How to Calculate BMI — and What Your Score Really Means

BMI — Body Mass Index — is the most widely used health screening tool in the world. Doctors, nutritionists, and health insurers all use it. This guide explains exactly how to calculate your BMI manually, what your result means, and where the formula has limitations.

The BMI Formula

BMI is calculated using a simple formula that divides your weight by the square of your height. There are two versions depending on whether you use metric or imperial measurements:

How to Calculate BMI Step by Step

Using Metric Units (kg and cm)

1. Measure your height in metres — divide your height in centimetres by 100. Example: 175 cm ÷ 100 = 1.75 m
2. Square your height — multiply it by itself. Example: 1.75 × 1.75 = 3.0625
3. Divide your weight by the result — Example: 70 kg ÷ 3.0625 = 22.9
4. Your BMI is 22.9 — look up the category in the table below

Using Imperial Units (lb and inches)

1. Convert your height to inches — multiply feet by 12 then add inches. Example: 5’9″ = (5 × 12) + 9 = 69 inches
2. Square your height in inches — Example: 69 × 69 = 4,761
3. Divide your weight in pounds by the result — Example: 154 ÷ 4,761 = 0.03234
4. Multiply by 703 — Example: 0.03234 × 703 = 22.7
5. Your BMI is 22.7

BMI Categories — What Your Score Means

The World Health Organization (WHO) defines four main BMI categories for adults aged 18 and over:

BMI for Different Groups

BMI for Women

The standard WHO BMI categories apply equally to men and women. However, women naturally carry a higher percentage of body fat than men at the same BMI. A BMI of 22 in a woman typically corresponds to around 28–32% body fat, while the same BMI in a man corresponds to around 20–24% body fat.

This means the BMI scale may slightly underestimate health risks in women, particularly post-menopause when fat distribution shifts toward the abdomen.

BMI for Men

Men tend to carry more muscle mass than women, which means BMI may overestimate body fat in muscular men. A male athlete with 10% body fat and significant muscle mass may score a BMI of 27 — technically “overweight” — despite being in excellent health.

BMI for Older Adults (Over 65)

For adults over 65, some research suggests a slightly higher BMI range (22–27) may be associated with better outcomes. This is partly because older adults with lower BMI may have lost muscle mass, which is a risk factor independent of body fat.

BMI for Different Ethnicities

The WHO standard BMI thresholds were developed primarily from data on European populations. Research shows that people of South Asian, East Asian, and some other ethnic backgrounds have higher body fat percentages at the same BMI compared to European populations. The WHO has proposed lower thresholds for these groups:

• Overweight threshold for South Asian and East Asian adults: BMI 23.0 (vs 25.0 in standard WHO)
• Obese threshold: BMI 27.5 (vs 30.0 in standard WHO)

BMI Examples at Different Heights

To give you a quick reference, here are the healthy weight ranges (BMI 18.5–24.9) for common heights:

The Limitations of BMI

BMI is a useful population-level screening tool, but it has well-documented limitations when applied to individuals:

1. BMI Does Not Measure Body Fat Directly

BMI measures weight relative to height — it cannot distinguish between fat mass and muscle mass. A rugby player or bodybuilder may have a BMI of 28–30 (technically “overweight”) while having very low body fat and excellent cardiovascular health.

2. BMI Does Not Account for Fat Distribution

Where you carry fat matters as much as how much you carry. Abdominal fat (visceral fat around the organs) carries significantly higher health risks than fat stored in the hips and thighs. Two people with identical BMIs can have very different health risk profiles depending on fat distribution.

Waist circumference is a useful supplement to BMI: men with a waist over 94 cm (37 inches) and women with a waist over 80 cm (31.5 inches) have elevated cardiometabolic risk regardless of BMI.

3. BMI Does Not Account for Age-Related Changes

As people age, muscle mass tends to decrease and fat mass tends to increase even if body weight stays the same. An older adult may have a “healthy” BMI while carrying excess fat — a phenomenon sometimes called “sarcopenic obesity.”

Better Alternatives to BMI

For a more complete picture of body composition and health risk, BMI is often used alongside:

• Waist-to-height ratio — waist circumference should be less than half your height. Simple and surprisingly accurate at predicting cardiometabolic risk.
• Waist circumference — directly measures abdominal fat. High risk: men over 102 cm (40 in), women over 88 cm (35 in)
• Body fat percentage — measured via DEXA scan, underwater weighing, or bioelectrical impedance. More accurate but less accessible than BMI.
• Waist-to-hip ratio — divides waist measurement by hip measurement. Associated with cardiovascular risk.

Frequently Asked Questions

The Bottom Line

BMI is calculated by dividing your weight in kilograms by the square of your height in metres. It is a simple, free, and widely validated screening tool — but it is not a measure of health. A BMI between 18.5 and 24.9 is considered healthy by WHO standards, but muscular individuals, older adults, and people of certain ethnicities may need to interpret their results differently.

Use BMI as a starting point, not a final verdict. Combine it with waist circumference, body fat percentage, and regular health checks for a complete picture.