How Much Sunscreen Should You Use? Free SPF Amount Calculator

Here's the skincare secret nobody puts on the bottle: the SPF number only works if you use enough. Most of us apply way less than the amount used in lab tests, which means that SPF 50 might be behaving more like SPF 15 on your skin.

This sunscreen amount calculator works out exactly how much you need for your face, body or both, how many teaspoons or finger strips that is, and how long your bottle will really last.

Why the amount of sunscreen matters so much

Every SPF number on a label comes from a lab test where sunscreen is applied at 2 milligrams per square centimeter of skin. That's a thick, even layer. In real life, studies have found that most people apply only a quarter to half of that. Because protection doesn't drop in a straight line, using half the amount can leave you with a fraction of the SPF you paid for.

The good news: you don't need a stronger sunscreen. You just need more of the one you already love.

How the sunscreen calculator works

1. Body surface area: your height and weight go into the Mosteller formula, the same one doctors use to estimate skin area.
2. Area percentages: each body zone is a share of that total. Your face is roughly 3 to 4%, each arm about 9% and your legs together about a third.
3. The SPF test amount: 2 mg for every square centimeter you're covering.

For an average adult, that's around 1.2 ml for the face alone and about 30 ml, roughly a shot glass, for the whole body. That matches the advice from dermatologists' associations to use about one ounce for full-body coverage.

Easy ways to measure without a spoon

• Two-finger rule: squeeze a line of sunscreen along the full length of your index and middle fingers. That's about right for your face and neck.
• Quarter teaspoon: a little over 1 ml covers an average face.
• Shot glass: about 30 ml is enough for your whole body.
• Apply in two layers if one big dollop feels heavy. It's easier to spread and gives more even coverage.

Reapplication: the step everyone skips

Sunscreen breaks down in sunlight and rubs off with sweat, towels and touching your face. The common guidance is to reapply at least every two hours when you're outside, and right after swimming or heavy sweating. Water-resistant formulas are tested for 40 or 80 minutes in water, not all day.

Wearing makeup? A sunscreen stick, a mist or an SPF powder makes touch-ups easier, but they're best as top-ups, not your main layer.

SPF, PA and broad spectrum, explained simply

• SPF measures protection from UVB, the rays that burn.
• Broad spectrum means the formula also protects against UVA, the rays linked to dark spots and skin aging.
• PA ratings (PA+ to PA++++), common on Japanese and Korean sunscreens, show the level of UVA protection. More plus signs, more protection.

UVA also passes through windows, which is why daily SPF is part of our 7 glow-up habits that make your skin look expensive even if you work indoors.

Make sunscreen part of your glow routine

SPF is the last step of your morning skincare and the step that protects every serum you use at night. Pair it with a simple night-time routine, and if you want a full reset, start the 30-day glow-up checklist. Day 4 is all about sunscreen.

Sunscreen FAQ

Is SPF in my foundation enough?

Usually not. To reach the SPF on the label you'd need to apply several times more foundation than anyone wears. Use a separate sunscreen underneath.

Do I need sunscreen on cloudy days?

Yes. A large share of UV rays pass through clouds, and UVA levels stay fairly steady all year.

Why does my bottle run out so fast now?

Because you're finally using the right amount. A 50 ml facial sunscreen used correctly once a day lasts about four to six weeks, not six months.