Question: How much sunscreen do I need to apply?

Answer:

Not only does your sunscreen have to be photostable, you also have to use enough sunscreen for it to offer adequate protection.

I used to use just a tiny amount of sunscreen for my face because good sunscreens are expensive and I wanted the sunscreen to last as long as possible. Little did I know, not using enough sunscreen leaves you unprotected (or, not as protected) from the sun's harmful UV rays.

1/4" teaspoon of sunscreen

Here is a general guideline for how much sunscreen you should use on your face and body:

Face: 1/4 teaspoon of sunscreen

Neck (front and back): 1/4 teaspoon of sunscreen

Arms: 1/2 teaspoon of sunscreen per arm

Legs: 1 teaspoon of sunscreen per leg

Chest: 1 teaspoon of sunscreen

Back: 1 teaspoon of sunscreen

Sunscreens are tested for SPF and PPD at the application rate of 2 milligrams of sunscreen for every square centimeter of skin (2 mg/cm2). This amounts to roughly 1/4 teaspoon of sunscreen for the average sized face. Because the above amounts are a general approximation, if you have a larger or smaller face or body, you may have to use more or less sunscreen. Similarly if you have a thinner sunscreen, like powdered sunscreen or sunscreens with a lot of alcohol like sunscreen sprays, you may need to apply more.

You can physically measure 1/4 teaspoon with a measuring spoon, but I tend to just eyeball it. If it helps, 1/4 teaspoon is approximately the size of a nickel if your sunscreen is runny and flat. If your blob of sunscreen is thicker and rises higher, it will be about the size of a penny. To show you what 1/4 teaspoon of sunscreen looks like, I measured out the exact amount of a physical sunscreen (which is a bit thicker in texture) in the below picture:

1/4" teaspoon of sunscreen compared to a penny

This may seem like a lot of sunscreen to use, especially on the face, but if you don't wear enough sunscreen, it won't form a protective enough layer on your skin to guard effectively against UVA and UVB rays. My friend (who's studying to be a dermatologist) explained this in the best way using a paint analogy:

Let's say you want to paint your room blue. You need 1 gallon of blue paint to get the job done, but for whatever reason, you only use 1/2 gallon of paint. You end up with a thin and uneven coat of paint on your walls that is not the shade of blue you want because you didn't use enough paint. The same idea applies to using enough sunscreen, as illustrated in the following graphic (please forgive my non-existent PhotoShop skills):

A graphic illustrating why applying the right amount of sunscreen is important

From the above graphic, you can see that when you apply less than the recommended amount of sunscreen, you get a weaker and more uneven sunscreen coverage. You want to use the right amount of sunscreen so you get an even and non-patchy shield on your skin. When your sunscreen forms this uniform protective film, it can do its job and effectively protect against UV rays. If your sunscreen is applied too thinly or unevenly, you will get uneven and less than full sun protection.

The moral of the story is: if you want good sun protection, make sure you are wearing enough sunscreen. The right amount varies slightly per person, but generally 1/4 teaspoon is good for the face. When it doubt, err on the side of applying sunscreen more liberally than you otherwise would.

For more information, check here to make sure you are practicing other techniques of proper sunscreen application. Wearing it right definitely makes a difference!