Sunburn timer

Offline — showing the last reading this device stored

Your settings

Under each choice: the time it gives you
Skin

SPF
Sweat

Start

Or click a time on the UV chart

UV through the day

Your burn dose

The sun

Weather

Which skin type am I?

The Fitzpatrick scale sorts skin by how it reacts to sun. Pick the one that sounds like you.

How does this work?

These estimates are based on dermatological research on UV exposure and erythema (skin reddening). Here's how it works:

1. UV energy input

The UV Index measures the intensity of skin-damaging radiation. Higher number means faster burn.

1 UVI = 25 mW/m² (milliwatts per square meter)

A 0.8× geometry factor is applied because you're a person standing and moving, not a flat sensor pointing at the sky.

2. Your skin's threshold (MED)

Each skin type has a Minimal Erythemal Dose (MED), the UV energy needed to cause visible reddening. Measured in J/m² (joules per square meter).

These are population averages. Your actual threshold varies with genetics, recent exposure, and other factors.

3. SPF as a divisor

SPF divides the UV reaching your skin. SPF 30 means only 1/30th of the UV energy gets through.

Real-world sunscreen rarely performs at lab-rated SPF due to application thickness and sweat. We model this degradation over time.

4. The calculation

Think of it as filling a bucket. Each minute adds UV damage, and at 100% you're burnt.

Damage % = (UV_Energy_Per_Min / (MED × SPF)) × 100

This runs minute-by-minute using hourly UV forecasts, interpolating between data points.

For low UV (UVI < 3), a smoothing curve accounts for reduced effective exposure at dawn/dusk sun angles.

The sun and your shadow

The lower the sun, the longer its light travels through the atmosphere and the weaker the UV. A quick check outdoors: if your shadow is shorter than you are, the UV is strong. The sun tile shows the sun's height through the day and how long your shadow is at any moment.

Sweat Index

Sweat Index is an outdoor comfort score based on temperature and dew point. Higher values mean hotter, muggier air where sweat doesn't evaporate as easily.

This does not change UV strength or your calculated sunburn time. Use it as context for outdoor comfort, sunscreen reapplication, and deciding whether conditions feel manageable for time outside.

References

Where are you?