SPF calculation formula
Here is how the SPF number is calculated:
Step 1:
Volunteers are asked to expose a portion of their bare skin to a UV lamp in a lab. Bare skin means no sunscreen is applied. The time it takes for the bare skin to burn under the UV lamp is noted.
Step 2:
Next, sunscreen is applied to the same volunteers, and the sunscreen-covered skin is exposed to the UV lamp. The time it takes for the skin to burn is noted.
Step 3:
The formula for calculating SPF is:
Time for sunscreen-covered skin to burn divided by Time for bare skin to burn
Example:
Let’s say…
Time for sunscreen-covered skin to burn = 200 minutes
Time for bare skin to burn = 10 minutes
So SPF = 200 / 10 = 20
SPF of the sunscreen used in this example is 20
So what does SPF measure…
Based on the calculation formula, would we be right in saying SPF measures how long the effect of that particular sunscreen lasts on the skin?
For the example used above, would we be right to say SPF measures how long it takes for sunscreen-covered skin to burn ~ 200 minutes?
For this example, would we be right to say the SPF number tells us we need to reapply the sunscreen above every 200 minutes?
Yes, we would be right – but that measure is not helpful to consumers.
When it comes to understanding what SPF number actually measures, ignore the formula. I will explain a bit further down what it actually indicates…
Here are two reasons why looking at SPF numbers, its formula and saying SPF measures how long the sunscreen’s effect will last is not helpful to everyday consumers:
Reason 1:
The UV lamp used in the lab to determine a sunscreen’s SPF number has constant UV radiation. But it is not the same in real life. The sun’s UV radiation is not constant like the UV lamp’s—it keeps changing based on the hour of the day and your location.
Reason 2:
Everyone’s bare skin burns at different times – some may last only 5 minutes, some may last 7 minutes, and some may last 10 minutes. Not many can last more than 10 minutes. So, how can we standardise how long SPF 20 lasts based on one volunteer’s time?
So what’s the conclusion so far…
SPF number though theoretically, is a measure of how long a sunscreen’s effect will last and when to reapply that sunscreen; this measure is not helpful to everyday consumers because of the two reasons explained in the previous paragraph.
So, what does SPF actually measure…
Regarding when an SPF should be reapplied, as advised by medical professionals, it is an industry-standard practice to reapply sunscreen at least every 2 hours, irrespective of the SPF number.
Reapply before 2 hours to be safe, especially on a hot day.
Consumers should not look at an SPF number to indicate how long the sunscreen lasts or when they should reapply it. Every sunscreen, irrespective of its number, should be reapplied at least every two hours.
Why two hours? Because that’s the standard set by medical professionals. At least every two hours (sooner than two hours if you want to be safe), irrespective of the SPF number.
So, what is an SPF indicator for everyday consumers?
Consumers should look at SPF numbers to understand how much UVB radiation it blocks.
Based on regulated testing, it is widely accepted that this is how much (below table) each of these SPF numbers shields skin from sun’s radiation:
| SPF 15 | blocks nearly 93% of UVB rays from affecting skin |
| SPF 30 | blocks nearly 97% of UVB rays from affecting the skin |
| SPF 50 | blocks nearly 98% of UVB rays from affecting the skin |
| SPF 100 | blocks nearly 99% of UVB rays from affecting the skin |
The percentages in the above table add a different perspective when viewed from a different angle:
| SPF 15 | Lets 7% of the sun’s UVB radiation into the skin. 7% of the sun’s UVB radiation reaches the skin and affects our skin. |
| SPF 30 | Lets 3% of the sun’s UVB radiation into the skin. 3% of the sun’s UVB radiation reaches the skin and affects our skin. |
| SPF 50 | Lets 2% of the sun’s UVB radiation into the skin. 2% of the sun’s UVB radiation reaches the skin and affects our skin. |
| SPF 100 | Lets 1% of the sun’s UVB radiation into the skin. 1% of the sun’s UVB radiation reaches the skin and affects our skin. |
The above table shows little difference in the protection SPF 30 and SPF 50 offer.
But when you view the figures in the above table from a different angle, it looks like this:
| SPF 50 | 2% of the Sun’s UVB radiation gets into the skin and affects the skin. |
| SPF 30 | 3% of the Sun’s UVB radiation gets into the skin and affects the skin. SPF 30 offers 50% less protection than SPF 50 (lets in 3% Versus lets in 2%) |
| SPF 100 | 1% of the Sun’s UVB radiation gets into the skin and affects the skin. SPF 100 offers 100% more protection than SPF 50 (lets in 1% Versus lets in 2%) SPF 100 offers 200% more protection than SPF 30 (lets in 1% Versus lets in 3%) |
Summary of what you should actually use SPF number for
Use the SPF number to indicate how much of the sun’s UVB radiation it can block (refer to the first table above).
The SPF number indicates how much of the sun’s UVB radiation can enter our skin (refer to the second and third tables above).
Reapply sunscreen, irrespective of its SPF number, every two hours.
If you use sunscreen with an SPF number of less than 30, please reconsider. A sunscreen with an SPF of 30 or above is recommended and safe.
A sunscreen with an SPF of less than 30, reapplied every 2 hours, is far less effective than a sunscreen with an SPF of 30 reapplied every 2 hours.