BMI Calculator
Body mass index from height and weight — metric or imperial — with the standard WHO weight category for adults.
BMI is the screening number behind a surprising amount of clinical and billing documentation. It supports obesity and overweight diagnoses, quality-measure reporting, anesthesia and surgical risk assessment, and coverage criteria for weight-management treatment. This calculator takes metric or imperial measurements and returns the BMI to one decimal place with its WHO category.
Calculate BMI
WHO adult BMI categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity — class I |
| 35.0 – 39.9 | Obesity — class II |
| 40.0 and above | Obesity — class III |
The formulas: metric — BMI = weight (kg) ÷ height (m)²; imperial — BMI = 703 × weight (lb) ÷ height (in)². The adult cut-points above do not apply to children and adolescents, whose BMI is interpreted as a percentile for age and sex, and they mislead at the extremes of muscle mass — athletes flagged as "overweight" and older adults with sarcopenia passed as "healthy" are the classic examples. In the chart, BMI pairs with a specific ICD-10 Z68 code by range, and obesity diagnoses drive risk adjustment and coverage decisions; our medical coding support team checks that the documented BMI, the Z code, and the assessment agree before the claim goes out.
Disclaimer: This calculator is educational only and is not medical advice. It does not replace clinical judgment or a full patient assessment. BMI is a screening measure; interpret it in the context of the individual patient.
Frequently asked questions
How is BMI calculated?
In metric units, BMI is weight in kilograms divided by height in metres squared. In imperial units, it is 703 times weight in pounds divided by height in inches squared. Both formulas give the same index, which is then read against the WHO adult categories.
Can I use this BMI calculator for a child?
No. The categories here are the adult WHO cut-points. For children and adolescents, BMI must be interpreted as a percentile on CDC growth charts for the child's age and sex — the same numeric BMI can be healthy for one age and concerning for another.
Why does the chart need a BMI if it is only a screening measure?
Documented BMI supports the ICD-10 Z68 code billed with the visit, feeds quality measures (such as adult BMI screening and follow-up), and is a required data point in coverage criteria for many weight-related treatments and procedures. A missing or inconsistent BMI is a common, avoidable documentation gap.