How to Accurately Measure BMI in Pediatric Patients
To accurately measure BMI in children, obtain precise height and weight measurements using standardized equipment and technique, calculate BMI using the formula weight(kg)/height(m)², then plot the result on age- and sex-specific CDC growth charts (for children ≥24 months) or WHO growth charts (for children <24 months) to determine percentile ranking. 1
Measurement Technique: The Foundation of Accuracy
Weight Measurement
- For infants and young children unable to stand: Use an infant scale with the child completely undressed, place clean paper liner in tray, calibrate to zero, and record weight to nearest 0.1 kg 1
- For children who can stand independently: Have child wear light clothing without footwear, calibrate scale to zero, position child in center of platform with feet flat and heels touching, standing as erect as possible, and record to nearest 0.1 kg 1
Height/Length Measurement
- Recumbent length (children up to ~24 months or unable to stand): Use infant stature board with fixed headboard and moveable footboard perpendicular to table surface, requires two measurers—one holds crown of head against fixed headboard with external auditory meatus and lower eye orbit margin perpendicular to table, second measurer grasps ankles and gently straightens legs while bringing footboard firmly against heels with toes pointing upward, record to nearest 0.1 cm 1
- Standing height (children able to stand unassisted): Use fixed stadiometer attached to wall, have child remove shoes and stand facing away from wall with heels together, back straight, arms down, heels/buttocks/shoulders/head touching wall, axis of vision horizontal with external auditory meatus and lower orbit margin aligned horizontally, place head projection at crown, perform 3 measurements within 0.2 cm of each other and use the average, record to nearest 0.1 cm 1
Critical pitfall: Measurement error is the primary source of BMI inaccuracy in children—adequate attention to data collection, appropriate equipment selection, and regular training and standardization of measurers are essential 2
BMI Calculation
Use one of these formulas 1:
- BMI = weight (kg) / height (m)²
- BMI = weight (kg) / height (cm) / height (cm) × 10,000
- BMI = weight (lb) / height (in) / height (in) × 703
Age- and Sex-Specific Interpretation: The Critical Step
BMI values are meaningless without age and sex adjustment in children because BMI varies dramatically with age—it increases during first months of life, decreases after first year, then increases again around age 6 years. 1
Growth Chart Selection
- Children <24 months: Plot on WHO growth standards 1, 3, 4
- Children ≥24 months through 20 years: Plot on CDC 2000 growth charts 1, 3, 4
Classification Thresholds
- BMI ≥95th percentile: Classified as obese 1
- BMI 85th-94th percentile: Classified as overweight (at risk of overweight in older terminology) 1
- BMI <5th percentile: May indicate undernutrition or underlying medical condition 3
The American Medical Association, CDC, and Maternal and Child Health Bureau expert committee concluded that BMI is sufficient for screening children because it can be easily calculated from height and weight, correlates strongly with direct measures of body fat (especially at higher BMI values), associates only weakly with height, and identifies those with highest body fat correctly with acceptable accuracy, particularly above the 85th percentile. 1
Important Limitations and Caveats
BMI Accuracy Issues
- Tends to underestimate overweight in tall individuals 1
- Tends to overestimate overweight in short individuals and those with high lean body mass (athletes) 1
- Generally correlates well with precise adiposity measures in individuals with BMI ≥95th percentile 1
- Recent research demonstrates that conventional BMI may not be valid for children younger than 17 years due to complex multi-scaling properties of weight-for-height relationships during growth 5
What BMI Does NOT Measure
- BMI cannot differentiate fat mass from fat-free (lean) mass—among overweight children, higher BMI can result from increased either fat or fat-free mass 6
- Alternative measures (skinfolds, waist circumference, bioelectrical impedance) are not recommended for routine clinical practice as there is little evidence they are sufficiently practicable or provide appreciable added information beyond BMI 2
- Direct body composition measures (DEXA, underwater weighing, MRI, CT) provide accurate fat mass estimates but are used mainly in research settings, not routine clinical care 1
Proxy Measures Should Be Avoided
Self-reported or parent-reported height and weight are much less preferred and should only be used with caution and cognizance of the limitations, biases, and uncertainties attending these measures. 2
Serial Measurements: Growth Velocity Matters
Growth velocity over time using multiple data points is more informative than single measurements and should be used in conjunction with medical and family history to assess appropriate growth. 3
- Plot all measurements on appropriate growth charts to visualize trends over time 3, 4
- Serial measurements every 3-6 months are recommended for tracking weight trajectory and response to interventions 3
- A child consistently tracking along a specific percentile curve (even if low) may be growing appropriately, whereas crossing downward through percentile lines indicates true growth faltering 3