How much sunscreen is enough? The two-finger rule, checked

SPF is tested with a generous layer: 2 mg of product on every square centimetre of skin. Under natural conditions people apply about 0.4 to 1 mg per square centimetre, which cuts the real protection factor a lot¹. A 2002 letter in the BMJ put it more bluntly: consumers typically apply roughly a third of the tested dose².

Where the two-finger rule comes from

The two-finger idea comes from that BMJ letter by Taylor and Diffey. They proposed squeezing sunscreen along the length of your index and middle fingers for each body area, borrowing from the dermatology 'fingertip unit', as a way to get close to the tested dose². A separate 2002 paper by Schneider set out a 'teaspoon rule' for the whole body. We can only point to that paper for the fact that it exists: it has no abstract online⁴.

The American Academy of Dermatology gives a similar face amount: at least one teaspoon, about what fits along the length of your index and middle fingers³.

A routine that follows the evidence

1. Squeeze a line of sunscreen along your index and middle fingers. That is your face and neck.
2. Apply it about 15 minutes before you go out, as the last step of your skincare³. The Skin Cancer Foundation says 30 minutes⁵.
3. Reapply every two hours when outdoors, and straight after swimming or sweating³⁵.
4. Under makeup, dab or spray a second layer on top instead of skipping it.

Aim for a broad-spectrum sunscreen of SPF 30 or higher³. A higher SPF does not last longer: SPF measures the level of protection, not how long it lasts⁷. For the whole body, the NHS says most people do not apply enough and suggests around 6 to 8 teaspoons for an adult⁶.