how long does spf 50 last

How Long Does SPF 50 Last?

By SkinCabin

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.

Consider SPF 50...Let's try to understand how long SPF 50 will last...

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 = 50
  • The time it takes 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:
50 = time for sunscreen covered skin to burn / 15
time for sunscreen covered skin to burn = 50 * 15 = 750 minutes

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

It can't be possible that SPF 50 lasts for 750 minutes irrespective of the time at which you apply, right? The intensity of the sun's rays is different throughout the day. If the intensity is low, the sunscreen lasts for a lot longer than it lasts when the intensity is very high.

And so, these 750 minutes have to be adjusted for UV index.

Let's say, UV index at 11 am is 9. This is how we adjust the 750 minutes:
time for sunscreen covered skin to burn (how long the SPF 50 sunscreen lasts) = 750 / 9 = 83 minutes = 1 hour 15 minutes

Let's say, UV index at 3 pm is 4
time for sunscreen covered skin to burn (how long the spf 50 sunscreen lasts) = 750 / 4 = 188 minutes = 3 hours

So for the example above, at 11 am SPF 50 lasts for 1 hour and 15 minutes. At 3 pm SPF 50 lasts for 3 hours.

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

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

UV index is usually very high in the middle part of the day (between 10 am and 2 pm).