Body Mass Index or BMI is a way of estimating how much of your body is made up of fat. A BMI estimate is not exact, but usually provides a good rough estimate; plus it's free, quick, and easy to do. Measuring BMI for children is a bit different from calculating BMI for adults because it's measured not just according to gender (boys and girls are calculated according to different charts) but also by age. You may see BMI for children referred to as BMI-for-age.
Step 1
Measure your child's height. Have him stand up straight against a wall, with his heels back against the wall, and mark where the top of his head reaches on the wall. Then have him step away so that you can use a tape measure or yardstick to measure the distance from floor to the mark you made. Note your child's height on a piece of paper.
Step 2
Weight your child on the most accurate scale you have handy. Generally medical scales are the most accurate, but a home scale can provide an accurate reading, too. Make sure to always weigh your child on the same scale for the sake of consistency. Note your child's weight on the same paper as his height.
Step 3
Sit down for a few calculations. If you've measured height and weight in kilograms and meters, you just need to square your child's height (in other words, multiply it by itself--so if he is 1.2 meters high, his height squared would be 1.2 times 1.2, or 1.44 meters). Next, divide his weight by his height. The resulting number is his BMI. The calculation is a bit more complicated if you've measured in pounds and inches. See the Tips section for the appropriate formula.
Step 4
Check the label on the BMI chart you're using--this information will probably be at the upper left of the chart--to make sure the chart is age and gender appropriate for the child you're measuring.
Step 5
Find your child's age along the bottom of the appropriate chart. Trace directly up from this point until you intersect with his BMI, located to either side of the chart. Mark a small dot here. Take note of which percentile line the dot falls on, or which percentile lines it's between.
Step 6
Compare your child's BMI percentile to the CDC's standards. If he's under the 5th percentile line, he is underweight. If he's between the 5th and 85th percentile lines he's a healthy weight. If he's between the 85th and 95th percentile lines he's overweight, and being at or above the 95th percentile indicates that he is obese.
Tips and Warnings
- To calculate BMI if you've measured your child's height in inches and his weight in pounds, first square his height; so if he's 40 inches tall, his height squared would be 40 x 40 = 1600 inches squared. Next, multiple this number by 703. Finally, divide his weight in pounds by the final product of your calculations above. The resulting number is his BMI. If you don't want to do the BMI calculations yourself, you can use an online BMI Calculator (see Resources).
Things You'll Need
- Measuring tape or yardstick
- Scale
- Pencil
- Paper
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