Should a rash guard be tight or loose?

Close-fitting, not tight. A rash guard should follow the body closely enough that it does not drag or ride up in water, but not so tightly that the fabric is stretched — stretching opens the gaps between fibers and lets more UV through. The check: a flat hand should slide between the fabric and the chest, and the hem should stay put when both arms go straight up.

Both errors cost you protection, in different ways. Too tight reduces the fabric's UPF directly. Too loose rides up and exposes the lower back, which is then the part that burns.

Why stretch reduces protection

UPF depends heavily on how tightly the fabric is constructed, because UV passes through the gaps between yarns. Stretching a knit pulls those yarns apart and enlarges the gaps — the same garment, physically thinner and more open, transmitting more UV.

This is not a marginal effect. A knit stretched significantly can lose a substantial share of its rating, which is why the label figure assumes the garment is worn as designed rather than as a compression layer. How UPF clothing works.

The thirty-second fit check

1. The flat-hand test. You should be able to slide a flat hand between the fabric and the chest. If you cannot, it is too tight.
2. The arms-up test. Raise both arms straight overhead. The hem should stay at or below the waistband. If it rides up and exposes the lower back, it is too short or too tight.
3. The shoulder test. Look at the fabric across the shoulders and upper back. If you can see the weave stretching and the color lightening, it is too tight.
4. The sleeve test. Sleeves should reach the wrist bone. Short sleeves leave the forearms — a high-exposure area — uncovered.
5. The pinch test. You should be able to pinch a small amount of fabric at the waist. No slack at all means it is stretched.

Sizing a child's rash guard

Buy the correct size, not a size up. Buying big is the standard parental instinct and it is wrong here for three reasons: an oversized rash guard billows in water and drags, it chafes under the arms, and it rides up and exposes the lower back — the area most likely to burn while a child is bent over at the water's edge.

If a child is between sizes, size up rather than squeezing into the smaller one, since a stretched garment loses protection outright while a slightly loose one only loses some coverage. Expect to replace it every season, which is simply the cost of the category. Best kids rash guards and sun shirts.

Length, and the gap nobody thinks about

The lower back is the most common gap. A rash guard that meets swim shorts when standing still can separate from them entirely when a child bends forward, and that is the position children spend most of a beach day in.

Look for a longer cut, or a two-piece swim set where the top overlaps the bottoms substantially. And check it in the bent-forward position rather than standing up straight in a fitting room.

Loose sun shirts are different

Everything above applies to rash guards, which are cut close for water. For a land sun shirt, looser is genuinely better: less stretch, more airflow, and a layer of moving air that keeps you cooler in direct sun. The only limit is that it should not flap enough to expose skin repeatedly. Rash guard versus swim shirt.

Keeping the fit over time

• Rinse in fresh water after salt or chlorine, which are what degrade elastane.
• Wash cool and line dry. Hot tumble drying destroys the stretch recovery that keeps a knit dense.
• Skip fabric softener, which coats fibers and interferes with the fabric's behavior.
• Replace when it goes slack. A garment that has lost its recovery stretches more easily, which means it protects less even at the same nominal size.