UPF at a glance
- UPF 30
- Blocks about 96.7 percent of UV
- UPF 50
- Blocks about 98 percent of UV
- US labelling
- ASTM D6603
- US test method
- AATCC TM183
- Australia and NZ
- AS/NZS 4399
How the number works
UPF is a fraction turned upside down. A UPF 30 fabric lets about one thirtieth of the ultraviolet radiation hitting it reach your skin, which is 3.3 percent through and roughly 96.7 percent blocked. UPF 50 lets one fiftieth through, so 2 percent gets past and 98 percent does not.
Which is why the gap between the ratings is smaller than it looks. Moving from 30 to 50 buys about 1.3 percentage points of extra blocking, not a doubling of anything. Anything testing above 50 is labelled 50+ rather than given a number, because the measurement stops being meaningful up there.
Which standard governs the label
In the United States the labelling is set by ASTM D6603, the specification for labelling UV-protective textiles, and the transmittance behind it is measured by AATCC TM183. Both are voluntary: nothing compels a maker to test a shirt or to print the result on it.
Australia and New Zealand, which have the most sun-conscious clothing market anywhere, use AS/NZS 4399, which covers the measurement and the classification together. Its output is the 15, 30 and 50+ banding that now turns up on garments worldwide.
What actually changes a fabric's UPF
Mostly how much light gets through the gaps. A dense weave or knit blocks more than a sheer one of the same weight, and deeper colors absorb more UV than pale ones. Fiber matters too: shiny polyesters reflect UV, unbleached cotton carries natural lignins that absorb it, and some fabrics add a mineral treatment such as titanium dioxide to lift the number further.
Two things lower it in use, and both are easy to forget. Stretching the fabric opens the gaps, so a top pulled tight across a shoulder protects less than the tag says. And a wet fabric transmits more UV than a dry one: a white cotton T-shirt rates about UPF 7 dry and about 3 soaked, which is worth knowing on exactly the day you are most likely to soak a shirt.
Does a base layer need a UPF rating?
It matters on the days the base layer is the only layer. A long-sleeved top worn alone above treeline is doing the job of sunscreen on your arms and shoulders, and doing it without needing to be reapplied, which is the real advantage clothing has.
Coverage beats the number, though. A UPF 50+ short sleeve protects nothing below the elbow, and a hood, a collar and long sleeves at UPF 30 cover far more skin than a higher rating on a smaller area ever will.
What this means when you're choosing
- A stated UPF number rather than the words sun protective on their own
- Coverage: sleeves, a collar and a hood matter more than the rating does
- A dense knit, since UPF drops when fabric is stretched or wet
- Whether the rating is tested to ASTM D6603 or AS/NZS 4399 or simply claimed
UPF vs SPF
| UPF | SPF | |
|---|---|---|
| Applies to | Fabric | Sunscreen on skin |
| Covers | UVA and UVB | UVB only |
| Based on | How much UV the material transmits | How long skin takes to redden |
| Wears off | No, though stretching and water lower it | Yes, needs reapplying every couple of hours |
Which to pick: Clothing is the more reliable of the two because it never needs reapplying. Use UPF fabric for what it covers and sunscreen for the face, neck and hands it does not.
The common mistake
Reading UPF as if it were SPF
They are different scales for different things. SPF is a sunscreen number built on how long UVB-exposed skin takes to redden; UPF describes what a fabric transmits, across UVA and UVB together. A UPF 50 shirt is not fifty times better than a UPF 30 one. It blocks 1.3 percentage points more.
Common questions
Is UPF 50 much better than UPF 30?
Marginally. UPF 30 blocks about 96.7 percent of UV and UPF 50 about 98, so the upgrade is worth 1.3 percentage points. How much skin the garment covers makes a far bigger difference than moving up a band.
Does UPF wash out?
A rating that comes from the weave and the fiber does not, and many fabrics tighten slightly with washing and rate a shade higher. A rating that leans on a mineral or chemical treatment can fade, so check whether the maker says the protection is inherent to the fabric.
Does an ordinary shirt have a UPF?
Every fabric transmits some UV, so yes, it simply is not measured. The Skin Cancer Foundation puts a white cotton T-shirt at about UPF 7, dropping to about 3 once it is wet, which is a long way below anything sold as sun protective.
Where this is defined
- ASTM D6603, standard specification for labeling of UV-protective textiles, ASTM International
- AS/NZS 4399:2020, sun protective clothing, evaluation and classification, Standards Australia and Standards New Zealand
- Sun-protective clothing and what UPF means, The Skin Cancer Foundation