[header] How to tell if your child is overweight [title] Figure out your child's weight-to-height ratio (or body mass index). [step] Body mass index (bmi) is age and sex specific, so it is often called bmi-for-age. Use a bathroom scale to measure your child's weight. 
Then, measure your child's height with a tape measure. Have your child stand against a wall, mark the top of their head with a pencil, and measure their height with a tape measure.