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BMI Calculator for Kids

Health

Estimate your child's BMI category by age using a simplified pediatric BMI-for-age approximation — includes a healthy weight guide for kids 2 to 19.

Unit System
Age
yrs
Sex
Height
cm
Weight
kg

Estimate only — not a clinical percentile. This tool uses a simplified age-adjusted approximation, not the official CDC/WHO BMI-for-age growth charts. For an accurate percentile, consult a pediatrician or use the official CDC growth chart tools.

Child's BMI

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What is a Kids BMI?

A BMI Calculator for Kids estimates a child's body mass index (BMI) and places it in context using an age-adjusted reference value, rather than the fixed adult BMI bands used by a standard BMI Calculator. BMI itself is calculated the same way for children as for adults — weight in kilograms divided by height in meters squared — but interpreting that number is very different for a growing child. A BMI of 17 might be perfectly healthy for a 9-year-old but underweight for a 17-year-old, because children's body composition changes substantially as they grow from toddlerhood through adolescence.

Clinically, pediatricians use CDC or WHO growth charts that track BMI-for-age percentiles built from large population datasets, broken down by age in months and by sex. Replicating those exact lookup tables is out of scope for a simple web calculator, so this tool instead uses a transparent, simplified approximation: it estimates how the median "healthy" BMI rises roughly linearly with age (from about 15.5 at age 2 to about 21 at age 19), applies a small adjustment for sex, and then compares your child's actual BMI to that reference as a ratio. The result is labeled clearly as an estimate, not a clinical percentile, and parents are encouraged to also check an Ideal Weight Calculator or consult their pediatrician for an official assessment.

How to use this Kids BMI calculator

  1. Choose a Unit System — Metric (cm, kg) or Imperial (ft/in, lbs) — using the toggle at the top of the form.
  2. Enter your child's Age in years, from 2 to 19.
  3. Select Sex (Girl or Boy) using the toggle — this applies a small adjustment to the reference BMI.
  4. Enter Height in centimeters, or feet and inches if using Imperial units.
  5. Enter Weight in kilograms, or pounds if using Imperial units.
  6. Review the result card: the large number is your child's BMI, the badge beneath it shows the estimated category, and the panel below shows the age-adjusted reference BMI used for comparison.
  7. Read the disclaimer note and treat the result as a starting point — for an accurate percentile, consult a pediatrician or an official CDC/WHO growth chart.

Formula & Methodology

Step 1 — Standard BMI:

BMI = weight (kg) ÷ [height (m)]²

Step 2 — Age-adjusted reference BMI (simplified estimate):

referenceBmiMedian = 15.5 + ((age − 2) ÷ 17) × (21 − 15.5)

This models median BMI-for-age rising roughly linearly from about 15.5 at age 2 to about 21 at age 19. A small sex offset is then applied: +0.3 for boys, −0.3 for girls.

Step 3 — Estimated category from the ratio:

ratio = BMI ÷ referenceBmiMedian

| Ratio | Estimated category |
|---|---|
| < 0.80 | Underweight (below ~5th percentile, estimated) |
| 0.80 – 1.10 | Healthy weight (estimated ~5th–85th percentile) |
| 1.10 – 1.25 | Overweight (estimated ~85th–95th percentile) |
| > 1.25 | Obese (estimated above ~95th percentile) |

Worked example: A 10-year-old girl who is 138 cm tall and weighs 32 kg has a BMI of 32 ÷ (1.38)² ≈ 16.8. The reference BMI is 15.5 + ((10 − 2) ÷ 17) × 5.5 − 0.3 ≈ 17.7. The ratio is 16.8 ÷ 17.7 ≈ 0.95, which falls in the "healthy weight" estimated range.

Important: this methodology is a simplified linear approximation designed for quick, directional estimates. It is not the CDC's official BMI-for-age percentile calculation, which relies on detailed age-in-months, sex-specific LMS growth-chart data. For clinical decisions, always consult a pediatrician or the official CDC growth charts.

Frequently Asked Questions

A BMI calculator for kids estimates a child's body mass index and compares it against an age-adjusted reference to suggest a weight category. Unlike an adult BMI calculator, it accounts for the fact that healthy BMI values change significantly as children grow, so a BMI of 18 might be normal for a 15-year-old but high for a 6-year-old. This tool uses a simplified age-adjustment formula, not the full CDC growth-chart percentile system.
The BMI formula itself is the same for children and adults: weight in kilograms divided by height in meters squared. The difference is in interpretation — a child's raw BMI number is compared against an age- and sex-specific reference value rather than the fixed adult bands of 18.5, 25, and 30, because what counts as healthy shifts as kids grow taller and add lean mass.
No. The official CDC BMI-for-age percentile uses detailed growth-chart lookup tables built from large population datasets for each age in months and each sex. This calculator uses a simplified linear approximation of how median BMI rises with age, which is useful for a quick estimate but is not a substitute for the CDC or WHO growth charts a pediatrician uses.
The standard [BMI Calculator](/bmi-calculator/) uses fixed adult bands (underweight, normal, overweight, obese) that apply the same cutoffs regardless of age. This kids-focused version adjusts the reference point by age — and optionally by sex — because a healthy BMI for a 4-year-old looks very different from a healthy BMI for a 17-year-old.
Age is used to compute an age-adjusted reference BMI, since median BMI rises steadily from early childhood through the teenage years. Sex is used for a small adjustment, since research shows boys and girls tend to track slightly different average BMI curves during growth, though this tool applies only a simplified offset rather than full sex-specific growth charts.
Select a unit system, enter your child's age between 2 and 19 years, choose their sex, then enter height and weight. The calculator instantly shows the estimated BMI, the age-adjusted reference BMI, and an estimated weight category badge — all of which update automatically as you adjust any field.
Yes, the tool supports ages 2 through 19, which covers the full pediatric-to-adolescent range typically tracked by growth charts. For children younger than 2, standard BMI-for-age tracking generally is not used, and weight-for-length charts are used instead.
The categories — underweight, healthy weight, overweight, and obese — are estimated by comparing your child's BMI to the age-adjusted reference BMI as a ratio. A ratio close to 1.0 suggests a value near the estimated healthy midpoint, while ratios notably below 0.80 or above 1.25 suggest the estimated category shifts toward underweight or obese respectively.
No. This tool provides a quick, simplified estimate for general awareness only. A pediatrician uses official CDC or WHO growth charts, tracks growth trends over multiple visits, and considers other factors like growth velocity and family history, all of which this calculator cannot replicate.
Because this calculator uses a simplified age-adjustment formula rather than the precise CDC LMS percentile tables your pediatrician references, results can differ — especially near category boundaries. Always treat the output here as a general estimate and defer to your child's growth chart from their doctor's office for clinical decisions.
Not necessarily. BMI does not distinguish between muscle, bone, and fat, so an athletic or muscular child can show a higher BMI without carrying excess body fat. This is one of several reasons this tool's output should be treated as an estimate rather than a diagnosis.
Treat any borderline or concerning result as a prompt to discuss growth with your child's pediatrician rather than as a diagnosis. A doctor can plot official percentiles, review growth history over time, and recommend next steps if genuinely needed.
Also known as
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