About this tool
Body Mass Index (BMI) is a standard heuristic proxy calculation used to categorise individuals into distinct body mass segments based on their total height and weight parameters. It was developed by Belgian mathematician Adolphe Quetelet in the 19th century and has since been adopted by the World Health Organisation (WHO) as a primary screening tool for identifying underweight, normal, overweight, and obese ranges.
Mathematical BMI Metric Formulation
BMI evaluates individual body dimensions using the standard metric formula:
When compiling using non-metric Imperial parameters (pounds and inches), the formula adds a scaling offset coefficient:
WHO Classification Ranges
- Below 18.5 — Underweight
- 18.5 – 24.9 — Normal Weight (healthy range)
- 25.0 – 29.9 — Overweight
- 30.0 and above — Obese
All calculations run strictly client-side inside your browser sandbox. None of your height, weight, or personal physiological metrics are ever stored or transmitted to external servers.
Frequently asked questions
Everything you need to know about BMI Calculator.
What BMI range is considered healthy for adults?
The World Health Organization defines: Underweight below 18.5, Normal weight 18.5–24.9, Overweight 25–29.9, and Obese 30+. These are population-level guidelines — individual healthy weight ranges depend on age, sex, muscle mass, and ethnicity. Asian health organisations use lower thresholds (overweight ≥ 23, obese ≥ 27.5).
Does BMI account for muscle mass and body composition?
BMI does not distinguish between fat and muscle. A heavily muscular athlete may have a BMI in the 'overweight' range despite low body fat. Conversely, a person with low muscle mass and high fat ('skinny fat') can have a normal BMI but be metabolically at risk. BMI is a population screening tool, not a complete individual health assessment.
How does the metric BMI calculation differ from the imperial formula?
Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = 703 × weight (lb) ÷ height (in)². The constant 703 is the unit conversion factor. Both formulas produce identical results — the calculator accepts either unit system.
Is BMI equally accurate for all body types and ethnicities?
No. BMI was derived from 19th-century European male data and has documented limitations for other groups. It underestimates health risk for East Asian, South Asian, and some other populations. For older adults, a slightly higher BMI (25–27) may be associated with better outcomes. Waist circumference and body fat percentage are more accurate individual measures.
What should I do if my BMI falls outside the healthy range?
Consult a healthcare professional before making any significant dietary or exercise changes. BMI is a starting point for discussion, not a diagnosis. A doctor can order more specific tests (body composition scan, metabolic panel) to give a complete picture of your health.