How Is SPF Number Calculated?

Sun Protection Factor, commonly called SPF, is a measure of how much longer it takes for protected skin to burn from UVB rays compared to how much longer it takes for unprotected skin to burn from UVB.

Protected skin is the skin covered with the sunscreen we are determining the SPF number for. Unprotected skin has no sunscreen on it. It is bare skin.

SPF number indicates how well the sunscreen protects our skin against sunburn. Sunburn is caused by Sun's UVB radiation.

How is SPF number calculated?

1. Human volunteers are exposed to light from a UV lamp with no sunscreen applied to their skin.  Time taken for their bare skin to burn is noted.
2. Human volunteers are now exposed to light from a UV lamp with 2mg of sunscreen applied to a square centimetre of their skin. Time taken for their sunscreen-covered skin to burn is noted.

SPF number = Time taken for sunscreen-covered skin to burn / Time taken for bare skin to burn

Example:
If it takes 10 minutes to burn without sunscreen and 200 minutes to burn with sunscreen then SPF number of that sunscreen is = 200 / 10 = 20

Word of caution:
After how minutes you should reapply your sunscreen is not the same as the SPF number. After how many minutes you should reapply can be less or slightly more than the SPF number. The reapplication time of sunscreen depends on other factors.