How to Calculate BMI: Formula, Charts, and What It Means

Body Mass Index (BMI) is one of the most widely used screening tools in medicine — and one of the most misunderstood. This guide covers exactly how it's calculated, what the numbers mean, and where it falls short.

What Is BMI?

BMI is a simple ratio of weight to height, designed to give a quick estimate of whether someone's weight falls in a healthy range for their height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet, long before modern medicine.

The formula produces a single number, typically between 15 and 40 for most adults, which is then compared against defined categories.

The BMI Formula

Metric (kg and cm)

BMI = (weight (kg)) / (height (m)^2)

Example: A person weighing 70 kg at 175 cm tall:

Convert height to metres: 175 ÷ 100 = 1.75 m
Square the height: 1.75² = 3.0625
Divide weight by height²: 70 ÷ 3.0625 = 22.9

Imperial (lbs and inches)

BMI = (weight (lbs) × 703) / (height (in)^2)

The factor 703 converts the result from lb/in² to the standard kg/m² scale.

Example: A person weighing 154 lbs at 68 inches (5'8") tall:

154 × 703 = 108,262
68² = 4,624
108,262 ÷ 4,624 = 23.4

BMI Weight Categories

| BMI Range | Category | |-----------|----------| | Below 18.5 | Underweight | | 18.5 – 24.9 | Normal weight | | 25.0 – 29.9 | Overweight | | 30.0 and above | Obese |

These categories were defined by the World Health Organization (WHO) and are used globally as a first-pass screening tool.

How to Calculate BMI by Hand

Step 1: Measure your weight and height accurately. Morning weight (before eating) and height measured barefoot give the most consistent readings.

Step 2: Convert units if needed. For metric, convert height from cm to m by dividing by 100. For imperial, ensure weight is in lbs and height is in total inches (not feet and inches separately — 5'8" = 68 inches).

Step 3: Apply the formula. Square your height, then divide your weight by that number.

Step 4: Compare to the chart above.

BMI by Age and Sex

The standard BMI categories apply to adults aged 18–65. For children and teenagers, BMI is calculated the same way but compared against age- and sex-specific percentile charts, since body fat and growth patterns vary significantly during development.

For adults over 65, some researchers argue the overweight range (25–27.9) may actually be associated with better health outcomes — a phenomenon sometimes called the "obesity paradox" in older adults.

Limitations of BMI

BMI is a useful population-level screening tool, but it has well-documented limitations as an individual health metric:

It doesn't distinguish muscle from fat. A highly muscular athlete may have a BMI in the "overweight" range despite very low body fat. Conversely, someone with a normal BMI may have high body fat if they have little muscle mass.

It doesn't account for fat distribution. Abdominal fat (around the waist) is far more metabolically dangerous than fat stored in the thighs or hips. Two people with identical BMI scores can have very different health profiles depending on where fat is stored.

It varies by ethnicity. Research shows that people of Asian descent tend to have higher health risks at lower BMI values, which has led some health organisations to recommend lower cut-off points (23 for overweight, 27.5 for obese) for Asian populations.

It ignores body frame and bone density. A large-framed person naturally weighs more than a small-framed person at the same height.

Better Alternatives to BMI

Waist circumference — abdominal fat is a stronger predictor of metabolic risk than total fat mass
Waist-to-hip ratio — divides waist measurement by hip measurement; values above 0.90 (men) or 0.85 (women) indicate elevated risk
Body fat percentage — measured via DEXA scan, bioelectrical impedance, or skinfold calipers; more accurate but requires equipment
Waist-to-height ratio — some researchers consider this the single best simple measure; a ratio above 0.5 indicates elevated risk regardless of sex or ethnicity

Calculate Your BMI Now

Use our free BMI calculator to get your result instantly, see your weight category, and compare against WHO ranges. The calculator supports both metric and imperial units.