This BMI calculator for kids estimates the body mass index according to child’s age, gender, height and weight in English or metric in order to offer the right BMI range. Read below more on the adaptation for children of the body mass formulas and also get an example of a weight calculation.

Gender: *
Child’s age: *
Child’s weight: *
Child’s height: *
Gender: *
Child’s age: *
Child’s weight: *
Child’s height: *

How does the BMI calculator for kids work?

This BMI calculator for kids returns the body mass index (BMI) value based on child’s age, gender, height and weight; together with the child’s weight status category and with the percentile range values. The tool has two tabs that are designed to make the calculations for the two measurements systems, English and metric. In order to determine the BMI percentile it needs the height, weight and age. What you need to take into consideration is that any small error that could occur during the measurements will lead to a large error in the results and therefore to an inaccurate determination. You should also consider which measurement you want to use and stick to it, so you take the weight in kg and the height in cm or the weight in lbs and the height in inches. The weight tool above can be used for children and teens with ages ranging from 2 to 19 years old.

Body Mass Index for children

The BMI is generically known as an index provided to determine the harmonious or not relationship between the weight and the height. However, when it comes to children, these index needs to be refined as to take into consideration the ratios of development for each age. Not to mention the gender as we all know that there are important differences in the way girls and boys develop, in regard to mass and height.

So the body mass index for kids gets calculated using the same types of measurements as the adult version but then the result is compared to typical values of the development of children at the same age. This is where BMI percentile calculation takes place, when comparing the result by age and gender.

How to calculate BMI for the young?

The form above provides both the BMI value and the corresponding BMI for age percentile on a CDC BMI for age growth chart. What there is to keep in mind is that a percentile below 5 means underweight while everything that is above 95 means obese. There is also a distinction made between 85 and 95 for overweight.

The basic BMI calculation isobtained through dividing the weight in kilograms by the square of the height in meters. BMI= mass (kg)/height (m)^2 but then this result needs to be compared with the appropriate tables for the age and gender of the child.

Example BMI calculation

Gender: girl

Age: 7 years and 3 months

Weight: 40 lbs

Height:  48 inches


Child’s BMI: 12.2

Child’s weight status category: Underweight

Child’s percentile is below 3rd percentile

Interpreting the results

This useful tool for all parents and pediatric specialists will return not only the mass index value but also the child’s weight status category according to the percentile range in which the BMI stands for the given case. This works as a great guideline to assess the current weight of a child or teen.

  • BMI 1st to 4th percentile: Underweight
  • BMI 5th to 84th percentile: Healthy Weight
  • BMI 85th to 94th percentile: Overweight
  • BMI 95th to 100th percentile: Obese

Why is childhood BMI important?

Using the BMI percentile in assessing your child’s weight and height can help you compare his or her development to the general tables and therefore to other children but is also related to important health risks. What it makes this specific calculation appropriate is that it uses other data to compare and determine whether the child is underweight, in a normal weight range or overweight. What parents should at all times remember that everything that is below the 5th percentile or above the 85th percentile can involve a certain health risk and that we shouldn’t focus just on obesity but also trigger some alarm in cases of mild underweight with nutritional deficiencies or mild overweight that can later on lead to obesity.

BMI implications and other studies

Studies say that six out of ten children and teens that have a BMI value placed above the 95th percentile are at risk of developing a cardiovascular disease at some point in their lives. From those six, two are said to have 2 or three risk factors for developing such life threatening conditions. Going below this percentile, the range between 85 and 94, which is currently considered as overweight also has similar health risk factors. It is worth mentioning that overweight children have a greater chance of becoming overweight or even obese adults.

Some other study has proven there is a connection between maternal employment and child BMI as a non standard work schedule for the mother and more working hours are said to be in correlation to higher BMI values for those children. However, such implications and other similar are still to be considered.


1) Kopelman PG. (2005) Clinical obesity in adults and children. Blackwell Publishing
2) WHO Multicentre Growth Reference Study Group. (2006) WHO Child Growth Standards: Length/height-for-age, weight-for-age, weight-for-length, weight-for-height and body mass index-for-age: Methods and development. Geneva: World Health Organization.
3) Speiser PW, Rudolf MC, Anhalt H et al. (2005) Childhood obesity. J. Clin. Endocrinol. Metab. 90 (3): 1871–87
4) Morrissey TW, Dunifon RE, Kalil A. (2011) Maternal employment, work schedules, and children's body mass index. Child Dev; 82(1):66-81.

21 Dec, 2014