How to Calculate BMI — And What It Really Means
BMI is one of the most widely used health screening tools in the world. Learn how it is calculated, what the numbers mean, and when to look beyond the number.
Body Mass Index (BMI) is a simple calculation that uses your height and weight to estimate whether you are in a healthy weight range. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has been used by health professionals ever since as a fast, low-cost screening tool.
The BMI Formula
The formula is straightforward: divide your weight in kilograms by the square of your height in meters. So if you weigh 70 kg and are 1.75 m tall, your BMI is 70 ÷ (1.75 × 1.75) = 22.9. If you work in pounds and inches, the formula is: (weight in pounds ÷ height in inches²) × 703.
BMI Categories
The World Health Organization (WHO) defines four adult BMI categories that apply to most populations:
- Underweight: BMI below 18.5
- Normal weight: BMI 18.5 – 24.9
- Overweight: BMI 25.0 – 29.9
- Obese: BMI 30.0 and above
These ranges are the same for men and women, though some research suggests the thresholds may differ slightly by ethnicity. For example, health organizations in Asia often use lower thresholds — overweight starting at 23 — because populations of Asian descent tend to carry more body fat at a given BMI.
What BMI Does Not Tell You
BMI is a screening tool, not a diagnosis. It cannot distinguish between fat mass and lean mass, which means a very muscular athlete might register as 'overweight' despite having low body fat. Conversely, an older adult with little muscle but excess fat might fall in the 'normal' range — a condition sometimes called 'normal-weight obesity'.
BMI also says nothing about where fat is stored. Abdominal (visceral) fat — the kind that accumulates around the organs — carries significantly higher health risks than fat stored under the skin elsewhere. Waist circumference measurements alongside BMI give a more complete picture of metabolic risk.
When BMI Is Useful
Despite its limitations, BMI remains valuable precisely because it is free, instant, and requires no equipment. At the population level, BMI correlates strongly with rates of diabetes, cardiovascular disease, and all-cause mortality. At the individual level, it is a reasonable starting point for a conversation with a healthcare professional.
If your BMI falls outside the normal range, it is worth discussing with your doctor — especially if combined with other risk factors like high blood pressure, elevated blood glucose, or a family history of metabolic disease. Used in context, BMI is a helpful flag, not a final verdict.
Tips for a More Complete Picture
- Measure your waist circumference: above 88 cm (women) or 102 cm (men) increases health risk regardless of BMI
- Track trends over time rather than focusing on a single reading
- Consider body fat percentage testing (DEXA scan, skinfold calipers) for a more precise assessment
- Use BMI as one data point among many, not the only measure of health
Calculate Your BMI Now
Use our free BMI Calculator to get your result in seconds. Just enter your weight and height — no sign-up, no email required. You will also find a calorie calculator, body fat calculator, and more in our Health & Fitness section.