Buying a size up is so standard it barely registers as a decision. The shoes are expensive. The kid will grow. The logic feels obvious.
This article isn't about opinion. It's about what happens to a child's foot and gait when a sports shoe doesn't fit — because sports shoes are a specific case, and the physics of the thing is worth understanding.
How fast children's feet actually grow
Children's feet don't grow slowly. Between the ages of three and five, the average child's foot grows approximately two sizes per year. Between five and twelve, growth typically slows to around one size per year, though this varies and growth spurts can compress that timeline significantly.
What this means in practice: a shoe bought half a size up in September may fit by December. A shoe bought a full size up may never fit correctly before the foot has grown past it. The window you're buying time for is shorter than it feels.
What a growth plate is and why it matters here
Children's feet contain multiple growth plates — areas of cartilage near the ends of the long bones that haven't yet hardened into bone. The last of these don't fully ossify until the mid-to-late teens.
Cartilage is softer and more compressible than bone. It responds to the forces placed on it differently. This is relevant to shoe fit because a shoe that causes the foot to move incorrectly — heel slipping, toes gripping, weight distributed unevenly — places repeated stress on these structures during exactly the kind of high-impact activity (running, cutting, jumping) that sports involves.
Paediatric podiatrists generally advise that ill-fitting footwear is a more significant issue in children than in adults for this reason. The bones are still forming. The window when the foot is most responsive to mechanical stress is also the window when fit matters most.
What actually happens in a too-big sports shoe
When a sports shoe is too large, three things happen that don't happen in a school shoe of the same size:
The heel slips.
Sports shoes are built for lateral movement and changes of direction. When the heel isn't held firmly, the foot slides inside the shoe on every plant. Within a session or two this produces blisters at the heel. A loose heel in a rugby boot or a football boot means the foot is effectively working against the shoe rather than being supported by it.
The toes grip.
This is the compensatory mechanism the foot uses when it's sliding. The toes curl down to try to grip the insole. This creates sustained tension in the plantar fascia and the small muscles of the foot. Over a full season of training and matches, this is a meaningful amount of unnecessary stress on developing tissue.
The gait changes.
A child running in shoes that are too large will shorten their stride and change their footstrike pattern to keep the shoe on. This isn't a conscious choice — it happens automatically. The result is that the child is practising movement patterns that don't reflect how they'd move in a well-fitting shoe. For a kid developing technique in any sport, this matters.
Why sports shoes are different from school shoes
The intuition most parents carry over from school shoe shopping doesn't transfer directly.
School shoes are fitted with a thumb's width of room at the toe because children's feet swell through the day, and because school shoes are worn for long periods at low intensity. The movement demands are minimal.
Sports shoes — football boots, running shoes, rugby boots — are performance-fit. They're engineered to transfer force efficiently between foot and ground. For a football boot, the fit is intentionally snug: you should feel the ball through the boot, not through a gap between your foot and the leather. Running shoes carry slightly more room, but are still tighter than a school shoe.
When you add a size "to grow into" to a sports shoe, you're not just adding room — you're undermining the reason the shoe is designed the way it is.
The cost-per-wear reality
The financial logic behind buying up a size runs: bigger shoe now = more wear before replacement = lower cost overall.
The actual maths tends to run differently. A shoe bought a size up and worn through a season of discomfort gets handed down or discarded. A shoe bought correctly-sized for now gets worn hard for the season it fits, then sold — if the condition is right — for a reasonable fraction of what it cost.
The shoes that come through KISH in the best condition are almost always the ones that fit the child they were bought for. The ones in the worst condition are usually the ones that were too big — worn hard, with internal damage that comes from a foot that couldn't stop moving inside them.
The one-rule version
Fit the shoe on the foot that will wear it, on the day you're buying it.
For sports shoes: the toe of the shoe should be within a thumb's width of the end of the longest toe when the child is standing, fully-laced, in the shoe. No more than that. Any more and the shoe is too big. If you can feel significant space between the heel and the heel cup when you press, it's too big.
Buy the correct size, use it for the season it fits, pass it on when it doesn't.
— The KISH team