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Body Composition

Obesity Calculator

Calculate your BMI and determine your WHO obesity class (Underweight, Normal, Overweight, Class I, II, or III Obesity) with health risk assessment and personalised weight loss targets. Supports kg/lbs and metric/imperial.

BMI & Obesity Class Calculator

WHO classification · Health risk · Weight targets

Unit System
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Body Mass Index

BMI is a useful population-level screening tool, but has limitations for individuals — particularly muscular athletes or those with very high or low height. For a comprehensive picture, use this alongside a body fat percentage calculator and waist-to-height ratio calculator.

WHO BMI Classification

BMIClassificationHealth Risk
< 18.5UnderweightModerate (malnutrition risk)
18.5 – 24.9Normal WeightLow
25.0 – 29.9OverweightIncreased
30.0 – 34.9Obesity Class IHigh
35.0 – 39.9Obesity Class IIVery High
≥ 40.0Obesity Class III (Severe)Extremely High

Understanding Obesity Classification in Practice

BMI-based obesity classification is a tool for population screening and clinical communication — not a precise individual diagnostic. The same BMI can reflect very different body compositions depending on muscle mass, age, sex, and ethnicity. Asian, South Asian, and Latin American populations have different metabolic risk thresholds; many researchers advocate lower cut-points for these groups. In clinical practice, BMI is always interpreted alongside waist circumference, blood biomarkers, and lifestyle factors.

Frequently Asked Questions

WHO classifies obesity into 3 classes: Class I (BMI 30–34.9) — elevated risk, commonly managed with lifestyle changes; Class II (BMI 35–39.9) — high risk, may require medical intervention; Class III (BMI ≥40) — also called "severe" or "morbid" obesity, associated with dramatically increased risk of cardiovascular disease, type 2 diabetes, sleep apnoea, joint problems, and early mortality. Class III typically qualifies for bariatric surgery consideration.
Even a 5–10% reduction from current body weight produces significant health improvements: blood pressure, blood sugar, cholesterol, and joint load all improve meaningfully. You don't need to hit "normal" BMI to get health benefits — improving from Class II to Class I obesity is clinically significant. Sustainable rate of fat loss: 0.5–1% of body weight per week.
No — BMI is poorly suited to athletes with above-average muscle mass. A 90 kg athlete at 170 cm/5'7" who is 10% body fat has a BMI of 31 (obese by WHO criteria) but is clearly not obese. For athletes, body fat percentage, waist circumference, and functional health markers are far more informative than BMI. BMI is designed for population-level health screening, not individual assessment.
No — the BMI result and obesity class will be identical regardless of which unit you choose. Select Metric (kg/cm) or Imperial (lbs/in) from the toggle. The calculator converts pounds to kg and inches to cm internally before computing BMI. The classification thresholds are universal — a BMI of 30 is Class I obesity in both the metric and imperial calculation.

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