UPF clothing works by reducing the amount of ultraviolet radiation that passes through the fabric and reaches the skin underneath. The level of protection depends on the fabric's construction and other characteristics, including its density, thickness, colour, stretch and condition.
A UPF rating measures this protective performance. Higher ratings indicate that less UV radiation passes through the tested material, but the rating only protects the skin actually covered by the garment.
How Does Fabric Block UV?
When sunlight reaches clothing, some ultraviolet radiation is absorbed or reflected by the material and some may pass through it. The structure of the fabric strongly influences how much reaches the skin.
Closely woven or knitted fabrics generally provide greater protection because there is less open space between fibres for UV radiation to pass through. Looser fabric structures can allow more radiation through those spaces.
This is why UV protection cannot be determined from the fibre name alone. Two polyester shirts, for example, can provide different levels of protection if their fabric structures, weights, colours or finishes differ.
What Does the UPF Rating Measure?
UPF stands for Ultraviolet Protection Factor. It describes how effectively a material reduces ultraviolet radiation reaching the skin beneath it.
A UPF 50 material allows approximately one fiftieth of the relevant UV radiation to pass through under the conditions used for its rating. That corresponds to approximately 98% being blocked.
The numerical rating deserves its own explanation. Read what UPF 50+ means for a closer look at how the rating should be interpreted.
What Determines How Protective a Fabric Is?
There is no single property that makes fabric UV protective. Its performance comes from several characteristics working together.
Important factors include:
- Fabric structure: tighter woven or knitted structures generally leave less open space for UV radiation to pass through.
- Thickness: thicker fabrics generally provide greater protection than thinner versions of otherwise similar materials.
- Colour: darker colours and higher concentrations of some dyes can absorb more UV radiation than lighter versions of comparable fabric.
- Stretch: stretching some fabrics opens the structure and can reduce protection.
- Moisture: wetness can change the UV protection provided by some fabrics.
- Condition: wear, fading and changes to the fabric over time can affect protection.
- Finishes and additives: some textiles incorporate compounds or treatments that increase UV absorption.
These variables are why appearance alone cannot reliably tell you how much UV protection a garment provides.
Why Does Fabric Density Matter?
Fabric contains fibres and spaces between those fibres. A dense construction reduces the amount of open space available for ultraviolet radiation to travel through.
This applies to both woven and knitted materials. A tightly constructed lightweight fabric can therefore provide substantial protection, while a visibly open or loosely constructed fabric may allow considerably more UV through.
Holding fabric up to visible light can reveal obvious gaps, but it cannot tell you its UPF rating. Human vision measures visible light, not the ultraviolet radiation used to determine UPF performance.
Does Colour Affect UV Protection?
Colour can influence UV protection. In otherwise comparable fabrics, darker colours and greater dye concentrations generally absorb more ultraviolet radiation than lighter colours.
Colour is only one variable, however. A tightly constructed light fabric can provide better UV protection than a loosely constructed dark fabric. It is therefore not reliable to judge a garment's complete UV performance by colour alone.
Does Stretch Reduce UPF Protection?
Stretching can reduce the UV protection of some fabrics, particularly knitted and elastic materials. As the material stretches, spaces within the structure can become larger, allowing more ultraviolet radiation to pass through.
This is particularly relevant to close-fitting technical clothing. A garment should not be assumed to retain exactly the same UV performance under every degree of stretch unless its rating and testing support that conclusion.
Correct sizing therefore matters for more than comfort and movement. Excessive stretch can alter the physical structure that contributes to UV protection.
Does UPF Clothing Still Work When Wet?
Wetness can change the UV protection provided by some fabrics, but the effect is not identical for every material. Fabric type, construction and treatment all influence what happens when the garment becomes wet.
This matters particularly for swimwear and waterwear because the garment is expected to become saturated during normal use. Do not assume that the dry performance of an untested fabric tells you exactly how it will behave when wet.
For watersports, verified performance and the manufacturer's stated rating are more useful than estimating protection from the fabric's appearance or composition.
Does All Clothing Have UPF Protection?
Most clothing creates some physical barrier between the skin and ultraviolet radiation, but that does not mean every garment has a tested UPF rating.
A stated UPF rating should be based on testing that quantifies the UV transmission of the material under an applicable test method. Without a tested rating, it can be difficult to determine the actual level of protection simply by looking at the garment.
Dense construction and greater coverage can still be useful characteristics in unrated clothing, but they should not be used to invent a numerical UPF value.
Why Does Garment Coverage Matter?
UPF only helps where the fabric covers the skin. A highly protective fabric on a small garment cannot protect areas that remain exposed.
This makes garment design part of practical sun protection. Long sleeves cover more of the arms than short sleeves, while higher necklines and longer body lengths can increase the area protected by clothing.
More coverage does not increase the UPF rating of the material itself. It increases the amount of skin positioned underneath that material.
UPF Clothing vs Sunscreen
UPF clothing and sunscreen protect the body in different ways. UPF describes protection provided by textiles, while SPF describes protection associated with sunscreen applied to the skin.
Clothing has the practical advantage of continuing to cover the same area while it remains properly positioned and maintains its protective characteristics. Sunscreen is important for exposed skin and needs to be used according to its directions.
The two should therefore be considered complementary rather than competing forms of protection. Our guide to SPF vs UPF explains the distinction in more detail.
Why UPF Matters Around Water
Watersports can involve long periods outdoors with limited shade, making clothing coverage particularly useful. Surfing, kitesurfing, swimming and other coastal activities can also make repeated sunscreen application less convenient than it is on land.
The importance of UV protection becomes particularly clear in sunny coastal environments where UV levels can become high. At destinations such as El Médano in Tenerife and Bonaire in the Caribbean, spending extended periods on or around the water can mean substantial sun exposure. UV levels vary with factors including season, time of day, cloud cover and atmospheric conditions, so current local conditions and the UV Index should always be checked rather than assuming a destination has a fixed level of exposure.
Technical waterwear can combine coverage with characteristics needed for active use in and around water. The Water Peak's Performance-Fit Coastech™ Rashguard uses UPF 50+ fabric in a close, second-skin performance fit, while the Regular-Fit Visibility Water Shirt uses UPF 50+ fabric with a more relaxed performance fit.
That does not make UPF clothing a complete sun-protection system. Areas not covered by the garment remain exposed, and appropriate sunscreen, shade, headwear and other protection can still be necessary.
Environmental exposure around water also deserves separate consideration. Read more about UV exposure around water and how to interpret the UV Index.
Can UPF Protection Change Over Time?
The protection provided by a garment can change as the material changes. Repeated wear, stretching, fading and physical deterioration can alter the fabric structure or other characteristics that contributed to its original performance.
The effect is not identical across all materials. Some fabrics can even change protection after washing, while prolonged wear and deterioration can reduce protection in others.
Follow the garment manufacturer's care instructions and replace heavily worn technical clothing when its fabric or construction has visibly deteriorated.
What Should You Look for in UPF Clothing?
Start with a verified UPF rating rather than trying to estimate protection from colour or fabric composition alone. Then consider how the garment will actually be worn.
- Check the stated UPF rating.
- Choose enough garment coverage for the conditions and activity.
- Use the correct size to avoid unnecessary fabric stretching.
- Consider whether the garment is designed for wet use when it will be worn in the water.
- Follow the manufacturer's care instructions.
- Continue protecting skin that remains exposed.
UPF Is a Property of the Complete Fabric
UPF clothing works because its material reduces the ultraviolet radiation that can reach covered skin. The effectiveness of that barrier depends on the complete fabric, not simply whether it is nylon, polyester, dark, light, thick or thin.
Fabric structure, thickness, colour, stretch, moisture, condition and treatments can all influence UV transmission. Testing brings those variables together into a measurable UPF rating.
For clothing used around water, that distinction is especially important. Choose protection based on verified performance, adequate coverage and the actual conditions rather than assuming that any technical-looking garment automatically provides a particular level of UV protection.