Every size chart behind this site turns a measurement into a size. None of them can turn an age into one, and Perlimpinpin says so on its own chart: age is for reference only. Here is how to take the measurements the charts ask for, and what each one decides.
Height
Perlimpinpin's published method is to measure barefoot, from the crown of the head to the floor, with the child standing against a wall. Height is where the snowsuit, base-layer and baby-suit charts all start, so it is the measurement to take first.
Chest, waist and hips
The same Perlimpinpin chart asks for the chest at its fullest point under the arms, and the waist at its narrowest point. The Deux par Deux base-layer chart adds hips. These are the measurements that say whether a size chosen on height actually suits the child's shape, and they are why one chart can select two different sizes for the same child. A child whose chest lands on one size and whose height lands on another is exactly the child a size chart fails, and saying so is more useful than resolving it quietly.
Both feet, standing
Deux par Deux's published method for its winter boot is to measure the foot in a standing position, from heel to big toe, add about 2 cm, then read the insole-length line on its chart. Habier reads big toe conservatively as the longest toe, whichever one that is. Measure both feet and keep the longer number, because the boot has to fit the larger foot.
Measure in the sock the child will actually wear in the boot. Habier records the sock weight with the profile and shows it back to you. No published source gives a centimetre value per sock thickness, so none is added to the measurement.
Hands
For mitts, measure from the wrist crease to the middle fingertip, and around the hand below the knuckles without the thumb. Nanö publishes a hand chart those measurements can be read against. Deux par Deux publishes no hand chart for its TEKNIK mittens and sizes them by age band instead.
Why age alone cannot answer
A chart maps a measurement to a size. Give it an age and there is nothing to look up. Habier's own rule is that no size is given for a piece of the kit unless a measurement for that piece has been supplied and matched to that product's current chart. A size from age alone is a guess, so we would rather show you the measuring instruction than the guess.
The same holds for the size the child wears now. A shoe size or a garment size in one brand is a fact about that brand. We record it as context and show it back, and we do not convert it, because those labels do not mean the same thing from one maker to the next.
See what each chart says
Enter a height or a foot length below and it will be read against each maker's own published chart, one chart at a time, with the source of each shown. Nothing is converted between brands.
For a size on every piece of the kit at once, with the gaps named where a chart cannot answer, send the measurements through the Fit Finder. To understand what the charts are publishing in the first place, read what each brand's size chart actually measures.