The math behind
your routine.

Sunscreen only delivers the SPF on the bottle if you apply the same thickness used to test it in the lab: 2 milligrams per square centimetre of skin. That sounds abstract, so this calculator turns it into grams, millilitres, and household measures for your body and the exact areas you are about to cover.

How to use it

Enter your height and weight, pick a preset — Everyday (face, neck, ears, hands), Active (sleeveless and shorts), or Beach (everything exposed) — or tap individual areas. Set how many hours you will be outside and your bottle size. The readout gives you the grams for a single coat, the familiar "shot glass" and "teaspoon" equivalents, how many times to reapply, and roughly how many days your bottle will last at that rate.

The science

We size you up with the Mosteller formula, the body-surface-area equation clinicians use: BSA in square metres is the square root of your height in centimetres times your weight in kilograms, divided by 3,600. For an average adult that lands near 1.8 m². We then take the slice of that surface you are actually covering using the dermatology "rule of nines" (each arm is 9% of the body, each leg 18%, the front and back of the torso 18% each, the head and neck 9%), and multiply the exposed area by the 2 mg/cm² standard.

The result lines up with the rules of thumb you may have heard. A single full-body coat comes out to about a shot glass (roughly 30 mL), and face-and-neck alone is close to the two-finger rule — two strips of sunscreen squeezed along your index and middle fingers, about 1.2 grams. The difference is that this tool is tuned to your body and your outfit instead of a one-size-fits-all guess.

The mistake almost everyone makes

Surveys of real-world use find people apply only a quarter to a half of the tested amount. Under-applying does not cut your protection in proportion — it cuts it far more steeply, which is exactly what the next calculator is for. If you want picks that make applying enough actually pleasant, our guide to the best Asian sunscreens covers lightweight SPF 50 formulas with no white cast.

A worked example

Say you are 5'7" and 154 lb, heading out for a half-day hike in a t-shirt and shorts. Your body surface area is about 1.8 m². The exposed skin — face, neck, ears, both arms, hands, lower legs, and feet — is roughly 60% of that. One coat is about 22 grams, and over five hours you should reapply two or three times, so a single 50 mL tube is more than half gone in a day. That is normal, and it is why a bottle that "lasts all summer" is usually a sign of under-application, not efficiency.

SPF is only half the label

This tool sizes the amount; the SPF number itself only describes UVB, the burning rays. For the UVA rays that drive aging and contribute to skin cancer, look for broad spectrum on a Western label or the PA+ rating on a K-beauty or J-beauty one — PA++++ is the highest. The amount you apply still governs how much of that protection you actually receive, so the grams below matter regardless of which rating system your sunscreen uses.

Here is the uncomfortable part. The SPF number is earned at that 2 mg/cm² lab thickness. Apply less and your protection does not slide down gently — it collapses. This calculator shows the SPF you are really wearing based on how thickly you apply.

How to use it

Choose the SPF printed on your bottle, then drag the slider to how thickly you apply. Most people live around 0.5–0.75 mg/cm² — a thin, fast layer. Watch the "effective SPF" and the percentage of UV blocked update as you move it. The two bars make the gap obvious: what the label promises versus what your skin actually receives.

The science

In a 2007 study in the British Journal of Dermatology, Faurschou and Wulf measured how SPF changes with the amount applied and found the relationship is roughly exponential. In practice, effective SPF equals the labelled SPF raised to the power of (amount applied ÷ 2). Apply half the recommended amount and you get the square root of the number on the bottle; apply a quarter and you get the fourth root. An SPF 16 applied at 1 mg/cm² behaves like an SPF 4. An SPF 50 applied the way most people apply it can act more like an SPF 7.

The percentages come from a simpler relationship: an SPF blocks (1 − 1 ÷ SPF) of UVB. SPF 30 blocks about 96.7%, SPF 50 about 98%. Those look almost identical — and they are, if you apply enough. The danger is the reverse: a small drop in dose creates a large jump in the UV getting through.

What to do about it

• Apply more. Use the amount from Calculator 01 — it is more than feels natural.
• Reapply. Every two hours outdoors, and after swimming or sweating.
• Buy high, wear high. Starting at SPF 50 gives you headroom for the inevitable under-application.
• Layer realistically. A second pass a few minutes after the first is the easiest way to reach a protective thickness.

So why bother with SPF 50?

If SPF 30 and SPF 50 block almost the same amount of UVB when applied perfectly, why pay for the higher number? Because nobody applies perfectly. The extra labelled SPF is a buffer against the real world — the thin layer, the spot you missed, the two hours that became four. Starting higher means that even your under-applied reality still lands somewhere protective rather than somewhere closer to a moisturiser.

The two-hour rule, explained

Reapplying every two hours is not because the filters expire on a timer. It is because sunscreen physically leaves your skin — rubbed off by clothing, towels, and hands, broken down by light, and carried away by sweat and water. Each loss thins the layer, and you now know what thinning does to SPF. Reapplication is simply topping the layer back up to the thickness this whole page is built around.

Sticker price tells you almost nothing about value. A $30 serum you finish in a month can be more expensive than a $60 serum that lasts half a year. This calculator converts any product into the number that matters — cost per use — and warns you when a product will expire before you can finish it.

How to use it

Enter the size, the price, and how much you use each time. Not sure? Tap a guide: Pea (~0.25 mL, a retinoid dot), Pump (~0.5 mL, a typical serum or lotion), Dropper (~0.7 mL, a full essence pipette), or Face SPF (~1.2 mL, the two-finger dose). Add how many times a day you use it and the product's PAO. You will see cost per use, per day, per month, and per year, how long the bottle lasts, and roughly when it runs out.

What PAO means

That little open-jar icon with a number like "12M" is the Period After Opening — how many months a product stays safe and effective once you break the seal. If the calculator says your jar will take 16 months to finish but its PAO is 12M, you are going to throw money away, and the tool flags it. Big tubs of active ingredients and sunscreens are the usual offenders.

How to use the number

• Compare like for like. Rank competing products by cost per use, not by price.
• Right-size your purchase. If a product expires before you finish it, buy the smaller jar.
• Decant bulk buys. Keep most of a large product sealed and work from a small portion to slow oxidation — and store delicate actives cool, which is what a beauty fridge is for.
• Spot the bargains. A "luxury" cream used in pea-sized amounts can quietly beat a drugstore tub you slather on.

A worked comparison

Two vitamin C serums. The first is $30 for 30 mL; the second is $55 for 50 mL. The cheaper bottle looks like the obvious win. But used at one 0.5 mL pump in the morning, the $30 bottle gives you 60 uses — about $0.50 per day — while the $55 bottle gives you 100 uses, or $0.55 per day. Nearly identical, so now the tiebreaker is the formula and the PAO, not the price. Flip the dose to twice daily and the gap narrows further. Cost per use is the only honest way to compare them.