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How to Calculate BMI — Formula, Chart & Examples

The Body Mass Index (BMI) is an internationally recognized anthropometric metric developed by Belgian mathematician Adolphe Quetelet. Used widely by the World Health Organization (WHO), physicians, and insurance providers, BMI offers a fast, non-invasive screening tool to evaluate whether an adult's weight falls within a healthy range relative to their height. This guide covers the mathematical BMI formula in metric and imperial units, details the official WHO weight category chart, provides worked calculation examples, highlights key clinical limitations, and shows you how to use our free BMI calculator.

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The BMI Formula

Calculating BMI requires only your weight and height. Depending on your preferred system of measurement, use one of the following two standard formulas: ════════════════════════════════════════════════════════ METRIC FORMULA (kg/m²): Formula: BMI = Weight (kg) ÷ Height (m)² Example: Weight = 70 kg, Height = 1.75 m Calculation: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.86 Result: BMI = 22.9 kg/m² (Normal weight) IMPERIAL FORMULA (lb/in²): Formula: BMI = (Weight (lbs) ÷ Height (in)²) × 703 Example: Weight = 154 lbs, Height = 69 in (5 ft 9 in) Calculation: (154 ÷ (69 × 69)) × 703 = (154 ÷ 4761) × 703 = 0.032346 × 703 = 22.74 Result: BMI = 22.7 lb/in² (Normal weight) ════════════════════════════════════════════════════════ Note on Centimeters: If measuring height in centimeters (e.g. 175 cm), divide by 100 first to get meters (1.75 m), or calculate: Weight (kg) ÷ (Height (cm) ÷ 100)².

BMI Categories Table (WHO Standard)

The World Health Organization (WHO) classifies adult Body Mass Index into standardized clinical categories to evaluate associated health and metabolic risks:

Category	BMI Range (kg/m²)	Health Risk Profile
Severe Underweight	< 16.0	Elevated risk of nutrient deficiencies and osteoporosis
Moderate Underweight	16.0 – 16.9	Increased vulnerability to illness and low stamina
Mild Underweight	17.0 – 18.4	Slightly below optimal nutritional status
Normal Weight	18.5 – 24.9	Lowest statistical risk for cardiovascular conditions
Overweight (Pre-obese)	25.0 – 29.9	Moderate risk; increased strain on joints and heart
Obese Class I	30.0 – 34.9	High risk of type 2 diabetes and hypertension
Obese Class II	35.0 – 39.9	Very high risk of metabolic and cardiac disease
Obese Class III (Severe)	≥ 40.0	Extremely high risk of chronic morbidity

BMI Calculation Examples

Here are three step-by-step examples demonstrating how real-world numbers translate into BMI categories: Person 1 (Healthy Normal Weight): • Parameters: Weight = 65 kg, Height = 1.70 m • Math: 65 ÷ (1.70 × 1.70) = 65 ÷ 2.89 = 22.49 • Classification: BMI 22.5 → Normal Weight (Optimal health zone) Person 2 (Overweight Classification): • Parameters: Weight = 90 kg, Height = 1.75 m • Math: 90 ÷ (1.75 × 1.75) = 90 ÷ 3.0625 = 29.39 • Classification: BMI 29.4 → Overweight (Approaching Class I Obesity) Person 3 (Lean Normal Weight): • Parameters: Weight = 55 kg, Height = 1.65 m • Math: 55 ÷ (1.65 × 1.65) = 55 ÷ 2.7225 = 20.20 • Classification: BMI 20.2 → Normal Weight

BMI Limitations & Clinical Considerations

While BMI is a reliable population-level indicator, it has recognized clinical limitations for individual diagnosis: 1. Muscle vs. Fat Mass: BMI cannot distinguish between dense muscle tissue and adipose tissue. Muscular athletes and bodybuilders often score in the 'obese' range despite possessing low body fat. 2. Age Differences: Older adults naturally experience bone density changes and sarcopenia (muscle loss), meaning a slightly higher BMI (25–27) may correlate with better health outcomes in seniors. 3. Gender Differences: Women naturally carry a higher percentage of essential body fat than men at identical BMI values. 4. Fat Distribution: BMI fails to evaluate visceral abdominal fat (waist circumference), which is the primary driver of cardiovascular and metabolic complications.

How to Use Our BMI Calculator

Determine your BMI and target healthy weight in seconds:

Step 1: Choose unit system

Select Metric (kg, cm) or Imperial (lbs, feet/inches) depending on your preference.

Step 2: Enter height and weight

Type your current weight and height into the respective input fields.

Step 3: Review BMI score

View your instant BMI score, highlighted WHO category badge, and ideal weight interval.

Step 4: Copy or track results

Copy your result to track personal fitness milestones or discuss with a physician.

Frequently Asked Questions

What is a healthy BMI?

According to the World Health Organization (WHO), a healthy BMI for adults is between 18.5 and 24.9 kg/m². This interval is statistically associated with the lowest risk of developing cardiovascular disease and type 2 diabetes.

How accurate is BMI?

BMI is an accurate general screening tool for 90% of the population. However, it does not distinguish between muscle and body fat, making it less accurate for competitive athletes, bodybuilders, pregnant women, and elderly individuals.

What is the BMI formula in kg and cm?

To calculate BMI using centimeters, divide height in cm by 100 to get meters, then divide weight in kilograms by height squared: BMI = kg ÷ (cm ÷ 100)². Example: 70kg and 175cm = 70 ÷ (1.75)² = 22.86.

What BMI is considered obese?

A BMI of 30.0 kg/m² or higher is clinically classified as obese. Obesity is sub-categorized into Class I (30.0–34.9), Class II (35.0–39.9), and Class III or severe obesity (40.0 and above).

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