So, what does that mean?
That SPF number represents the length of time you can stay in the sun, without burning.
For example: SPF 15
A person who would normally start to burn in 10 minutes (without sunscreen) could theoretically have 150 minutes of sun-time with a SPF 15 sunscreen.
That number on the bottle is only telling you one thing: how long it will offer you protection from the sun (this can range anywhere from 1-5 hours).
Please remember that the strength of SPF you choose is less important than frequency of application! Especially if you’re out in the sun for prolonged periods of time, sweating, in contact with water, etc.
Also, The SPF number only relates to ultraviolet B (UVB) protection. It does not measure ultraviolet A (UVA) protection.