Baby BMI Calculator (0–5 yrs): WHO Percentile & Z-Score

How Is Baby BMI Calculated?

Body Mass Index (BMI) for babies and children is calculated with the same basic formula as for adults — weight in kilograms divided by height in metres squared (kg/m²) — but the result is interpreted completely differently. A single BMI number means little on its own for a child. What matters is how that number compares with other children of the same age and sex, expressed as a percentile and a z-score.

Our calculator uses the WHO Child Growth Standards (2006) for ages 0–5 years. For each age and sex, the WHO publishes three numbers — L (skew), M (median) and S (variation), known as the LMS method — that convert your child's BMI into a z-score (how many standard deviations they sit from the median), which then maps to a percentile.

How Do I Work Out My Baby's BMI?

BMI = weight (kg) ÷ [height (m) × height (m)].
Z-score = compare that BMI with the WHO median for your child's exact age and sex using the LMS values.
Percentile = the share of healthy children of the same age and sex whose BMI is lower than your child's.

Worked Example

Suppose you have a 12-month-old boy who weighs 10 kg and is 75 cm long:

Height in metres = 0.75 m, so 0.75 × 0.75 = 0.5625 m²
BMI = 10 ÷ 0.5625 = 17.8 kg/m²
Against the WHO median BMI for a 12-month-old boy (≈17.1), this gives a z-score of about +0.5
That places him near the 68th percentile — comfortably in the healthy weight range

BMI Categories for Children (WHO Standards)

Category	Z-score	Approx. percentile
Severely underweight	Below −3	Below 0.1%
Underweight	−3 to −2	0.1% – 2.3%
Healthy weight	−2 to +1	2.3% – 84%
Possible risk of overweight	+1 to +2	84% – 97.7%
Overweight	+2 to +3	97.7% – 99.9%
Obese	Above +3	Above 99.9%

CategorySeverely underweight

Z-scoreBelow −3

Approx. percentileBelow 0.1%

CategoryUnderweight

Z-score−3 to −2

Approx. percentile0.1% – 2.3%

CategoryHealthy weight

Z-score−2 to +1

Approx. percentile2.3% – 84%

CategoryPossible risk of overweight

Z-score+1 to +2

Approx. percentile84% – 97.7%

CategoryOverweight

Z-score+2 to +3

Approx. percentile97.7% – 99.9%

CategoryObese

Z-scoreAbove +3

Approx. percentileAbove 99.9%

Most healthy children fall between the 2nd and 98th percentiles. A single reading outside that band is not a diagnosis — the trend over time matters far more than any one measurement.

Length vs. Height: Why It Matters

For children under 2 years, the WHO standards expect length measured lying down (recumbent). From 2 years onward they expect height measured standing. The difference is small but real (lying length runs slightly longer than standing height), so use the method that matches your child's age for the most accurate percentile. This calculator switches the label automatically.

Is BMI Accurate for Babies Under 2?

BMI-for-age is a recognised screening tool from birth under the WHO standards, but it is exactly that — a screen, not a diagnosis. In the first two years, weight-for-length is often used alongside BMI-for-age, and rapid changes are normal as babies go through growth spurts. Always interpret a single number in the context of your child's overall growth curve, feeding, and development.

Frequently Asked Questions

How do I calculate BMI for a baby?

Divide weight in kilograms by height in metres squared (kg/m²), then compare that BMI against the WHO median for your child's exact age and sex. For example, a baby weighing 10 kg at 75 cm has a BMI of 10 ÷ (0.75 × 0.75) = 17.8. Our calculator does this and the percentile conversion for you.

Why is BMI different for children than adults?

Children's BMI is interpreted using age and sex-specific percentiles because body composition changes naturally as they grow. A healthy BMI for a 6-month-old differs from a 3-year-old, so fixed adult cut-offs do not apply.

What is a healthy BMI percentile for a baby or toddler?

A z-score between -2 and +1 (roughly the 2nd to 84th percentile) is generally considered healthy under WHO standards. Most healthy children fall between the 2nd and 98th percentiles, and the trend over time matters more than any single reading.

What does a BMI z-score mean?

A z-score shows how many standard deviations your child's BMI is from the median for their age and sex. Zero is exactly average; +2 to +3 indicates overweight and above +3 indicates obesity, while -2 to -3 indicates underweight.

Is BMI accurate for babies under 2 years old?

BMI-for-age is a recognised WHO screening tool from birth, but it is a screen, not a diagnosis. Under 2 years, length is measured lying down and weight-for-length is often used alongside BMI. Always read a single value in the context of the whole growth curve.

When should I be concerned about my child's BMI?

Consult your pediatrician if your child's z-score is below -2 or above +2, or if their growth curve shifts sharply across percentiles. A single out-of-range reading is not a diagnosis on its own.

Methodology and Sources

This calculator implements the WHO Child Growth Standards: BMI-for-age (0–60 months). Z-scores are derived with the WHO LMS method, and values beyond ±3 SD use the WHO-recommended restricted adjustment so extreme readings stay realistic. The category cut-offs (overweight above +2 SD, obese above +3 SD, wasting below −2 SD) follow WHO and are consistent with American Academy of Pediatrics (AAP) guidance on growth monitoring. For children over 5, the CDC growth charts become the standard reference.

Important Considerations

BMI is one screening indicator, not a verdict on health. Genetics, prematurity, muscle versus fat, feeding patterns and overall development all shape a child's growth. This tool provides general guidance based on population standards and cannot replace a professional medical evaluation.

Talk to your pediatrician if your child's z-score is below −2 or above +2, if their growth curve shifts sharply across percentiles, or if you have any concerns about their weight, feeding or development.

Baby BMI Calculator