How SPF Numbers Are Calculated
How SPF Numbers Are Calculated — SPF, or Sun Protection Factor, is a laboratory-measured rating of how much longer protected skin resists UVB-induced redness compared with unprotected skin. It is determined by comparing the UV dose needed to cause minimal sunburn with and without a set amount of sunscreen applied.
Key points
- SPF is calculated as the ratio of the sunburn-inducing UV dose on protected versus unprotected skin.
- Tests apply a standardized 2 mg per square centimeter, which is thicker than most people use.
- SPF primarily reflects UVB protection, not the full range of UVA rays.
- SPF 30 filters roughly 97 percent of UVB and SPF 50 about 98 percent, a small numerical gap.
- Real-world protection is usually lower than the label because application is often too thin.
Frequently asked
Is SPF 100 twice as protective as SPF 50?
No; SPF 50 blocks about 98 percent of UVB and SPF 100 about 99 percent, so the added protection is marginal rather than double.
Why is my real-world SPF lower?
Lab ratings assume a generous, even application that most people do not match, so applying too little or missing reapplication reduces effective protection.
Related topics
This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.
