Sunscreen Calculator: SPF, UV Index and Safe Exposure Time

Calculate safe sun exposure time before burning based on Fitzpatrick skin phototype, local UV index, and sunscreen SPF rating.

UV index, skin type and SPF: estimating time to sunburn

The tool estimates time to sunburn without sunscreen by dividing a reference time for each skin type (10 to 60 minutes at UV index 10) by the day's UV index, then shortening it for ground reflection. It also shows the share of UVB filtered by the sunscreen, (SPF − 1) / SPF, which is 96.7% for SPF 30.

Formulas

  • Time without sunscreen (min) = reference time × 10 / UV index, rounded. Reference time at UV 10: type I 10 min, II 15, III 20, IV 30, V 45, VI 60.
  • Reflection: time divided by 1.25 (water), 1.20 (sand) or 1.80 (snow), with a 2-minute minimum.
  • UVB filtered (%) = (SPF − 1) / SPF × 100. The "50+" option is computed as SPF 60.
  • Reapplication: every 120 minutes, or 80 minutes when swimming or doing sport.

Example

Skin type II, UV index 7, SPF 30, in town: 15 × 10 / 7 = 21.4, i.e. 21 minutes to sunburn without sunscreen; the sunscreen filters 96.7% of UVB and lets 3.3% through. On snow the time drops to 21 / 1.8 = 12 minutes. With SPF 15, filtering is 93.3%.

UV index categories

0-2 low; 3-5 moderate; 6-7 high; 8-10 very high; 11 and above extreme. The WHO advises seeking shade around midday from 3 and avoiding being outside at midday from 8.

Limits

The reference times by skin type and the reflection factors are orders of magnitude, not verified against an official source. SPF is measured in the lab with 2 mg of sunscreen per cm² of skin (ISO 24444, not verified); people often apply less, and multiplying the unprotected time by the SPF does not give a safe exposure time. Sunscreen is no reason to stay longer in the sun. The quantities shown (about 30 ml for the body) are indicative. The result is an estimate and does not replace a dermatologist's advice.

Sources and dates: WHO, Global solar UV index practical guide of 13 June 2002 (WHO, Global solar UV index: a practical guide) and UV index Q&A of 20 June 2022 (WHO, Radiation: the ultraviolet (UV) index); European Commission Recommendation 2006/647/EC of 22 September 2006 on the efficacy of sunscreen products (protection categories, "50+" for SPF 60 or more) and ISO 24444:2019, cited without a link and not verified online (checked on 14/10/2026)

Frequently Asked Questions

How long can I stay in the sun at UV index 7?

For fair skin (type II), the tool estimates about 21 minutes before sunburn without protection at UV index 7, and 12 minutes on snow. The WHO advises seeking shade around midday from a UV index of 3.

What is the difference between SPF 30 and SPF 50?

SPF 30 filters about 96.7% of UVB and SPF 50 about 98%, based on (SPF − 1) / SPF. The gap looks small, but the share getting through falls from 3.3% to 2%. These figures assume 2 mg of sunscreen per cm² of skin, the dose used in lab testing.

Can I stay in the sun SPF times longer with sunscreen?

In theory SPF 30 multiplies by 30 the UVB dose needed to redden the skin, but that assumes a thick, intact layer. Sweat, rubbing and applying too little reduce protection: the tool advises reapplying every 2 hours, or every 80 minutes when swimming or doing sport.

What does a UV index of 8 or more mean?

UV index 8 to 10 is rated very high and 11 or more extreme. At these levels the WHO advises avoiding being outside around midday. At UV index 8 the tool estimates 19 minutes to sunburn without protection for skin type II and 13 minutes for type I.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.