Apply Sunscreen, Step by Step

An ordered routine: measure about 2 mg per square centimetre, spread evenly over exposed skin, and reapply about every two hours and after water.

Start by measuring the dose. The dose used in FDA sunscreen testing is 2 mg per square centimetre of exposed skin, and covering the whole body needs about 35 mL, roughly one ounce or six teaspoonfuls. Pour the amount out first so you can see whether you are near the target before you begin spreading.

Spread the product evenly over every exposed area until the film is even. Cover the ears, the neck and the backs of the hands, which are routinely missed, and check the hairline and the tops of the feet. Uneven spreading leaves bare patches that receive no protection at all, and those patches burn first.

Wait before dressing so the sunscreen sets on the skin rather than rubbing off on clothing. A thin application that drops the tested SPF is the most common failure: there is an exponential relation between SPF and the amount of sunscreen applied in vivo, so a half-dose loses far more than half the protection.

Reapply about every two hours of sun exposure, and reapply after being in the water even if the label says water resistant. Towelling off removes product as well, so a fresh coat follows drying. Use the same measured dose on the reapplication as on the first coat.

Higher SPF products do not stay effective longer on the skin than lower SPF ones, so the reapplication clock does not change with the number on the bottle. The routine above holds for SPF 15, SPF 30 and SPF 50 alike.

• Measure about 2 mg per square centimetre of exposed skin.
• Expect about 35 mL, roughly six teaspoonfuls, for the whole body.
• Spread evenly; cover ears, neck and hands.
• Wait before dressing so the film sets.
• Reapply about every two hours and after water, at the same dose.

Further reading

• What the SPF Number Means
• Reapplication, Water and Aerosol Warnings