How to Calculate the SPF You Need

We all look forward to the time of year when the sun comes out and we can go outside and enjoy getting a tan or just relaxing out in the sunshine. However it is something of a double edged blade and with all the benefits of the sun come also some downsides. The most notable of these is of course that the sun can cause sun burn, and that spending too much time out in the sun can result in us damaging our skin and contributing potentially towards developing cancer. As such it is highly important that we use sun screen or sun block when we are enjoying the sun shine, and for this to be effective it is equally as important that we use the correct SPF.

Understanding SPF

SPF stands for ‘Sun Protection Factor’ and it corresponds to the amount of protection that we can get from a specific bottle of sun screen or sun block. In other words then, this number corresponds to the percentage of the sun’s rays that will be absorbed or blocked by that product.

SPF ranges from 2-60 and this corresponds to how much of the sun’s rays it absorbs or blocks. However it is important to recognise that the SPF is not on a fixed scale and is not relative which is to say that SPF 4 is not twice as strong as SPF 2. This makes it somewhat complicated to know precisely how much sun screen to use, but as such it is advisable whenever you are out for extended periods of times to use the highest possible to avoid skin damage.

As a rough guide it is important to always use at least SPF 30 if you are out for 20 minutes or more in the direct sun. Anything higher though will only provide you with more protection and will be better for you in the long run. In terms of percentages the following guide may be helpful:

  • SPF 2: 50% of the sun’s rays blocked.
  • SPF 15: 94% of the sun’s rays blocked.
  • SPF 30: 97% of the sun’s rays blocked.

Of course there are other factors to bear in mind, and that also come into play when deciding what SPF to use. Here we will address a few.

Makeup: Some makeup and other cosmetics such as moisturisers include some sun-screen or sun block in their composition. However this is generally no more than around SPF 15 meaning that it is not sufficient to keep the sun completely at bay. Find out what SPF your cosmetics are, and then make sure that you add a little sun block or sun screen to make the difference.

Weather: On an overcast day there is of course a layer of cloud between the sun and your skin which blocks out many of the sun’s rays. However again some will still get through so you should still wear a sun screen – though not necessarily as high.

Skin tone: While your skin tone will affect how easily you burn, that does not mean that you can afford to forego sun screen if you have dark skin. You might burn less but the cell damage done by the sun will not change making it absolutely paramount that you still use a good sun block. It is a common mistake for those with darker skin tones to believe that they do not need sunscreen or sun block and to this way do damage to their skin cells.

Water: Going in the water can wash off your sunscreen and render it useless meaning that you need to use more. As such, if you intend to spend your day swimming then you need to look for a sunscreen that says either ‘waterproof’ or ‘water-resistant’ on the bottle. If it says waterproof this will mean that it lasts underwater for 80 minutes or more, while if it says water-resistant this will mean that it lasts for 40 minutes. As such you should still also replace your sunscreen every hour when you are spending the day swimming.

Comments 1
  1. This article is entitled "How to Calculate the SPF You Need" but it doesn't tell me how to calculate the SPF I need, it provides general information. I want something relating to UV Index, fair to medium skin, time of day and exposure time. More specifically, 20 minutes at 1pm, medium skin when the UV index is 8. If you don't provide that then you are waffling and anyone can do that.

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