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A full adult body needs 30-40ml per application (six-plus teaspoons): most people apply a quarter to half that, which quietly turns SPF30 into SPF10 or worse. Reapply every 2 hours and after swimming or towelling, whatever "water resistant" claims. NHS advice: SPF30+, UVA 4-5 stars, and the shade-hat-shirt trio doing the work sunscreen shouldn't do alone.
What The Sunscreen Calculator Works Out
- Total sunscreen: people x days x applications x 35 ml. The SPF on the label was measured at 2 mg per square centimetre of skin, which is 30 to 40 ml (six-plus teaspoons, or a shot glass) for an adult body. Real-world users apply a quarter to a half of that, which collapses SPF30 toward single digits; under-dosing, not product failure, is behind the classic "I wore factor 30 and still burned" holiday mystery.
- Bottles to pack: the total divided into 200 ml bottles, rounded up. Two adults, a week, two applications a day is nearly a litre: five bottles.
- Reality check: the single family bottle that comes home half-full is the audit that convicts almost everyone.
- Skin exposed (advanced): scales the adult dose for how much skin is out: about 25 ml in shorts and a vest, 15 ml in shorts and a T-shirt, and 8 ml for face, neck and hands on a city break or a British spring day. A dry T-shirt outperforms most applied SPF, which is why clothing carries the primary load in NHS advice.
- Children (advanced): counts some of the group as children at a smaller dose (about 20 ml for a primary-age child in swimwear). Rash vests and hats solve much of the problem for children entirely; babies under six months should be kept out of direct sun.
- Bottle size and price (advanced): cabin-bag 100 ml tubes through to 400 ml family pumps, with the total cost and the price per 100 ml for comparing brands. Supermarket own-label SPF30 performs as well as premium brands in consumer tests; check for SPF30-plus and a 4 or 5-star UVA rating or the circled UVA logo.
- Spare margin (advanced): adds a percentage for the inevitable extra application after a swim or a towelling. Reapply every two hours and after water regardless of "water resistant" claims, and treat "once a day" formulations with the same scepticism.
- Per application and per bottle (advanced): the adult dose in teaspoons and how many applications a bottle really holds, so you can see how quickly a tube should run out. Apply 15 to 30 minutes before exposure for chemical filters, and remember the ears-feet-lips-parting checklist of the commonly missed.
- Caveats: sunscreen is the third line, not the first: shade in the 11am to 3pm window and clothing do the main work, with sunscreen covering what they cannot. Check the open-jar symbol (12M means last summer's bottle is a coin-flip), remember UK UV burns through cloud from April, take vitamin D as a supplement rather than from burning, and get any new, changing or odd-looking mole checked early.
Guidance: NHS: sunscreen and sun safety.