 minutes before the first sign of sunburn.
An estimate for planning, not a promise. Clouds, water, sand, altitude and some medication change the real number.
Phototype III, UV index 7, SPF 30, realistic amount.
How it works.
- Pick your Fitzpatrick phototype, the day's UV index and the SPF you are wearing.
- Read the minutes before sunburn, then press Start countdown to track the real time as it runs.
- Watch the reapply reminder every 2 hours and the warning that appears under 10 minutes, then reset for a new session.
The Fitzpatrick scale and the minimal erythema dose
The Fitzpatrick scale sorts skin into six phototypes, from type I, pale skin that always burns and never tans, to type VI, deeply pigmented skin that almost never burns. Each phototype tolerates a different minimal erythema dose, the amount of UV energy that produces the first faint redness. This timer uses typical mid range figures: 200 joules per square metre for type I, 250 for type II, 300 for type III, 450 for type IV, 600 for type V and 1000 for type VI. Real tolerance in one person can sit well above or below these averages, which is why the same UV index and the same SPF give different outcomes for two people of the same phototype. Repeated unprotected exposure also shows up later as a tan that hides the real dose, and over years as pigment spots that are far easier to prevent than to remove.
How the UV index becomes minutes
The World Health Organization's UV index runs from 0 to 11 and beyond, in five bands: 1 to 2 low, 3 to 5 moderate, 6 to 7 high, 8 to 10 very high, 11 or more extreme. This tool treats one UV index point as 25 milliwatts of erythemal irradiance per square metre, then divides the minimal erythema dose by that irradiance to get the unprotected time in minutes. Sunscreen extends it by the SPF factor, but the number on the bottle assumes 2 milligrams per square centimetre, a thickness most people never apply. Field studies put a typical layer at about half that amount, so the realistic setting halves the labelled SPF, while the full amount setting keeps the printed number. Either way, sunscreen wears off through sweat, water and rubbing, so dermatology guidance calls for reapplication every 2 hours regardless of SPF, which is why the countdown repeats that reminder and stops counting past 480 minutes, 8 hours.
What this timer is not
This is a planning estimate, not a permission slip and not a diagnosis. Cloud cover still lets through 50 to 80 percent of UV radiation, water and sand reflect it back onto skin already exposed from above, and UV intensity rises by roughly 10 percent for every 1000 metres of altitude, so a mountain afternoon can burn faster than the number suggests. Certain medications raise photosensitivity far beyond the averages used here, a child's skin behaves differently from an adult's, and pregnancy can change how skin responds to sun. If marks left by past sun exposure already concern you, the guide on age spots explains what accumulated UV damage looks like later and what actually treats it. For anything unusual on your skin, see a doctor rather than a timer.
Frequently asked.
What is the Fitzpatrick phototype scale?
It is a six point scale from type I, skin that always burns and never tans, to type VI, skin that almost never burns. Dermatologists use it to estimate baseline sun sensitivity, and this timer uses it to choose a minimal erythema dose for the calculation.
Where do the minutes before sunburn come from?
They come from dividing your phototype's minimal erythema dose by the UV index converted to irradiance, then multiplying by your effective SPF. The result is an average for your phototype, not a measurement of your own skin.
Why does the realistic application setting change the number so much?
SPF ratings assume 2 milligrams of sunscreen per square centimetre of skin, roughly a shot glass for a full body. Most people apply about half that amount, so the realistic setting halves the labelled SPF to match how sunscreen is actually used.
Does the countdown mean I can stop paying attention?
No. It is a reminder, not a guarantee. Reapply every 2 hours regardless of what the clock shows, move to shade sooner if your skin looks or feels different, and treat the number as a planning aid rather than a target to reach.
What is saved when I use this tool?
Only your phototype selection, in your browser's local storage under one entry. The UV index, SPF, application setting and any countdown are not saved, and reset the next time you open the page.
Guides behind this tool
Skin and Cosmetics
Tanning: colour is not dose
Laser and Skin
Pigment spots: the cause decides
Laser and Skin
Age spots: the skin's light accountMore tools
This timer estimates an average time to the first sign of sunburn from population data; it does not measure your skin, diagnose a condition or replace advice from a doctor or dermatologist. This page is for general information and does not replace medical or professional cosmetic advice.