How Often to Reapply Sunscreen?

SPF number indicates how well the sunscreen protects our skin against sunburn. Sunburn is caused by Sun's UVB radiation.

Assume the following:
Let's say it takes 15 minutes for your skin to burn without sunscreen. You have SPF 30 sunscreen on.
Question: How much longer can you be out in Sun before your skin starts to burn?

The formula for SPF number is = Time taken for sunscreen-covered skin to burn / Time taken for bare skin to burn...
And so...
30 = time for sunscreen covered skin to burn / 15
time for sunscreen covered skin to burn = 30 * 15 = 450 minutes

So based on the above calculations, would it be correct to say you have to reapply sunscreen every 450 minutes?
No. In fact, nowhere close.

Sun's UV intensity changes throughout the day. The intensity is indicated by a number called UV index. We need to adjust this 450 minutes to take the UV index into account.

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

Let's say, UV index at 12 pm is 11
time for sunscreen covered skin to burn (reapplication time) = 450 / 11 = 40.9 minutes

So for the example above, at 10 am you have to reapply every 56 minutes. At 12 pm you have to reapply every 41 minutes.

But of course, if you go out at 10 am, the UV index will keep rising for some time as the day progresses, and so, in the above example, always best to reapply much sooner than 56 minutes.

How often should I reapply my sunscreen = SPF number (x) time for your bare skin to burn / UV index

How long does it take for my bare skin to burn?
Everyone is different - but it usually takes around 15 minutes for bare skin to burn.

Quick recap with another example: If the UV index is 9 at midday and if it takes 15 minutes for your bare skin to burn, you need to reapply SPF 30 every = 30 * 15 / 9 = 50 minutes.

What if you don't have access to the UV index?
The shadow rule helps. Not accurate but better than nothing. If your shadow is smaller than your height, you are in high UV index time. This means frequent reapplication of sunscreen is needed.