Buying for a boy
who's growing fast
Children in the middle years grow around five to six centimetres a year, which is enough to make last summer's swim shorts useless and enough to tempt every parent into the same mistake.
Updated 26 July 2026 · 5 min read
Why two sizes up doesn't work
It is the obvious move — buy big, get two or three summers out of one pair — and it fails for reasons that only become apparent once the child is wearing them.
Swim shorts scale in two directions at once. A size up adds waist, but it also adds length, and length is the dimension that goes wrong first. Two sizes too long on a child means the hem sits below the knee, which drags in the water, catches when they climb out, and makes running on wet tile harder than it needs to be. The drawstring can take in a waist. Nothing can take in a leg.
There is also the part parents underestimate: a child who thinks they look ridiculous will refuse to wear them, and you will buy another pair anyway. A garment that does not get worn has an infinite cost per wear.
One size up is the sweet spot. The elastic waistband plus drawstring absorbs the extra width with no visible slack, and the extra length is small enough to read as a normal fit rather than a hand-me-down. That single size typically buys you two seasons instead of one.
Go by height, not age
Youth sizes are labelled by age, which is the least reliable guide available. Children of the same age routinely differ by fifteen centimetres. A tall eight-year-old and a short ten-year-old often need the same size, and the label will tell you neither.
Measure the child against a wall, then size from the height. If they are close to the top of a band, take the next one up — that is the one-size-up rule applied automatically.
Buy at the start of the season
This sounds trivial and it is worth several months of wear. A pair bought in May and sized one up will still fit the following June. The same pair bought in August fits for four weeks, then sits in a drawer over the winter while the child grows through it.
If you are buying late in the season for next year, size up from the size they need now, not from the size they wore last year.
Growth spurts break the rule
Steady growth is predictable. Puberty is not — boys can add ten centimetres in a year during a spurt, and no amount of buying ahead will keep up with that. If a boy is visibly in one, do not buy ahead at all. Buy what fits now, accept one season from it, and expect to be in men's sizing sooner than you think.
When to move to men's sizing
Around 156 cm. Above that height, the largest youth sizes start running short in the leg and tight through the seat, and a men's Small will fit better and look more age-appropriate. Teenagers usually notice this before their parents do.
The cost-per-wear argument
Swim shorts are unusually forgiving compared with most children's clothing, because the waistband adjusts and there is no fastening to outgrow. Bought one size up at the start of a season, a single pair reasonably covers two summers of regular swimming. That is a better run than almost anything else in a child's wardrobe, and it is worth remembering when the temptation to buy the cheapest possible pair arrives.
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