What Does SPF Mean?
SPF stands for Sun Protection Factor. It describes the level of sunburn protection provided by a sunscreen when the product is applied and used as directed.
SPF testing compares the amount of ultraviolet radiation required to produce sunburn on protected skin with the amount required to produce sunburn on unprotected skin.
Because sunburn is caused mainly by UVB radiation, SPF is primarily an indication of protection against UVB.
One important point is often misunderstood: SPF does not tell you how many hours you can safely stay in the sun.
The amount of ultraviolet radiation reaching your skin changes according to factors such as the time of day, season, location, altitude, cloud cover and reflection from surfaces such as water, sand and snow.
How SPF Ratings Work
A simple way to understand different SPF ratings is to consider the proportion of UV radiation that reaches the skin under controlled testing conditions.
- SPF 15 allows approximately 1/15 through, filtering about 93%.
- SPF 30 allows approximately 1/30 through, filtering about 97%.
- SPF 50 allows approximately 1/50 through, filtering about 98%.
The difference between SPF 30 and SPF 50 can look quite small when expressed as a percentage. However, SPF 50 allows less ultraviolet radiation to reach the skin than SPF 30.
These figures also assume that the sunscreen has been applied at the amount used during SPF testing. Applying too little sunscreen reduces the protection actually achieved.
Does a Higher SPF Mean You Can Stay in the Sun Longer?
No. SPF should not be used as a countdown clock for deciding how long it is safe to remain in the sun.
For example, SPF 30 does not simply mean that someone who would normally burn after 10 minutes can safely remain outside for 300 minutes.
UV intensity changes throughout the day, and sunscreen can also rub off, wash off or be affected by sweating and swimming.
A higher SPF provides greater protection from sunburn when used correctly, but it does not remove the need to reapply sunscreen or use other forms of sun protection.
What About UVA Protection?
SPF primarily measures protection against UVB-related sunburn. UVA radiation is also important because it contributes to cumulative skin damage, premature skin ageing and skin cancer.
For this reason, look for sunscreen labelled broad spectrum, or the equivalent UVA protection designation used in your country.
Broad-spectrum sunscreen is designed to provide protection against both UVA and UVB radiation.
You can learn more about the filters used to provide UV protection on our sunscreen ingredients page.
SPF 30 Versus SPF 50
The difference between SPF 30 and SPF 50 is sometimes misunderstood.
SPF 30 filters approximately 97% of UV under test conditions, while SPF 50 filters approximately 98%.
Another way of looking at the same numbers is that SPF 30 allows roughly 1/30 of the relevant UV exposure through, while SPF 50 allows roughly 1/50 through.
That means SPF 50 still provides greater protection even though the percentage difference appears relatively small.
What SPF Should You Choose?
Sun-protection recommendations vary somewhat between countries and health authorities, but broad-spectrum sunscreen with an SPF of at least 30 is commonly recommended when sunscreen is required. Some national authorities recommend SPF 50 or 50+.
Whichever SPF you use, correct application matters. Sunscreen should be applied generously to exposed skin and reapplied according to the directions on the product.
Reapplication is particularly important after swimming, sweating, towelling off or other activities that may remove sunscreen from the skin.
Water-Resistant Does Not Mean Waterproof
No sunscreen should be regarded as completely waterproof.
Products described as water-resistant are tested to maintain their stated protection for a defined period while swimming or sweating. The label should tell you how the product was tested and when it needs to be reapplied.
If you are swimming, exercising or sweating heavily, choose an appropriate water-resistant sunscreen and follow the reapplication instructions.
Is Sunscreen Enough on Its Own?
No sunscreen blocks 100% of ultraviolet radiation.
Sunscreen works best as one part of a broader sun-protection approach. Depending on UV conditions, this can also include:
- seeking shade
- wearing sun-protective clothing
- wearing a broad-brimmed hat
- using sunglasses that provide appropriate UV protection
- reducing unnecessary exposure when UV radiation is particularly strong
Clothing can provide an additional physical barrier against ultraviolet radiation. You can read more about this in our guides to UPF ratings and sun-protective clothing.
SPF: The Key Points
- SPF means Sun Protection Factor.
- SPF primarily indicates protection against UVB-related sunburn.
- SPF is a measure of relative sunburn protection, not the number of hours you can safely spend in the sun.
- SPF 30 allows approximately 1/30 of the relevant UV exposure through under test conditions.
- SPF 50 allows approximately 1/50 through.
- A higher SPF provides greater sunburn protection when correctly applied.
- Look for broad-spectrum protection to help protect against UVA as well as UVB radiation.
- Applying enough sunscreen and reapplying it correctly are essential.
- No sunscreen provides complete protection from ultraviolet radiation.
More About Sunscreen
For more information, see our guides to sunscreen effectiveness, types of sunscreen, sunscreen ingredients and using sunscreen correctly.
You can also read about the history of sunscreen and SPF.
Information on this website is provided for general educational purposes and is not a substitute for individual medical advice or assessment by a qualified healthcare professional.