Every parent has bought the wrong size. You order what seems right, it arrives, and it's either dragging past his knees or cutting into his waist before the trip even starts. Swim trunks sizing for boys isn't intuitive — kids are all built differently, and brands size inconsistently.
This guide is built around Le Club Original's boys' swim trunks sizing: six sizes from XS to XXL, mapped to age ranges and outseam measurements, so you can match the right trunk to your son before you buy.
Le Club Original Boys' Swim Trunks Size Chart
Le Club Original sizes boys' swim trunks by age and outseam length. Use the age range as your starting point, then cross-check the outseam against a pair he already wears well.
| Size | Age Range | Outseam Length |
|---|---|---|
| XS | 1–2 | 9½" |
| S | 3–4 | 10" |
| M | 5–6 | 10½" |
| L | 7–8 | 11½" |
| XL | 10–12 | 12½" |
| XXL | 14+ | 14" |
How to measure outseam: Have him stand straight. Measure from the top of the waistband down the outside of the leg to where you want the hem to hit — typically just above or at the knee for a clean, proportionate look.
Size by Age: What to Expect at Each Stage
XS — Ages 1–2 (Outseam 9½")
At this age, fit is mostly about comfort and staying put. Toddlers move constantly and need a trunk that won't slide down or bunch up. The XS outseam of 9½" hits at a natural mid-thigh on most toddlers. Look for an adjustable drawstring — Le Club Original's boys' trunks include one — so you can cinch the waist to fit his build rather than relying on age alone.
Common sizing issue: Many 2-year-olds run bigger than the XS. If your son is tall or has a wider build at 2, size up to S.
S — Ages 3–4 (Outseam 10")
Boys in this range are usually past the toddler belly and moving into more active play. The S at 10" outseam sits just above the knee on most 3–4-year-olds, which is the ideal length — enough coverage to move freely without excess fabric.
Common sizing issue: If your son is 4 and on the taller side, the M may fit better and give him room to wear it next summer too.
M — Ages 5–6 (Outseam 10½")
At 5–6, boys are swimming more independently and spending real time in the water. The M outseam of 10½" provides coverage without restricting movement. This is a size where the build varies a lot — two kids the same age can differ significantly in waist and height.
Tip: At this age, measure the outseam of a pair of shorts he currently wears well. That number is more reliable than his age.
L — Ages 7–8 (Outseam 11½")
The jump from M to L (10½" to 11½") is a full inch — worth noting if your son is right on the edge. At 7–8, boys are often in their most active swimming years and put real wear on their trunks. The L outseam of 11½" hits at or just above the knee for most kids in this range.
Common sizing issue: If he's 8 and growing fast, go ahead and size up to XL. A slightly longer trunk still looks intentional; one that's too short looks like he's outgrown it.
XL — Ages 10–12 (Outseam 12½")
Boys in this range are starting to care how they look, and fit matters more. The XL at 12½" outseam is proportionate for most 10–12-year-olds — it reads as a proper swim trunk, not a toddler short or an adult hand-me-down. This is also the age where many boys start gravitating toward the same styles as their dad.
Tip: If he's 12 and you're already buying XL, consider whether adult sizing is a better fit for next season. Le Club's men's trunks start at XS/S — measure and compare.
XXL — Ages 14+ (Outseam 14")
At 14 and up, many boys are close to adult proportions. The XXL at 14" outseam is a proper length trunk — it will hit at or just above the knee for most teens. At this stage, it's worth comparing the XXL boys' outseam to Le Club's men's sizes and buying whichever fits better. Teen fit varies more than any other age group.
Tip: A 14-year-old who wears an adult small may actually prefer the men's S or XS — the cut is slightly different and may sit better on his build.
Between Sizes? Here's What to Do
If your son falls between sizes, these guidelines apply:
- He's between sizes and still growing: size up. A slightly longer trunk looks fine; a too-short one looks like he outgrew it last summer.
- He's between sizes and has a narrow waist: size to the outseam/height, then use the drawstring to cinch the waist.
- He's between sizes and has a wider build: size to the waist — comfort wins. A slightly longer trunk is always the better trade-off.
Matching Styles: Father & Son
Le Club Original makes coordinating swim trunks for fathers and sons — same prints, proportioned for each age. If you're buying matching trunks for a vacation or a photo, use this guide to size your son accurately, then choose the same print in your own size from the men's swim trunks collection.
Browse the Father & Son matching styles — available in Le Club's signature prints, sized from toddler through adult.
Frequently Asked Questions
What size swim trunks should I buy for my 5-year-old? Le Club Original's size M is designed for boys ages 5–6, with a 10½" outseam. For a 5-year-old of average height and build, M is the right starting point. If he's taller or bigger for his age, go with L (10–12 and 11½" outseam) for a better fit and more room to grow.
What size swim trunks for a 7-year-old boy? Size L (ages 7–8, 11½" outseam) is the right size for most 7-year-olds. If he's on the smaller side for his age, check the M outseam (10½") — if a pair of shorts he wears well has a similar length, M may fit better.
What size swim trunks for a 10-year-old? Le Club Original's XL is designed for ages 10–12, with a 12½" outseam. Most 10-year-olds fit XL well. At this age, the best approach is to measure the outseam of a pair of shorts he already wears comfortably and compare it to the chart.
How do I measure my son for swim trunks? Stand him straight and measure from the top of his waistband down the outside of his leg to where you want the hem — just above or at the knee is standard. Compare that number to the outseam column in Le Club Original's boys' size chart: XS (9½"), S (10"), M (10½"), L (11½"), XL (12½"), XXL (14").
Should I size up in boys' swim trunks? If your son is between sizes or growing fast, yes — size up. A longer trunk still looks proportionate. A trunk he's already outgrown doesn't. The adjustable drawstring on Le Club Original's boys' trunks lets you dial in the waist regardless of which size you choose.
At what age do boys switch to men's swim trunks? There's no set age — it depends on build. Many boys transition around 13–15. Le Club Original's boys' XXL (14+, 14" outseam) bridges the gap, but if your teen is closer to adult proportions, compare the XXL outseam to the men's XS or S before deciding.
Do Le Club Original boys' swim trunks match the men's styles? Yes — Le Club Original's Father & Son collection offers matching prints in boys' and men's sizes. The prints coordinate exactly, making them popular for vacations and family photos. See the Father & Son collection for available styles.
More sizing guides: How Long Should Men's Swim Trunks Be? · Women's Swimwear Size Guide
Shop Le Club Original's boys' swim trunks — sized XS through XXL, available in matching Father & Son styles. Free domestic shipping on orders over $50.