SPF is not a one size fits all proposition.
Think of it this way — a very fair person, say a redhead, may burn without sun protection in as little as three minutes whereas sunburn on a darker skin might take as long as an hour or more to appear.
The SPF number on a sun protection product is actually part of a formula that tells you exactly how much time you, as an individual, can spend in the sun before re-application is required. In order to calculate that you must know how long it actually takes your skin (and your skin alone) to burn in an unprotected state.
Contrary to popular belief, a higher SPF does not necessarily provide more protection, but rather is an indicator as to the amount of time you can spend in the sun unprotected before reapplication is necessary. This varies based on individual sensitivity and skin tone.
Let’s do the math
If you are a redhead, who burns in three minutes, and are planning on being outside for a while, you may actually need an SPF 50. Using our previous formula of the number of minutes it takes for your skin to burn multiplied by the SPF rating (in this case, 3 x 50) your SPF 50 gives you the same duration of protection (150 minutes) as someone who burns in 10 minutes using an SPF 15.
On the other hand, if your skin starts to get pink (the first sign of an impending burn) at about the 10 minute mark an SPF 15 allows you to spend (10 x 15 =) 150 minutes in the sun before you need to dip back in your beach bag for your product and slather on another layer. Make sense?