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BMI for Kids

BMI for Kids: calculated with the published clinical method, with result interpretation. A reference tool that does not replace a doctor's advice.

ℹ Reference calculation based on the published method. It does not replace a clinical examination or a doctor's decision.

BMI for Kids

Body weightkg
Heightcm
Ageyears
Girl (1 = yes)
BMI
16.7966kg/m²
IOTF cut-offs (Cole et al., BMJ 2000) correspond to adult BMI 25 and 30Underweight is not assessed here before age 18: in children it is determined with WHO/CDC percentiles for age and sexBMI for Kids: calculated with the published clinical method, with result interpretation. A reference tool that does not replace a doctor's advice.
Assessment (IOTF)
Below the overweight cut-off
Overweight cut-off
17.9kg/m²
Obesity cut-off
20.6kg/m²

Adult BMI categories don't work for children, because a healthy BMI changes as a child grows. This calculator finds your child's BMI and checks it against the international IOTF cut-offs for their exact age and sex.

How the calculation works

The calculator first computes standard body mass index: weight in kilograms divided by height in metres squared. It then compares the result with the cut-offs of the International Obesity Task Force (IOTF), published by T. J. Cole and colleagues in the BMJ in 2000. These curves were built from growth data in six countries and, for every age, give the BMI value that tracks into an adult BMI of 25 (overweight) or 30 (obesity) at age 18.

The cut-offs are tabulated every half year for boys and girls separately, and the calculator interpolates linearly between them. The inputs are body weight in kg (to 0.1 kg), height in cm, age in years (2 to 10, in steps of 0.5) and a Girl switch (1 = girl, 0 = boy). Working in pounds and inches? 1 lb = 0.4536 kg and 1 in = 2.54 cm.

You get the BMI, the overweight and obesity cut-offs for that age and sex, and an assessment: below the overweight cut-off, overweight or obesity. The tool does not classify underweight in children; that is judged with WHO or CDC growth percentiles.

Worked example

A 7-year-old boy weighs 25 kg and is 122 cm tall. BMI = 25 / 1.22² = 16.8 kg/m². For boys aged 7 the IOTF overweight cut-off is 17.9 and the obesity cut-off is 20.6, so 16.8 falls below both and the result reads “below the overweight cut-off”. At the same height but 28 kg, BMI would be 18.8, which is in the overweight range.

Things to keep in mind

  • Enter age to the nearest half year. The cut-offs for 7 and 7.5 years differ enough to change a borderline result.
  • Measure height without shoes against a wall and weigh in light clothing, ideally in the morning. A 2 cm error in height shifts BMI by roughly half a unit.
  • Pediatricians in the US usually plot BMI on CDC percentile charts (85th and 95th percentiles). IOTF cut-offs are a separate international standard, so the label may occasionally differ near the boundary.
  • A single reading says little; the trend on a growth chart over several years matters more.
  • This is a reference tool, not a diagnosis. Talk to your child's doctor before changing their diet.

More about: BMI for Kids

What it calculates

The “BMI for Kids” calculator computes BMI in kg/m² from 4 parameters: body weight (kg), height (cm), age (years), girl (1 = yes).

Used by professionals and patients to assess health metrics, dosages, and risks.

Example calculation

With parameters Body weight = 25 kg, Height = 122 cm, Age = 7 years, Girl (1 = yes) = 0 the result is 16.8 kg/m² (IOTF cut-offs (Cole et al., BMJ 2000) correspond to adult BMI 25 and 30).

How to use

  1. Enter body weight, height, age and girl (1 = yes) — each field above is adjustable with a slider.
  2. BMI (kg/m²) is calculated automatically as you type.
  3. Check the worked example below to see the formula applied to real numbers.
  4. Copy the result or bookmark this calculator.

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FAQ

Why can't I use the adult BMI chart for my child?
In childhood BMI naturally falls until about age 5–6 and then rises again. A BMI of 18 signals overweight in a five-year-old but is normal in a teenager, so cut-offs must depend on age and sex.
What ages does this calculator cover?
Children from 2 to 10 years. Older children and teens need cut-offs for those ages, where boys and girls diverge more.
Does “below the overweight cut-off” mean my child's weight is fine?
It means the child is not overweight or obese by IOTF criteria. It does not rule out underweight, which this tool does not assess; ask your pediatrician to check the percentile charts if your child seems thin.
How is IOTF different from CDC percentiles?
CDC charts label children at or above the 85th and 95th percentiles of a US reference population, while IOTF links child cut-offs to the adult values of 25 and 30. Both usually agree, but they can disagree for borderline cases.

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