1. Franz Greiter: The Student Who Created SPF After a Sunburn in the Alps
In 1938, a chemistry student named Franz Greiter was said to have suffered severe sunburn while climbing in the Swiss Alps.[3] He later founded a company named Piz Buin after the mountain he climbed. Their first product, called "Glacier Cream," was released in 1946 and had an SPF of 2 from the start.[3]
While this number is small compared to today’s products, it introduced the idea of expressing strength with a single number. The concept of SPF itself is thought to have been developed by Greiter around 1962 and improved in 1974.[8][7] Because sources differ on exact years, it is safer to treat the year as approximate. But what does this number actually mean? Can we say that SPF 30 means you can stay outside 30 times longer? We will check this in the next section.
2. Does SPF 30 Mean You Can Stay in the Sun 30 Times Longer?
The answer is no. According to the Kanagawa Prefectural Institute of Public Health and Environmental Science, SPF is an index for preventing erythema (sunburn) caused by UVB rays.[1] The U.S. Food and Drug Administration (FDA) states this even more clearly: SPF relates to the amount of UV radiation needed to cause sunburn on protected skin, not directly to the time spent in sunlight.[2]
You cannot express protection simply in minutes because the amount of UV rays changes depending on whether it is noon in summer or evening in winter. While you may see calculations like "multiply SPF by 20 minutes," this is not an official definition and does not match the FDA’s explanation.[2]
Instead, think of the number as a guide to the fraction of UV rays that reach your skin. If you apply the test amount correctly, SPF 15 means the amount of UV radiation needed to cause sunburn is reduced to about one-fifteenth.[2] The FDA describes it as the ratio of the amount of UV needed to burn protected skin versus unprotected skin. So, what does this "one-fifteenth" look like when converted into a percentage of blocked rays?
3. How Much More UV Does Sunscreen Block When the SPF Number Goes Up?
The percentage of UV rays blocked can be calculated using the formula: 1 minus (1 divided by SPF). For SPF 2, this is 50%. For SPF 15, it is about 93.3%. SPF 30 blocks about 96.7%, and SPF 50 blocks 98%.[7] These are calculated values based on applying the standard test amount; actual protection may be lower depending on how you use it.
Even though the number increases from 15 to 50 (about 3 times), the blocked percentage only rises from about 93.3% to 98%, a difference of less than 5 points.[7] Comparing SPF 30 and SPF 50, the difference is just 1.3 points.[1] Therefore, saying "SPF 50 is twice as strong as SPF 30" is not supported by this calculation.
However, looking at it from another angle changes the impression. If you look at the amount of UV rays that reach your skin, SPF 30 lets about 3.3% through (1/30), while SPF 50 lets 2% through (1/50).[7] This means the amount reaching your skin with SPF 50 is 60% of the amount with SPF 30, or about 40% less. It is good to read both perspectives: "the difference in blocked percentage is small, but the difference in rays reaching the skin remains."
This calculation assumes you apply the exact test amount. But does your everyday application produce these same numbers?
4. Does the SPF on the Bottle Hold Up When You Apply Sunscreen Normally?
In SPF testing, the amount of sunscreen applied is strictly defined. The Japan Cosmetic Industry Association (JCIA) standard requires weighing 2.00 mg/cm² of the sample before spreading it on the skin (with a margin of error of ±2.5%).[4]
A study using Asian skin found that applying 2.0 mg/cm² resulted in the labeled SPF value. However, when less was applied, the SPF dropped. At the amount people typically use—about 0.5 mg/cm²—it was difficult to predict the resulting SPF.[5] It is said that people actually apply only about one-quarter of the test amount.[5]
This means the number on the container reflects what happens when you apply the full test amount. If you apply less, you cannot expect the labeled level of protection. Also, because sunscreen can rub off due to contact with hands or clothes and sweat, the Kanagawa Institute advises reapplying as needed.[1]
Does this mean looking at the SPF number alone tells you everything about a sunscreen?
5. Does SPF Tell You Everything About How Well a Sunscreen Protects?
SPF indicates protection against sunburn, which is related to UVB.[1] Protection against UVA rays is indicated by a separate index called PA.[1]
In Japan, SPF labels go up to "50+". The "+" means "greater than 50," and no number higher than this appears on the label. Even if a product has an SPF over 50, it is labeled as 50+.[1]
The labeling of SPF and PA in Japan is not based on national law but on voluntary industry standards set by the JCIA. The testing methods also follow these industry standards. In 2013, the label "PA++++" was introduced.[6]
There are several hidden limits to these numbers. Rather than choosing a sunscreen based on one number alone, considering how much you apply and how often you reapply is closer to real-world use.
6. Try It at Home: Calculate How Much UV Reaches Your Skin With Your Sunscreen
If you have sunscreen at home, look for the SPF and PA labels on the container. You can do a simple calculation with the numbers you find. Divide 1 by the SPF to find the fraction of rays that reach your skin. For example, if the SPF is 50, then 1 ÷ 50 = 0.02, which is 2%.[7] Subtracting this value from 1 gives you the calculated percentage of blocked rays.
If you have multiple bottles, compare their numbers and calculations. You can use a calculator to verify that even if the SPF number doubles, the percentage of blocked rays changes very little. On the back of the container, you should find instructions on how to apply it and when to reapply.[1] Reading these while thinking about the difference between the test amount and what you usually use will help you understand the meaning of the numbers better.
If you want to learn more about the tests and standards behind SPF, the Kanagawa Institute’s "Eiken News" summarizes this in Japanese.[1] You can also research how the Glacier Cream’s numbers changed over time or how Greiter’s products displayed SPF labels.