Contents
Dose, SPF and reapplication
Sunscreen is tested at 2 mg per square centimetre of exposed skin; a whole body needs about 35 mL, roughly six teaspoonfuls. Higher SPF does not buy you extra time on the skin, so the clock starts at reapplication, not at the number on the bottle.
Use 2 mg per square centimetre of exposed skin, which comes to about 35 mL for a whole body.
The dose used in FDA sunscreen testing is 2 mg per square centimetre of exposed skin. Every SPF number printed on a bottle is measured at that dose, so the label value only holds if you apply the same amount.
Covering the whole body needs about 35 mL, roughly one ounce or six teaspoonfuls of product. That is far more than most people pour out, and a thin film drops the protection below the printed SPF.
There is an exponential relation between SPF and the amount of sunscreen applied in vivo, so under-dosing does not cut protection in half — it cuts it much harder than a linear reading would suggest.
SPF 15 means 1/15 of the burning radiation reaches the skin at the tested dose.
SPF 15 means 1/15 of the burning radiation reaches the skin when sunscreen is applied evenly at a dose of 2 mg per square centimetre. The number is a fraction, not a strength rating you can stretch by applying less.
Higher SPF products do not stay effective longer on the skin than lower SPF ones. The number describes how much burning radiation gets through at the tested dose, not how many hours of protection you have bought.
Because the tested dose is fixed at 2 mg per square centimetre, a half-dose application moves you to a different, untested point on the curve.
Reapply about every two hours, and immediately after swimming or heavy sweating.
Higher SPF products do not stay effective longer on the skin than lower SPF ones and must be reapplied about every two hours. The two-hour clock applies whether the bottle says 15 or 50.
Sunscreen should be applied thoroughly and reapplied during the day, especially after being in the water. A fresh coat restores the film that water has washed away.
The dose on the second application counts too: the same 2 mg per square centimetre of exposed skin, not a token swipe.
Higher SPF products do not stay effective longer on the skin than lower SPF ones — the number changes the fraction, not the clock.
| SPF on the label | Burning radiation reaching the skin | Reapplication interval |
|---|---|---|
| SPF 15 | 1/15 of the burning radiation | About every 2 hours |
| SPF 30 | 1/30 of the burning radiation | About every 2 hours |
| SPF 50 | 1/50 of the burning radiation | About every 2 hours |
| Whole-body dose | 35 mL at 2 mg per cm² | Re-measure for each coat |
| Tested dose | 2 mg per square centimetre | Fixed by FDA testing practice |
| After water | Film washed away | Reapply immediately |
| Thin application | Exponential drop in SPF | Not a valid substitute for dose |
A thin film drops the tested SPF and shortens the time before burning.
There is an exponential relation between SPF and the amount of sunscreen applied in vivo, so a thin film loses protection faster than the dose falls. Half the dose does not mean half the SPF.
Because higher SPF products do not stay effective longer on the skin than lower SPF ones, a thin coat of a high-number product can perform worse than a proper dose of a lower-number one.
Uneven spreading leaves bare patches that receive no protection at all, and those patches burn first.
Confirm dose, coverage, and the reapplication alarm before you step into the sun.
Pour out the measured amount first: about 35 mL for a whole body, or the proportional share for the area you are covering.
Work the product over every exposed area until the film is even, then wait before dressing so the sunscreen sets on the skin.
Set a reminder for about two hours and plan where the bottle will be after swimming, so the extra coat is not skipped.
Figures on this page trace to FDA testing practice, published studies, and independent laboratory testing.