Sunscreens and moisturizers with SPF are tested at a standardized density. That density is 2 milligrams of sunscreen per square centimeter of skin. If we want protection closer to what’s on the label, we should be using sunscreen or moisturizers with SPF at the density they’re tested at too.
Most of us don’t know the surface area of our skin and most of us don’t know the density of our sunscreens either. This has led to techniques and recommendations, like using 2 or 3 finger lengths of sunscreen, using 1/4 teaspoon of sunscreen, or applying our sunscreen twice. These techniques are all meant to encourage a more generous application of sunscreen, because when unprompted people tend to not apply enough.
With sunscreen, I think it is better to err on the side of applying too much rather than not enough. A higher density and thicker film of sunscreen generally means higher protection. It also makes sense to use more, because some of the sunscreen we apply will remain on our fingers, palms, or tools.
In some cases, I think these techniques might be leading people to use much more sunscreen than they might need. A sunscreen that might have been acceptable at a lower density might leave a strong cast, be too greasy, or pill (when a formula adheres to itself and rubs off the skin). This is especially relevant for people with deeper skin tones evaluating sunscreens for cast.
I’ve made a rough estimate of the surface area of my face, as well as the density (grams per millilitre) of two different sunscreens (IGTV: ‘How much sunscreen do you need?’). One is a cream and the other is a free-flowing milky texture. For both sunscreens, I need about 0.8 millilitres (mL) of sunscreen to get about the 2 milligrams of sunscreen per square centimeter of skin for my face. If I used a 1/4 teaspoon (1.23 mL), I’d be dispensing about half more than I need to protect my face.
With the cream sunscreen, applying 3 finger lengths of sunscreen dispensed about 3 mL of sunscreen. That’s almost 4 times more sunscreen than I might need. With the milk sunscreen, applying 3 finger lengths of sunscreen dispensed about 1.5 mL of sunscreen. That’s about 2 times more sunscreen than I might need. There’s also going to be differences in the thickness of our ‘lines’. That depends on things like how hard we squeeze, how slow we dispense, and the packaging.
If I apply the milky sunscreen with my palms and fingers the amount that ends up on my face might be close to the density I need. With 3 fingerlengths of the cream sunscreen, the finish is greasy and leaves a strong cast. The sunscreen is unusable for me with this much. But If I apply around 0.8 mL of the cream sunscreen, the finish is much nicer and I don’t notice a cast.
There’s no right or wrong method, they’re all just recommendations to encourage “proper” sunscreen use. Measuring your skin’s surface area, sunscreen density, and then each dose is going to be impractical for most people. But we might think about taking another look at sunscreens that may have been overused and left an unusable finish. They might not have at a density closer to the one it was tested at.
One of the ways we can encourage use of sunscreen is modeling realistic use. Many people can’t stand wearing heavier coats of sunscreen, and that’s OK. Some sun protection is better that none.
On the opposite end, brands need to take responsibility for their marketing, and show actual and proper use – especially since they’re the ones testing it at the right density. A smidge might look nice in an advertisment, but we all know it’s not enough and misleading. In the US, sunscreens are drugs not cosmetics – brands need to respect that and stop playing around.
Cheers to @FiddySnails for popularizing the 2 and 3 finger methods. They work for her because her 3 finger-lengths are not as chonky as mine. As well, when using 3 finger-lengths she uses a cushion puff to apply – which is absorbing some of the sunscreen.
If they made 1/6th teaspoons, I’d be golden. You don’t need to overthink this like I did! Our first application of sunscreen doesn’t need to be “perfect” if we reapply throughout a very UV-exposed day. All these techniques, besides precise measurements, are just estimates or rules of thumb – so some common sense and adjustments are sometimes needed!