Reviewed guide
What SPF Means
Understand what the SPF number describes—and what it does not tell you about a sunscreen.
Reviewed 2026-08-24 · SunShieldLab editorial review
SPF is a tested label value
The FDA describes SPF as a measure based on the ultraviolet exposure required to cause sunburn with the sunscreen compared with exposure without it. It primarily reflects UVB sunburn protection.
SPF is not a time multiplier
SPF does not mean you can stay in the sun a fixed multiple longer. Exposure intensity, application amount, rubbing, swimming, sweating, and reapplication all affect real-world use.
Check broad spectrum separately
A broad-spectrum label indicates that the product passed the applicable test for UVA protection relative to UVB protection. Do not infer broad-spectrum status from the SPF number alone.
Follow the directions
Use the labeled directions for application and reapplication. If swimming or sweating, choose a product labeled water resistant and follow its stated duration.
Sources and further reading
Reviewed evidence used in this guide
A broad-spectrum sunscreen has passed the applicable test for protection across UVA and UVB radiation; broad-spectrum status should be checked separately from SPF.
SPF primarily indicates a sunscreen’s tested level of protection against sunburn associated with UVB radiation; it is not a time multiplier.
- U.S. Food and Drug Administration — Sunscreen: How to Help Protect Your Skin from the Sun (What do SPF and Broad Spectrum mean?)