Kids' sizes are about 3 inches apart, so half an inch of error can put your kid in the wrong one. Getting it right takes two minutes, a wall, a hardcover book and a metal tape measure.
The steps
Take off shoes, hats and bulky clothes. Undo ponytails, braids or anything else that adds height.
Find a hard floor, not carpet, next to a flat wall with no baseboard molding.
Stand your kid with feet flat and together, heels against the wall, legs straight and arms at the sides.
Head, shoulders, bottom and heels should touch the wall if they can. Have your kid look straight ahead.
Lay the book flat on top of the head with one edge against the wall, so it makes a right angle with the wall.
Crouch so your eyes are level with the book, and make a light pencil mark on the wall under it.
Measure from the floor to the mark with the metal tape. Write it down to the nearest eighth of an inch.
These steps follow the CDC's guidance for measuring children's height at home.
Mistakes that add or lose half an inch
Carpet. Soft flooring gives under the feet and under the tape, and the number moves.
Chin up. A kid who looks up at you tips the top of the head back and changes the number. Eyes straight ahead.
A tilted book. If it is not square to the wall, the mark lands high or low.
A cloth tape measure. It stretches and sags. Use a metal one.
Reading the mark from above. Get your eyes down to its level.
Measure twice. If the two numbers are more than a quarter inch apart, do it a third time.
Babies and kids under 2
Kids under 2 are measured lying down, and that number is called length. It needs two people and is hard to do well at home, so use the length from the last checkup. The calculator starts at age 2, when standing height takes over.
Every three months is enough, and always right before you buy ahead. A kid of 5 to 9 adds about 2.5 inches a year, so three months is a little over half an inch. That is enough to move from the middle of a size to the top of it, which changes what to buy for next year.