When it comes to SPF calculation, there are two aspects:
Number one: How to calculate how much protection an SPF number offers
I will use SPF 15 as my first example to calculate the protection it offers.
Step 1
100 divided by 15 = 6.67
Step 2
100 – 6.67 = 93.33
I will approximate this number to 93.
Conclusion:
SPF 15 offers protection from 93% sun’s UVB rays.
This can also be said as SPF 15 blocks 93% of the sun’s UVB rays from affecting our skin.
An important conclusion to remember based on the above calculation is that SPF 15 allows 7% of the sun’s UVB radiation to affect the skin.
7% because 100 – 93 = 7
Now, I will use SPF 30 as my second example to calculate the protection it offers.
Step 1
100 divided by 30 = 3.33
Step 2
100 – 3.33 = 96.67
I will approximate this number to 97.
Conclusion:
SPF 30 offers protection from 97% sun’s UVB rays.
This can also be said as SPF 30 blocks 97% of the sun’s UVB rays from affecting our skin.
An important conclusion to remember based on the above calculation is that SPF 30 allows 3% of the sun’s UVB radiation to affect the skin.
3% because 100 – 97 = 3
| SPF | How much protection it offers | How much it allows | Comparison |
|---|---|---|---|
| 15 | 100 / 15 = 6.67 100 – 6.67 = 93.33 (approximately 93) It offers protection from 93% of the sun’s UVB rays that hit our skin. | 100 – 93 = 7 It allows 7% of the sun’s UVB rays to hit and affect our skin. | |
| 30 | 100 / 30 = 3.33 100 – 3.33 = 96.67 (approximately 97) It offers protection from 97% of the sun’s UVB rays that hit our skin. | 100 – 97 = 3 It allows 3% of the sun’s UVB rays to hit and affect our skin. | SPF 30 allows 3%, and SPF 15 allows 7%, 4% less. Though it’s only 4% less, it greatly affects our skin. SPF 30 offers 133% more protection than SPF 15. |
| 50 | 100 / 50 = 2 100 – 2 = 98 It offers protection from 98% of the sun’s UVB rays that hit our skin. | 100 – 98 = 2 It allows 2% of the sun’s UVB rays to hit and affect our skin. | SPF 50 allows 2%, and SPF 30 allows 3%, 1% less. Though it’s only 1% less, it greatly affects our skin. SPF 50 offers 50% more protection than SPF 30. SPF 50 offers 250% more protection than SPF 15. |
Number two: How do the manufacturers arrive at the SPF number of a sunscreen
We, as consumers, will not be very interested in this calculation. But still, if you would like to find out more, I will explain the manufacturers’ method in this section.
Manufacturers have a SPF number in mind before putting the sunscreen formulation together. Once they put it together, they need to test if the sunscreen they have put together comes with the SPF number they were hoping for.
I have outlined the manufacturers’ calculation for testing SPF numbers in 3 easy steps.
Step 1
Bare skin
Human volunteers expose a portion of their bare skin to a UV lamp. Bare skin has no sunscreen applied to it.
The time it takes for this bare skin to burn is noted.
Step 2
Sunscreen-covered skin
Next, the volunteers expose a portion of their sunscreen-covered skin to a UV lamp.
The time it takes for sunscreen-covered skin to burn is noted.
Step 3
How to calculate the SPF number?
To find the SPF number of the sunscreen:
The time it takes for sunscreen-covered skin to burn is divided by The time it takes for bare skin to burn
Recap of the SPF formula
The time it takes for bare skin to burn
divided by
The time it takes for sunscreen-covered skin to burn
Example 1
The time it takes for sunscreen-covered skin to burn under a UV lamp = 200 minutes
The time it takes for bare skin without sunscreen to burn under a UV lamp = 10 minutes
SPF number of the sunscreen used here is = 200/10 = 20
Example 2
The time it takes for sunscreen-covered skin to burn under a UV lamp = 150 minutes
The time it takes for bare skin without sunscreen to burn under a UV lamp = 5 minutes
SPF number of the sunscreen used here is = 150/5 = 30
Example 3
The time it takes for sunscreen-covered skin to burn under a UV lamp = 140 minutes
The time it takes for bare skin without sunscreen to burn under a UV lamp = 7 minutes
SPF number of the sunscreen used here is = 140/7 = 20
I will cover some key facts about SPF numbers in the following sections.
If you are wondering what SPF stands for, it is the Sun Protection Factor.
Should we use the SPF number as a guide to tell us how long the sunscreen’s effect lasts on the skin?
No!!
Let me explain…
The SPF number tells us how much protection it offers from UVB rays. For example, SPF 30 protects us from 97% of the sun’s UVB rays, while SPF 50 protects us from 98%.
However, the intensity of these rays differs in different parts of the world. Again, in a particular region, the intensity is different at different times of the day. And finally, everyone’s skin’s resistance to the sun’s UV rays differs.
The SPF number tells us that sunscreen protects us from a certain percentage of UVB rays, 97% in the case of SPF 30 and 98% in the case of SPF 50, regardless of the intensity of UVB rays.
The SPF number does not indicate how long the sunscreen’s effect lasts on the skin. The sunscreen’s protection duration depends on the UV intensity in that region, which depends on its latitude, longitude, and time.
How long will the sunscreen’s effect last? Once its effect wears off, we need to reapply. Knowing when to reapply is important.SPF number is of no help in this area.
Medical professionals recommend that we should reapply sunscreen at least every two hours.
The SPF number indicates how vigorously sunscreen protects our skin during those two hours.
SPF 30 offers more protection than SPF 15 during those two hours.
SPF 50 offers more protection than SPF 30 during those two hours.
Regardless of the SPF number, sunscreen must be reapplied every two hours. Its protection wears off after two hours.
However, if it is a peak time for the sun’s rays and you live in a scorching area, reapply slightly below two hours if possible.
Does sunscreen protect us from all the sun’s rays?
One important thing to be aware of is that the SPF number only indicates the sunscreen’s protection from the Sun’s UVB radiation. Two types of rays affect our skin: UVA and UVB. Both these types age our skin and can cause skin cancer.
The SPF number does not indicate sunscreen’s protection against UVA rays.
When you buy sunscreen, always look for the letters PA next to the SPF number. The letters PA will have one or more + signs next to them. PA stands for protection grade. The number of plus signs indicates how much sunscreen protects against UVA rays.
If PA is not mentioned on the sunscreen, don’t buy that sunscreen.
When it comes to sunscreen, don’t settle for less. You need a sunscreen that offers protection against both UVA and UVB. This type is called a ‘broad-spectrum sunscreen ‘. You should always buy this type of sunscreen, as it provides comprehensive protection against the two types of rays that can harm your skin.
Is high SPF pointless?
There are two reasons why sunscreens higher than SPF 50 are not recommended much…
Number one: The formulation of sunscreens above SPF 50 is undesirable. It does not feel good on the skin, and so many don’t prefer it.
Number two: This is a very important reason. Sunscreens above SPF 50 are misleading. Many incorrectly assume SPF 100 provides double the protection of SPF 50, so reapply it much less frequently. The reality is that the increase in protection of SPF 100 over SPF 50 is only 1%. It does not offer the double protection.
Unfortunately, many people fall prey to these misconceptions. Hence, in some countries, the regulatory boards don’t recommend sunscreen above SPF 50 or 60.
The best and safest route is to apply SPF 50 and reapply it regularly. If you can’t get SPF 50, use SPF 30 and reapply it regularly.