Sunscreen SPF Math: Percent of UVB Blocked, How Much to Apply, and When to Reapply
October 29, 2026
Sun protection factor (SPF) compares the dose of UVB radiation needed to cause sunburn on protected skin to unprotected skin. The percentage of UVB blocked is 1 − 1/SPF: SPF 15 blocks 93.3%, SPF 30 blocks 96.7%, SPF 50 blocks 98%, and SPF 100 blocks 99%. The gains diminish: doubling from 30 to 60 reduces the transmitted UVB from 3.3% to 1.7%, a small absolute gain that still halves the exposure.
SPF measures protection against UVB (which causes sunburn). Look for 'broad-spectrum' on the label to cover UVA as well.
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SPF is tested at 2 mg of sunscreen per square centimeter of skin. An adult's body surface area is about 1.7 to 1.9 m² (17,000 to 19,000 cm²), so a full-body application is about 2 mg × 17,000 cm² = 34 g, close to 1 ounce (30 mL, a shot glass). The face and neck alone need about a quarter teaspoon to a half teaspoon (1.25 to 2.5 mL) each. Most people apply only a quarter to half of the tested amount, which cuts the actual protection drastically: applying half the amount of SPF 30 may give protection closer to SPF 5 to 6 (the effect is roughly exponential in the thickness).
| SPF | UVB blocked | UVB reaching skin | Theoretical time if you burn in 10 min |
|---|---|---|---|
| 15 | 93.3% | 6.7% | 150 min |
| 30 | 96.7% | 3.3% | 300 min |
| 50 | 98.0% | 2.0% | 500 min |
| 100 | 99.0% | 1.0% | 1,000 min |
Reapplication Matters More Than the Number
The 'time to burn x SPF' calculation is theoretical because sunscreen wears off from sweating, swimming, and rubbing, and UV intensity changes. Reapply every 2 hours and after swimming or heavy sweating, even with water-resistant products (rated for 40 or 80 minutes). A high SPF applied once in the morning protects less than SPF 30 applied properly and reapplied. Sunscreen should be applied 15 to 30 minutes before going out. Layer it with shade, hats, protective clothing, and avoiding midday sun (10 am to 4 pm).
Choosing and Using
Dermatology organizations recommend a broad-spectrum SPF 30 or higher, water-resistant for outdoor activity. Check the expiration date; products typically last up to three years, less if exposed to heat. Spray sunscreens must be applied until the skin glistens and rubbed in, avoiding inhalation. People with darker skin still need sun protection. Tanning does not indicate health.
Cautions
Sunscreen supports but does not replace other protection, and no product blocks 100%. Infants under 6 months should be kept out of direct sun. This article is educational; talk to a dermatologist about skin cancer risks, moles, and prescription needs.
Frequently Asked Questions
How much UVB does SPF 30 block?
About 96.7%.
How much sunscreen should I apply to my body?
About 1 ounce (30 mL).
How often should I reapply?
Every 2 hours, and after swimming or sweating.
Does SPF protect against UVA?
Not by itself; look for broad-spectrum.
Is SPF 100 twice as good as SPF 50?
No, it blocks 99% versus 98% of UVB.