What is BMI for children?

BMI stands for Body Mass Index. It’s a quick ratio that relates weight to height. The math is the same for kids and adults, but the interpretation is different for children. That’s because kids are growing — their body composition changes with age, and boys and girls often develop at different rates.

So when someone says “child BMI,” they usually mean two steps: (1) calculate BMI from height and weight, then (2) compare that BMI to a BMI-for-age percentile chart for the child’s exact age and sex.

• BMI = weight (kg) ÷ [height (m)]²
• Example: 32 kg and 1.35 m → BMI = 32 ÷ (1.35²) ≈ 17.6

• BMI = 703 × weight (lb) ÷ [height (in)]²
• The factor 703 converts pounds/inches into the same scale as kg/m².

The BMI number alone does not “diagnose” anything. It’s a screening tool — a quick way to notice potential patterns. For children, what matters is where that number sits compared to other kids of the same age and sex.

How this calculator works

This page is designed to be fast and practical. Here’s what happens when you tap “Calculate Child BMI”:

• Step 1: Convert your inputs into a consistent unit system.
• Step 2: Calculate BMI using the standard BMI formula.
• Step 3: Show a kid-friendly interpretation guide (percentile ranges) and practical next steps.
• Step 4: Enable quick sharing (WhatsApp, Telegram, Twitter/X, Facebook, LinkedIn, Copy, Native Share).

A viral little trick: parents often share a screenshot of the result card in family group chats or with coaches. That’s why the result box is formatted to look good as a screenshot.

Note: We don’t estimate BMI percentiles automatically in this version, because it requires official growth chart tables (which vary by month of age and sex). Instead, we make the “what next” step extremely clear so the BMI number is still useful.

Worked examples (so you can sanity-check)

A 9-year-old child (sex doesn’t change the BMI math) is 135 cm tall and weighs 32 kg. Convert height: 135 cm = 1.35 m. Compute BMI:

• BMI = 32 ÷ (1.35²) = 32 ÷ 1.8225 ≈ 17.6

Next step: take BMI 17.6 + age 9 + sex to a BMI-for-age chart to find the percentile. Depending on the percentile, it may fall in the healthy range (5th to <85th), but charts are the final word.

A 12-year-old is 4 ft 10 in tall and weighs 95 lb. Convert height to inches: (4×12) + 10 = 58 in. Compute BMI:

• BMI = 703 × 95 ÷ (58²) = 66785 ÷ 3364 ≈ 19.9

Again, BMI 19.9 is a number — you then compare it to a BMI-for-age chart for a 12-year-old boy or girl.

Two kids can have the same BMI but different interpretations because they’re different ages. A BMI of 19 might be typical for an older teen but high for a much younger child. That’s why pediatric growth charts use percentiles: they “normalize” the BMI by age and sex.

How to interpret a child’s BMI (the practical way)

If you only remember one thing: look at patterns over time, not a single BMI reading. Growth is not linear — kids grow in spurts, appetite can change, activity shifts with seasons, and puberty changes body composition.

• Measure consistently: similar time of day, similar clothing, same scale if possible.
• Use the same units: switching between cm/in and kg/lb can cause confusion.
• Track trends: compare every few months, not daily.
• Consider context: recent illness, sports season, growth spurt, sleep, appetite changes.
• Use percentiles: BMI-for-age percentiles are the standard for children and teens.

If a child’s percentile is shifting quickly (for example, crossing multiple major percentile lines), or if you see concerns like fatigue, dizziness, poor growth, or rapid weight change, it’s worth talking to a pediatrician. BMI is just one screening signal — clinicians consider many factors.

Frequently Asked Questions

• Is BMI accurate for children?

  BMI is a useful screening tool, but it’s not a perfect measure of body fat. It does not distinguish muscle from fat, and it can’t account for individual growth patterns. For kids, BMI is used with age/sex percentiles, plus clinical context.

• Why can’t we use adult BMI categories for kids?

  Adult cutoffs (like 18.5–24.9) assume fully grown bodies. Children’s height, muscle, and body composition change rapidly. Percentiles compare a child to peers of the same age and sex, which is why pediatric guidelines use BMI-for-age.

• What ages is this for?

  BMI-for-age percentiles are commonly used for children and teens ages 2 through 19. This calculator accepts ages 2–20 for convenience, but pediatric charts are typically 2–19.

• What if my child is athletic?

  Athletic kids can carry more lean mass (muscle), which can increase BMI without implying excess fat. That’s why you shouldn’t interpret BMI alone — percentiles and overall health indicators matter.

• How often should we calculate BMI?

  For most families, every 2–3 months is plenty. Weekly or daily checks can create unnecessary stress and doesn’t reflect true long-term growth patterns.

• Can this calculator diagnose underweight or obesity?

  No. This is an educational tool. Diagnosis and treatment decisions should be made by qualified health professionals using percentiles, growth history, and additional clinical information.

• What’s the best “next step” after I get the BMI number?

  Use the BMI number with age and sex on a BMI-for-age percentile chart (or ask your pediatrician). If you’re worried, bring the BMI number and measurements to your next visit and ask about the trend.