How Long Does SPF 30 Last?

Sunburn is caused by Sun's UVB radiation. Sun Protection Factor, commonly called SPF, tells us how long a sunscreen protects our skin from UVB radiation.

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

In our case:

• SPF number = 30
• Time taken for bare skin to burn = if you know you can input the value in minutes here - if you don't know, it usually takes around 15 minutes for bare skin to burn

Substitute the values in the formula:
30 = time for sunscreen covered skin to burn / 15
time for sunscreen covered skin to burn = 30 * 15 = 450 minutes

Please note that the time for sunscreen-covered skin to burn is the same as how long sunscreen lasts.

This 450 minutes has to now be adjusted for UV index.

Sun's UV intensity changes throughout the day. The UV index measures the sun's UV intensity.

Let's say, the UV index at 10 am is 8. This is how we adjust the 450 minutes:
time for sunscreen-covered skin to burn (how long the spf 30 sunscreen lasts) = 450 / 8 = 56.25 minutes

Let's say, the UV index at 12 pm is 10
time for sunscreen-covered skin to burn (how long the spf 30 sunscreen lasts) = 450 / 10 = 45 minutes

So for the example above, at 10 am you have to reapply every 56 minutes. At 12 pm you have to reapply every 45 minutes.

What if you don't have access to the UV index?
This approximate rule can help. If your shadow is smaller than your height, you are in high UV index time.
Very High UV index: 8 to 10
High UV index: 6 to 7
Moderate UV index: 3 to 5
Low UV index: 1 to 2

How long does sunscreen last = SPF number (x) time for your bare skin to burn / UV index

In high UV index time, SPF 30 lasts slightly less than one hour.

Moderate UV index:
Let's say, UV index is 7
how long the spf 30 sunscreen lasts = 450 / 7 = 64 minutes
In moderate UV index time, SPF 30 lasts slightly over an hour.