SPF Sun Safe Exposure Time Calculator

SPF (Sun Protection Factor) measures how much longer you can stay in the sun before burning compared to wearing no sunscreen.

The formula:

Safe Time = Unprotected Burn Time × Effective SPF

The unprotected burn time is not a fixed number for your skin. It depends on how strong the sun is, so this calculator works it out from the minimum erythemal dose (MED), the amount of UV energy each Fitzpatrick Skin Type absorbs before it reddens. One UV Index unit is 25 milliwatts per square metre of burn-weighted ultraviolet, which delivers 1.5 joules per square metre every minute:

Burn minutes = MED ÷ (UV Index × 1.5)

These are the same figures the UV Safe Sun Time, Sunscreen SPF and Sun Safety calculators on this site use, so all four give the same burn time for the same skin and sky.

Effective SPF, not label SPF

The number on the bottle assumes 2 mg of product on every square centimetre, a visible layer. Field studies keep measuring people at about half that, and Faurschou and Wulf (2007) found that protection then falls off as a power of the dose: effective SPF = label SPF ^ (applied dose ÷ 2). At half the dose that is the square root, so SPF 30 behaves like about 5.5 and SPF 50 like about 7.

Example:

Type II skin at UV 7 burns in about 24 minutes with nothing on. SPF 30 applied the way people actually apply it multiplies that by 5.5, which is about 2 hours 10 minutes. Applied at the full test dose it would be 30 times, nearly 12 hours, which is the figure the label implies and nobody gets.

However: the reapplication clock usually wins.

Sunscreen wears off on the skin whatever its rating. The American Academy of Dermatology says to reapply every 2 hours, and after swimming or sweating as the label directs. In the US a water-resistant label must say 40 or 80 minutes, and past that time the product is assumed gone. So the calculator gives the shorter of the burn window and the clock.

In practice, sunscreen effectiveness is also reduced by:

• Towel drying: removes sunscreen mechanically
• Missed patches: ears, the back of the neck and the tops of the feet burn first
• Reflection: fresh snow sends back up to 80% of UV, dry sand about 15% and water about 10%, so the real dose on a beach or a ski slope is higher than the UV Index says
• Peak UV hours: roughly 10am to 4pm, when the UV Index is at its highest

UV Index and burn times (no sunscreen, Type II skin):

The best sun protection strategy:

1. Avoid peak UV hours (10am–4pm) when possible
2. Apply SPF 30+ generously, 1 oz (30ml) for full body coverage
3. Reapply every 2 hours regardless of SPF rating
4. Wear UPF-rated clothing, wide-brim hat, and sunglasses