SPF stands for sun protection factor and the number indicates the length of time that your skin’s natural sun protection is extended by. This means that you can stay in the sun for longer periods with your skin protected than you otherwise could without any sunscreen.
If you usually burn after 10 minutes, for example, a properly applied, effective SPF 15 would allow you to stay in the sun for fifteen times longer (10 minutes x 15 = 150 minutes).
If you have skin that burns after 20 minutes, this time length would change to 20 minutes x 15, which is 300 minutes. As you can see, the effective timescale of an SPF will differ depending on skin type.
The other important thing to note is that the SPF rating is only telling you how high the protection is against UVB (burning) rays. There is currently no rating for UVA but you can find out more in the ‘What are Broad-spectrum Sunscreens?’ section on how to identify sunscreens that contain UVA protection.
So here it gets a little bit confusing: SPF indicates the length of time that you’re protected, it’s not telling you the percentage that your protection offers. SPF 15 protects you (for 15 times longer than your usual sunburn rate) against 93% against UVB rays.
SPF 30 only protects your skin 4% more than SPF 15 (97% against UVB rays). This is often where people get confused. The higher factor does not mean that you are more protected, but that you’re protected for longer. This means that a high SPF does not give you a huge amount more protection than SPF 15, which is fine for daily usage.