What does that SPF number on the front of the tube actually mean?

Many people think SPF 50 gives twice the protection of SPF 25 (DW I used to think this too before MOTHER SPF). But it's simply not true. This confusion comes from not fully understanding how SPF works.

SPF measures how well a sunscreen protects against UVB rays, which cause sunburn and contribute to skin cancer. The number refers to how long you can be in the sun without getting burned compared to not wearing sunscreen at all.

Quick tip: The SPF number does not measure protection from UVA rays, which cause skin aging. We’ll explain how to guard against UVA in our next post but for now let’s break down what SPF really means.

What Does SPF Actually Stand For?

SPF stands for Sun Protection Factor, it tells you how long you can stay in the sun without burning compared to no sunscreen at all.

Think: burn time, not strength time. 

If your skin normally burns after 20 minutes, SPF 30 means (in theory) you could stay in the sun 30 times longer, that’s about 10 hours. But that’s only true in perfect lab conditions where SPF gets tested. 

In real life, things like swimming, sweating and towel drying all wear off your sunscreen. Plus, most people don’t apply enough in the first place.

So, while SPF 30 might offer 10 hours of protection on paper, you’ll need to reapply it every 2 hours and apply the correct motherdose for it to really work.

Why the Misunderstanding?

People tend to interpret SPF values linearly (thinking 50 is double 25), but SPF protection works on a logarithmic curve. As the SPF increases, the incremental protection gains shrink as we see in the graph below. 

So what does this all mean?  

How you apply sunscreen matters more than the SPF number. Even the highest SPF won’t work if you don’t use enough or don’t reapply. Apply generously and reapply every 2 hours, or immediately after swimming, sweating, or towel-drying

Getting the amount and timing right is key to real protection.

Up next in the series:

We’ll take a closer look at how SPF is tested, what “broad spectrum” really means, and what role UVA protection plays in long-term skin health.

Have questions? We’re always happy to help.