How It Works
BMI (Body Mass Index) is a simple weight-to-height ratio defined as mass in kilograms divided by height in meters squared: BMI = kg/m². It's a screening tool — not a diagnosis — that bins adult weight into Underweight (< 18.5), Normal (18.5–24.9), Overweight (25–29.9), and Obese (≥ 30). Enter your weight and height in any supported unit (kg/lb for weight, cm/in/ft for height) and the calculator converts to metric internally before applying the formula.
Example Problem
A 30-year-old adult is 175 cm tall and weighs 70 kg. Calculate the body mass index and identify the WHO classification.
- Convert height to meters: 175 cm / 100 = 1.75 m.
- Square the height: 1.75 m × 1.75 m = 3.0625 m².
- Divide weight by height squared: 70 kg / 3.0625 m² ≈ 22.9 kg/m².
- Compare to the WHO categories: 22.9 falls in 18.5–24.9 → Normal weight.
Key Concepts
BMI was developed in the 1830s by Adolphe Quetelet as a population-level statistic, not a body-fat measure. It correlates with body fat percentage on average but doesn't distinguish muscle from fat, so it misclassifies athletes and people with dense bone structure as overweight even when their body fat is healthy. WHO ranges apply to adults aged 20+ of average build; the CDC uses age- and sex-specific percentile charts for children and teens. For populations of South Asian descent, lower thresholds (e.g., overweight at ≥ 23) are recommended by several health bodies.
Applications
- Initial weight-status screening in primary care.
- Eligibility checks for clinical trials and population studies.
- Tracking weight trends over time in epidemiological research.
- Calibrating drug doses where therapeutic windows depend on body size.
- Setting baseline metabolic numbers before computing BMR and TDEE.
Common Mistakes
- Mixing units — entering height in centimeters but weight in pounds without converting first. The calculator handles unit conversion automatically if you set both selectors correctly.
- Interpreting BMI as a direct body-fat measurement. It's a ratio of mass to height, not adiposity.
- Applying adult thresholds to children, teens, or pregnant women. Use age-specific growth charts instead.
- Reading 0.1 increments as meaningful clinically — BMI doesn't carry that precision; round to whole numbers when comparing to category cutoffs.
- Forgetting that high muscle mass can push BMI into the overweight or obese range without indicating health risk.
Frequently Asked Questions
How do you calculate BMI?
Divide weight in kilograms by height in meters squared: BMI = kg / m². If you have imperial units, the equivalent is BMI = 703 × pounds / inches², but it's easier to let a calculator convert internally.
What is the formula for BMI?
BMI = weight(kg) ÷ height(m)². Example: a 70 kg person at 1.75 m has BMI = 70 / (1.75)² = 70 / 3.0625 ≈ 22.9.
What BMI is considered healthy?
The WHO classifies 18.5–24.9 as normal (healthy) weight for adults aged 20+. Below 18.5 is underweight; 25–29.9 is overweight; 30 and above is obese.
Is BMI accurate for muscular people?
No. BMI doesn't distinguish muscle from fat, so very muscular people (athletes, bodybuilders) often land in the overweight or obese range despite low body fat. Body-fat percentage or waist-to-height ratio is a better screen for that population.
What's the difference between BMI for adults and children?
Adult BMI uses fixed cutoffs (18.5, 25, 30). Children and teens are evaluated against age- and sex-specific percentile curves from the CDC or WHO — a child with BMI 20 might be at the 85th percentile (overweight) at one age and the 50th percentile (normal) at another.
Does BMI vary by ethnicity?
Yes. Health authorities recommend lower thresholds for some populations (e.g., overweight ≥ 23 and obesity ≥ 27.5 for adults of South Asian descent) because cardiometabolic risk rises at lower BMI in those groups. Check your country's clinical guidelines.
Related Calculators
- BMR Calculator — basal metabolic rate via the Harris-Benedict equation
- Body Fat Percentage Calculator — estimate body fat from total weight and lean body mass
- TDEE Calculator — total daily energy expenditure across activity levels
- Weight Loss Calculator (Comprehensive) — BMI, BMR, body fat, and TDEE in one tool
- Heart Rate Calculator — max HR, reserve, and Karvonen target zones
Related Sites
- Medical Equations — Hemodynamic, pulmonary, and dosing calculators
- InfantChart — Baby and child growth percentile charts
- Hourly Salaries — Hourly wage to annual salary converter
- Dollars Per Hour — Weekly paycheck calculator with overtime
- Percent Error Calculator — Calculate percent error between experimental and theoretical values
- CameraDOF — Depth of field calculator for photographers