Method
The whole calculation, written down.
There is no proprietary algorithm here. It is a length, an allowance and a growth rate. You can do it on paper, and if you disagree with one of our numbers you can see exactly which one.
1. Internal length
Every EU size corresponds to a fixed internal length. One EU point is two thirds of a centimetre, so the inside of the shoe measures EU size × 20 ÷ 3 millimetres. That is a convention, not an opinion, and it is the same across the whole catalogue.
2. The allowance
Subtract the allowance for the line, and you have the longest foot that size fits. Different lines carry different allowances, because a rain boot worn over a thick sock and a bare-foot sandal are not the same problem:
- First Walkers — 10 mm. Feet this small change fast but the shoe is short, so 10 mm is already a large share of the total length. More than that and the foot moves inside the shoe.
- Everyday Kids — 12 mm. 12 mm covers a normal stride plus a few months of change, without leaving enough room for the foot to slide.
- School Shoes — 12 mm. Same 12 mm as everyday shoes. Growth has slowed by this age, so the allowance buys more months than it does at three.
- Sandals — 8 mm. Sandals are worn without socks and are usually a short-season pair, so 8 mm keeps the foot seated instead of sliding forward on straps.
- Rain & Winter — 14 mm. 14 mm, because these are worn over a thick sock. Measure the foot bare, then let the extra 2 mm be the sock, not the growing room.
3. The recommendation
We take the measured foot length, add the allowance, and pick the smallest size whose internal length covers it. Not the closest size — the smallest sufficient one. Rounding up by habit is how a child ends up half a size adrift for six months.
4. Wear time, as a range
We keep 4 mm as the point at which a shoe has stopped fitting. The room above that, divided by a typical growth rate for the age, gives a number of months. We then widen it by about thirty per cent in each direction and show you the range, because a single number would be a prediction we are in no position to make.
The growth rates we use, in millimetres per month:
- 1 to 3 years — 1.3
- 3 to 6 years — 0.85
- 6 to 10 years — 0.62
- 10 to 12 years — 0.5
These are population averages. Your child is not a population average, which is the whole reason the answer is a range and the reason we ask you to measure again rather than trust the estimate.
5. What we refuse to do
- Recommend more than one size above the measured fit. Too much room makes the foot slide forward and the toes grip.
- Suggest buying next year’s size now. Feet do not grow evenly and the shape changes as well as the length.
- Publish UK or US conversions. They differ between makers, and a conversion is one more place for the number to go wrong.
- Send you a reminder. The re-measure date is written on the results page for you to note down, and it never leaves your browser.
We describe how our shoes are made. We don't give advice about your child's feet. If you have concerns, ask a healthcare professional.
Worked example
A 148 mm foot, four years old, buying Everyday Kids.
- 148 mm measured, standing, longer foot
- + 12 mm allowance for this line
- = 160 mm of internal length needed
- EU 24 has 160 mm inside — the smallest that covers it
- 160 − 148 − 4 = 8 mm of usable room
- 8 ÷ 0.85 ≈ 9 months, shown as 6–12 months
| EU | Inside | Fits at +8 mm | Fits at +14 mm |
|---|---|---|---|
| 20 | 133 mm | 125 mm | 119 mm |
| 24 | 160 mm | 152 mm | 146 mm |
| 28 | 187 mm | 179 mm | 173 mm |
| 32 | 213 mm | 205 mm | 199 mm |
| 36 | 240 mm | 232 mm | 226 mm |
Insoles, and what they are not
The 2 mm Growth Insole takes up a little length while a foot catches up with a size. It changes how much room is left in a shoe. It does not do anything to a foot, and it is not an orthotic device.