How much sunscreen do you need?

What the number means

The printed SPF on a bottle is a laboratory result. To get that protection in real life you have to apply the same thickness the lab used, and most people put on much less. The result here is the weight of sunscreen that corresponds to the lab thickness over the skin you leave uncovered, plus how many times you should apply it during your time outdoors. It deliberately does not claim to know how long you can stay in the sun: that is not something SPF tells you.

The method

Skin area = body surface area × the share of skin left uncovered. Body surface area uses the Mosteller formula, the square root of height in cm times weight in kg divided by 3,600, as on our body surface area calculator, converted from m² to cm² (1 m² = 10,000 cm²). Amount per application = skin area × 2 mg/cm², the FDA's test standard. Applications = time outdoors divided by 2 hours, rounded up, since the FDA says to reapply at least every two hours. What gets through = 100 ÷ SPF percent of the sunburn-causing UV, at the test thickness.

Worked example

A 70 kg, 175 cm adult has a surface area of 1.84 m² = 18,447 cm². Covering all of it at 2 mg/cm² takes 18,447 × 2 = 36,893 mg = 37 g, or 1.3 oz, which agrees with the FDA's "at least one ounce". Four hours outdoors need 2 applications, 74 g (2.6 oz) in all. If only half the skin is uncovered for six hours with SPF 50: 18 g each time, 3 applications, 55 g in total, and about 2 percent of the sunburn-causing UV passes at the test thickness. For a 20 kg, 112 cm child, one full application is 16 g (0.56 oz). Ask a pediatrician about sunscreen for babies and small children.

Limits of this method

Thinner than the test means less protection. The FDA SPF test applies 2 mg/cm²; the FDA's consumer advice of an ounce for the whole body is the same amount. If you apply half of it, you do not get half the protection or a neat fraction, and we do not claim a number. SPF is mostly about UVB. The FDA says SPF values primarily indicate UVB protection; choose a broad-spectrum product for UVA. Body surface area is an estimate. Formulas like Mosteller average over many bodies, and the share of skin you leave uncovered is your own guess. Sweat, water, towels and sand remove sunscreen, so the two-hour schedule is a minimum. No burn-time estimate is made. No official table of burn doses by skin type is available to cite, and the UV Index describes the sun, not your skin.

Common mistakes

Applying sunscreen once in the morning and counting on it all day; skipping ears, the back of the neck, feet and the part in the hair; using last year's bottle without checking its date; assuming a high SPF permits a longer stay; and trusting a cloudy sky: check the UV Index forecast instead of the weather.

Questions

How much sunscreen should I use?

The FDA says an average-sized adult or child needs at least one ounce, about a shot glass, to cover the body from head to toe. That matches the amount used in the SPF test, 2 milligrams per square centimeter of skin: for a person of 70 kg and 175 cm (1.84 m² of skin) it is 37 grams, or 1.3 ounces, for one application over the whole body. The FDA does not recommend sunscreen for babies under 6 months; keep them out of the sun and ask a doctor.

How often should I reapply?

The FDA advises reapplying at least every two hours, and more often after swimming or sweating, even for a water-resistant product. Reapplying does not extend the protection beyond what the product gives; it restores the layer that sweat, water, towels and clothing remove. Apply 15 minutes before going out, as the FDA recommends.

Does SPF 30 let me stay in the sun 30 times longer?

No, and the FDA calls this a misconception: SPF is not directly related to the time spent in the sun but to the amount of solar exposure. SPF is the ratio of the sun dose needed to cause a light sunburn with sunscreen to the dose without it, measured in a lab at 2 mg/cm². Real sunlight changes through the day, and real application is thinner and uneven, so a time multiplied by SPF would be false precision. This calculator therefore does not predict a time to burn.

What does SPF 30 or SPF 50 actually block?

SPF is about the fraction of the sunburn-causing UV that gets through: about 1/SPF when the product is applied at the test thickness. SPF 15 lets 6.7 percent through, SPF 30 about 3.3 percent, and SPF 50 2 percent. The FDA states that SPF values mainly indicate UVB protection, so look for a broad-spectrum label to cover UVA as well. The gain from 30 to 50 is small compared with the gain from applying the full amount and reapplying.

What is the UV Index?

It is a forecast of the strength of sunburn-causing UV radiation. The National Weather Service model weights the UV at each wavelength by the McKinlay-Diffey erythema action spectrum, adds it up, divides by 25, and rounds. In physical terms, a UV Index of 1 is 25 milliwatts per square meter of sunburn-weighted UV; at UVI 8 that is 200 mW/m², or 720 joules per square meter in one hour of midday sun.

What does the EPA advise at each UV Index?

At a UV Index of 1 to 2 no protection is needed for most people. At 3 to 7 the EPA advises seeking shade from late morning to mid-afternoon, using broad-spectrum SPF 15 or higher sunscreen, and wearing protective clothing, a wide-brimmed hat and sunglasses. At 8 and above extra protection is needed. Its shadow rule is also a quick check: if your shadow is shorter than you are, UV exposure is high.

Why did you not include a time to sunburn?

A reliable burn time needs the minimal erythema dose (MED) of the person, and the published ranges by skin type disagree between sources and come from lab lamps rather than sunlight. We found no official table from the EPA, FDA or WHO that we could cite, and a made-up number is worse than none. If you want a rule, use the UV Index guidance above and watch your own skin: pink skin means you have already had too much.

For general information only — not medical advice. Sources: U.S. Food and Drug Administration, "Sunscreen: How to Help Protect Your Skin from the Sun" (one ounce for the body, reapply at least every two hours, SPF 15 or higher, apply 15 minutes before, SPF is not directly related to time of exposure) and 21 CFR 201.327 / the SPF test method (2 mg/cm²); U.S. EPA, "UV Index Scale" and "Calculating the UV Index" (McKinlay-Diffey weighting, division by 25, the Shadow Rule); Mosteller RD, "Simplified calculation of body-surface area", N Engl J Med 1987. Read October 27, 2026. Related: body surface area calculator, sun angle calculator, heat index calculator. How we check our calculators.