Body Mass Index (BMI) is a ratio of weight to height-squared — a quick population-level screening tool for categorising weight ranges. The formula is weight (kg) ÷ height (m)². While it is widely used, BMI does not account for muscle mass, bone density, age, or fat distribution — it is a screening indicator, not a clinical diagnosis.
The WHO BMI classifications for adults: Below 18.5 — Underweight; 18.5–24.9 — Normal weight; 25.0–29.9 — Overweight; 30.0 and above — Obese (subdivided into Class I, II, III). These thresholds were calibrated on predominantly white European populations and may underestimate risk for Asian populations (where the cut-off for overweight is often applied at 23) and overestimate risk for people of Black African heritage. Many health authorities now treat BMI as one data point among several.
BMI is a quick screening tool, not a health measure. A bodybuilder with low body fat can have a high BMI due to muscle mass. Conversely, someone with a "normal" BMI can still have high body fat. Use it alongside waist circumference, blood pressure, and clinical assessment.
The standard range is 18.5–24.9 for most adults. Some guidelines lower the overweight threshold to BMI 23 for East Asian populations. Talk to a healthcare provider for personalised guidance.
Children are still growing, so BMI must be interpreted against age and sex percentile charts rather than fixed thresholds. This calculator is for adults (18+).
BMI = weight (kg) ÷ (height in metres)². In imperial: BMI = 703 × weight (lb) ÷ (height in inches)².
See also the BMR Calculator for metabolic rate, Calorie Calculator for daily energy needs, and Body Fat Calculator.
⚠ Disclaimer: This calculator is for informational and educational purposes only. Results are estimates and should not be relied upon as financial, medical, or professional advice. Always consult a qualified professional before making financial or health decisions.