SPF Calculator
Our SPF calculator estimates how long your sunscreen will protect you from sunburn based on four factors: your skin type (Fitzpatrick scale I–VI), your chosen SPF value, the current UV index, and your activity type. The result shows the maximum protected time before reapplication is needed, with a hard cap at 4 hours since UV exposure accumulates and no sunscreen lasts indefinitely.
Sunscreen is one of the most evidence-based skin cancer prevention tools available. Daily use of SPF 15 or higher reduces the risk of developing melanoma by 50% and squamous cell carcinoma by 40% according to longitudinal studies. However, sunscreen alone is not sufficient — UV protection also requires seeking shade during peak hours (10am–4pm), wearing protective clothing (UPF-rated fabrics block 97.5% of UV), and using polarised sunglasses that block UV.
Frequently Asked Questions
What does SPF number mean?
SPF (Sun Protection Factor) indicates how much longer you can stay in the sun before getting a sunburn compared to unprotected skin. SPF 30 means it takes 30× longer to burn. In practice: if your unprotected skin burns in 10 minutes, SPF 30 provides about 300 minutes of theoretical protection. However, real-world protection is lower due to incomplete coverage and sweating.
What is the Fitzpatrick skin type scale?
The Fitzpatrick scale categorises skin into 6 types based on melanin content and UV sensitivity: Type I (very fair, always burns, never tans), Type II (fair, usually burns, sometimes tans), Type III (medium, sometimes burns, gradually tans), Type IV (olive, rarely burns, tans easily), Type V (brown, very rarely burns, tans deeply), Type VI (dark, almost never burns). Lower types need higher SPF.
Does SPF 100 provide twice the protection of SPF 50?
No — the difference is much smaller than the numbers suggest. SPF 50 blocks 98% of UVB rays; SPF 100 blocks 99%. The 1% difference means SPF 100 allows through only half as many UV rays as SPF 50 — but both block more than 98%. For daily use, SPF 30–50 is considered adequate; SPF 50+ is recommended for fair skin, long outdoor exposure, and high UV environments.
What is UV index and how does it affect sunscreen needs?
UV index (UVI) measures ultraviolet radiation intensity: 1–2 = low; 3–5 = moderate; 6–7 = high; 8–10 = very high; 11+ = extreme. At UVI 6+, unprotected fair skin can burn in 10–15 minutes. The WHO recommends SPF 30+ at UVI 3+, SPF 50+ at UVI 6+. UV index peaks at solar noon and is highest in summer, at altitude, near snow/sand/water (reflection).
How much sunscreen should I apply?
The standard dose is 2 mg/cm² of skin surface. In practice, this means: face and neck — 1 teaspoon (5ml); full body adult — 35–40ml (about 6–7 teaspoons or approximately one-quarter of a 150ml bottle). Most people apply only 25–50% of the recommended amount, significantly reducing actual protection. Applying too little is the most common sunscreen mistake.
Does sunscreen expire?
Yes. Sunscreen has a shelf life of 2–3 years from manufacture. After expiry, UV-filtering compounds degrade and protection is reduced. Store sunscreen below 25°C and away from direct sunlight. Sunscreen left in a hot car (60–80°C) can degrade significantly within days. Check the expiry date on the bottle — an "open jar" symbol with a number (e.g., 12M) indicates months after opening.
Is sunscreen safe for children?
For children under 6 months: avoid sunscreen — protect with clothing, hats, and shade instead. Over 6 months: use SPF 30+ broad-spectrum sunscreen. Mineral sunscreens (zinc oxide, titanium dioxide) are preferred for children as they sit on the surface of skin rather than being absorbed. Avoid sunscreens with oxybenzone or retinyl palmitate in children.
What is broad-spectrum sunscreen?
Broad-spectrum means protection against both UVA and UVB rays. UVB causes sunburn and most skin cancers. UVA (longer wavelength) penetrates deeper, causing premature skin ageing and contributing to melanoma. The SPF number only measures UVB protection. Always choose "broad-spectrum" sunscreens, which have passed additional UVA protection testing.
Does sunscreen prevent vitamin D production?
Regular sunscreen use does slightly reduce vitamin D synthesis, but not as much as feared. Studies show that even regular sunscreen users produce sufficient vitamin D because: application is rarely complete (uncovered areas still produce D), and brief incidental sun exposure (10–15 min on arms/face daily) provides adequate synthesis. Vitamin D deficiency is better addressed through diet and supplements than by skipping sun protection.
What is water-resistant sunscreen?
Water-resistant sunscreen maintains efficacy for 40 minutes in water (standard) or 80 minutes (very water resistant). After this time, or after towelling off, reapplication is essential. No sunscreen is "waterproof" — this claim was banned in many countries. For swimming and water sports, use SPF 50+ with very water-resistant formula and reapply after exiting the water.
Can I use last year's leftover sunscreen?
Check the expiry date first. If within date, the sunscreen should still be effective if stored properly (cool, dark place). If it has separated, become watery, developed a strange smell, or changed colour, discard it — the formula has degraded. As a practical rule: buy a new bottle at the start of each sun season rather than relying on opened bottles from previous years.