Swimwear fabric blocks UV based on how tightly it is knit, not on the fiber name alone. Look for a certified UPF 50+ label, a dense knit, and a fit that does not stretch thin on your child. Wet, stretched or pale fabric lets in more UV. Sunscreen and shade are still needed on top.
This guide covers how fabric type, weave, color, stretch and water affect UV protection in children’s swimwear. It also gives a label checklist you can use at the shop or at checkout.
TL;DR
- A UPF 50 fabric blocks 98% of UV rays and lets 2% through. Look for a UPF rating on the label, not a vague “sun protective” claim.
- Weave density matters most. Tighter knit means better protection. Dark or bright colors absorb more UV than whites and pastels.
- Stretching and wetting a swimsuit lowers its UPF. Choose a size that fits snugly without pulling the fabric thin.
- Swimwear does not replace other protection. Add sunscreen, a hat, sunglasses and shade.
- BANZ® UV swimwear is rated UPF 50+ and starts from 15 USD.
Table of Contents
- Can UV rays go through swimsuits?
- What UPF means on a swimsuit label
- How fabric type affects UV protection
- Why stretch and water lower UPF
- Two quick checks you can do at home
- Swimsuit label checklist for parents
- Perspective: how we think about swim fabric
- Build a full sun plan around the swimsuit
- Shop BANZ® UV swimwear
- FAQ
- Sources
Can UV rays go through swimsuits?
Yes. Ordinary swimsuit fabric can let UV through, and the amount depends on the knit and the condition of the fabric. ARPANSA, the Australian radiation protection agency, notes that a UPF rating drops when clothing is wet or stretched compared with its dry, relaxed state.
For a child, that is the normal state of a swimsuit. The fabric is wet, and it stretches every time your child climbs, kicks, crouches or pulls at a strap. A label rating is measured on the fabric under test conditions. Real use is harder on it.
The point is not that swimwear fails. The point is to pick fabric that starts with a high rating and has enough margin to stay protective in use.
What UPF means on a swimsuit label
UPF stands for Ultraviolet Protection Factor. According to the Skin Cancer Foundation, it measures how much UV radiation, both UVA and UVB, passes through fabric to reach skin.
| UPF rating | What it means |
|---|---|
| UPF 50 or 50+ | Excellent. Blocks about 98% of UV; about 1/50th reaches skin |
| UPF 30 to 49 | Very good protection |
| Under UPF 30 | Not enough to count as sun protective |
For comparison, the same Skin Cancer Foundation page says a typical white cotton T-shirt has a UPF of about 7, and about 3 when wet. Many parents assume a T-shirt over a swimsuit is enough. It is not.
A plain “SPF” claim on clothing is not the same thing. SPF applies to sunscreen. For fabric, look for “UPF” and, ideally, a mention of the testing standard. ARPANSA explains that its swing tags show a product was tested to the Australian and New Zealand standard AS/NZS 4399:2020.
If you want a deeper explanation of the number, see our guide to what the UPF 50 rating really means.
How fabric type affects UV protection
Most swimwear is a blend of a synthetic base fiber and a stretch fiber such as spandex or elastane. The base is usually nylon or polyester. Fiber chemistry plays a part in UV blocking, but it is not the deciding factor on its own.
Fiber: synthetic versus natural
Synthetic knits sit at the dense, smooth end of the range. Natural fibers like cotton and linen are often loosely knit or woven and can let a lot of UV through. A peer-reviewed study of summer apparel published on PubMed found a significant portion of everyday summer clothing fell under UPF 30, including some cotton and linen blends.
That is why a cotton cover-up from the dresser is a weak substitute for a rated swim shirt.
Weave and knit density
The Skin Cancer Foundation lists several things that affect how well clothes block UV: color, fabric construction, weight, stretch and wetness. The CDC gives the same practical advice: tightly woven fabric offers the best protection.
When two suits use the same fiber, the one with the denser knit blocks more UV. Gaps between yarns are where UV passes through.
Color
Dark or bright colors, including red, black and navy, absorb more UV than whites and pastels, according to the Skin Cancer Foundation. A pale pastel swimsuit starts at a disadvantage unless it is built for a rated UPF.
Treatments and finishes
Some fabrics reach a high UPF through a tight construction. Others use added UV-absorbing treatments. Either way, the number that matters is the tested UPF on the finished garment. The fiber name on the tag does not tell you that.
Why stretch and water lower UPF
Two things happen in a pool.
Stretching. When fabric is pulled, the spaces between yarns open. ARPANSA says stretching can reduce a fabric’s UPF substantially. Laboratory work on knitted fabrics in the dry state and the wet state tested relaxed and stretched samples for this reason. Those studies used cotton and cotton blend knits, not swimwear, but the mechanism is the same: open the knit and more UV gets through.
Wetting. Water changes how a fabric interacts with UV. Wet clothing generally has a lower UPF than dry. If your child stays in the water for long stretches, plan for some loss from the label number.
What this means when you buy
- Start high. A UPF 50+ fabric has more margin than a UPF 30 fabric when it gets wet or stretched. Our comparison of UPF 50 versus UPF 30 shows the difference.
- Choose the right size. A suit that is too small stretches tight across the shoulders and back. A suit that is a little loose can gape and expose skin. Aim for a snug fit that moves with your child without pulling thin.
- Cover more skin. Long-sleeve rash guards and longer legs add coverage in places where sunscreen is easy to miss or wash off.
We explain wet performance in more detail in does UPF clothing still protect kids when it gets wet.
Two quick checks you can do at home
The light test. Hold the fabric up to a bright light. The Skin Cancer Foundation suggests this simple check: if you can easily see light through it, the weave is loose and protection is lower.
The stretch check. Pull a section of the fabric gently, as your child would when climbing out of the pool. If it turns see-through or the color lightens, the knit is opening up. Hold it to the light again while stretched.
These checks do not give you a UPF number. They help you spot weak fabric. A tested and labeled rating is still the better guide.
Swimsuit label checklist for parents
Before you add a swimsuit to your cart, check:
- A stated UPF of 50+ on the product page or tag
- A tested rating, not just a “sun protective” or “UV blocking” phrase
- Dark or bright colors if you want extra margin
- Coverage: sleeves, longer legs, higher neckline
- Size chart and fit notes, so the fabric does not overstretch
- Care instructions, since washing can affect fabric over time
On care, our guide to keeping UPF 50+ clothing working through repeated washes covers rinsing and drying habits.
Perspective: how we think about swim fabric
We started BANZ® because a family in Australia could not find suitable protection gear for their children. That origin shapes how we talk about swimwear fabric.
We do not treat the fiber name as the answer. A parent comparing suits needs three facts: the UPF rating, how the garment holds that rating in use, and whether it fits a real child who moves around. A rating with no fit behind it is a number on a tag.
That is why we build our UV swimwear to UPF 50+. A high starting rating leaves room for the wear a suit gets at the pool. We also pair swimwear with hats and sunglasses, because a child’s face, ears and eyes need cover too.
We also tell parents plainly that clothing is one layer. It works best next to sunscreen on exposed skin and shade during peak hours.
— Shari M. Murphy
Build a full sun plan around the swimsuit
The CDC recommends combining protective clothing, a wide-brimmed hat, sunglasses, broad-spectrum sunscreen with SPF 15 or higher, and shade. Its guidance for peak sun hours is 10 a.m. to 4 p.m., as summarized in the CDC’s skin cancer prevention Q&A.
For swimming days:
- Put a UPF 50+ swimsuit or rash guard on first.
- Apply sunscreen to all exposed skin: face, neck, hands, feet and any gaps.
- Add a hat and UV-rated sunglasses.
- Set up shade for breaks.
- Reapply sunscreen after swimming and at least every two hours.
For babies, the FDA advises keeping infants under 6 months out of direct sunlight. Talk to your pediatrician about sunscreen and sun protection for infants.
A checklist for the whole day is in our pool day sun gear list. For wider gear ideas, see sun safety gear examples for kids.
Shop BANZ® UV swimwear
BANZ® Feel No Flare™ UV Swimwear is rated UPF 50+ and starts from 15 USD. It is made for kids who spend long days in and around water. You can pair it with Feel No Flare™ Sun Hats (from 19 USD) and See No Glare™ Sunglasses (from 17 USD) for head-to-toe cover.
Browse sizes, styles and current availability on the BANZ® USA store, then add your choice to the cart and check out.
FAQ
What is the best fabric for UV protection?
No single fiber wins on its own. A dense knit with a tested UPF 50+ rating matters more than the fiber name. Dark or bright colors add margin, and tightly woven fabric blocks more than loose weaves, according to the CDC.
What is the healthiest fabric for swimwear?
From a sun-safety view, choose fabric with a labeled UPF 50+ rating and a fit that does not overstretch. If your child has sensitive skin or a skin condition, ask your pediatrician or dermatologist about fabrics and care. Rinse and dry the garment as the care label says.
What is the best UV protection swimwear for kids?
Choose a UPF 50+ garment with good coverage, such as a long-sleeve rash guard or a one-piece with higher neckline, in a snug size. Our guide to UV protection swimsuits for kids and caregivers goes through styles and what to look for.
Does UPF swimwear replace sunscreen?
No. Cover with fabric where you can and use broad-spectrum sunscreen on skin the fabric does not cover. Reapply it after swimming, as the CDC advises.
Sources
- Sun-Protective Clothing, Skin Cancer Foundation
- Dress to Protect: 5 Things that Affect How Well Your Clothes Block UV Rays, Skin Cancer Foundation
- Australian sun protective clothing, ARPANSA
- Sun protection using clothing, ARPANSA
- Sun Safety Facts, CDC
- Questions and Answers on Skin Cancer Prevention, CDC
- Tips to stay safe in the sun, FDA
- Ultraviolet protection of summer apparel, PubMed
- Stretching and UV protection of knitted fabric, dry state, PMC
- Stretching and UV protection of knitted fabric, wet state, PMC