Sunscreen Calculator

Sunscreen protects your skin from harmful ultraviolet (UV) radiation emitted by the sun. UV rays — particularly UVA and UVB — can cause sunburn, premature skin aging, and increase the risk of skin cancer. Daily sunscreen use significantly reduces these risks even on cloudy days.

Look for a broad-spectrum sunscreen rated SPF 30 or higher, which blocks about 97% of UVB rays. SPF 50 blocks around 98%. For outdoor activities or swimming, choose a water-resistant formula and always check the expiry date. Dermatologists generally recommend SPF 30–50 for everyday use.

The standard guideline is approximately 2 mg of sunscreen per cm² of exposed skin. In practice, this works out to roughly 35 ml (about 7 teaspoons) for a fully exposed adult body, or around 1/4 to 1/3 of a standard 150 ml bottle per application. Most people apply far less than needed, which reduces the effective SPF.

Body surface area can be estimated from your height and weight using the Mosteller formula. Once you know your exposed surface area (adjusted for clothing), multiply by 2 mg/cm² to get the amount needed per application. This calculator does all of that for you automatically.

Not exactly. SPF refers to how much longer you can stay in the sun before burning compared to unprotected skin. If unprotected skin starts to burn in 10 minutes, SPF 30 theoretically gives 300 minutes of protection — but only under lab conditions. In reality, sweating, swimming, and incomplete application reduce this significantly, which is why reapplying every 2 hours is recommended.

You should reapply sunscreen at least every 2 hours when outdoors. If you are swimming or sweating heavily, reapply every 40–80 minutes depending on whether the product is water-resistant. No sunscreen provides 100% protection, so combine it with shade, a wide-brimmed hat, and UV-protective clothing.

A 150 ml bottle provides roughly 4 full-body applications for an average adult (using the recommended 35 ml per application). If you're outside 6 hours a day and reapplying every 2 hours, that's about 4 applications per day — meaning one 150 ml bottle lasts roughly one day of full sun exposure. Pack more than you think you need.

For a week outdoors (6–8 hours of sun per day), an average adult applying the recommended amount will use 3 to 5 standard 150 ml bottles. This calculator estimates the exact number based on your body size, clothing coverage, activity level, and bottle size so you never run short.