How to Calculate BMI and What Your Number Actually Means

What Is BMI?

Body Mass Index (BMI) is a number calculated from your height and weight. It's used by doctors, nurses, and health organisations worldwide as a quick screening tool for weight-related health risks.

Despite its limitations, BMI remains the most practical large-scale health screening tool available — it requires only two measurements and correlates reasonably well with body fat percentage at a population level.

The BMI Formula

BMI is calculated as:

For example: weight 75kg, height 1.75m → BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5

In imperial units: BMI = (weight in lbs × 703) ÷ height in inches²

BMI Ranges for Adults

The WHO classification system (used in the UK):

• Below 18.5 — Underweight. May indicate malnutrition or an underlying health condition.
• 18.5–24.9 — Healthy weight. Associated with lowest health risks at population level.
• 25–29.9 — Overweight. Increased risk of some conditions; lifestyle changes often recommended.
• 30–34.9 — Obese (Class I). Significant increased risk of type 2 diabetes, heart disease, and some cancers.
• 35–39.9 — Obese (Class II). High risk; medical intervention usually recommended.
• 40+ — Severely obese (Class III). Very high risk; specialist care typically required.

BMI for Different Ethnicities

Research shows that people of Asian, Black African, and some other ethnic backgrounds have higher health risks at lower BMI values. NHS guidance uses adjusted thresholds for South Asian, Chinese, and other Asian populations:

• Overweight: BMI 23+ (vs 25+ for white European populations)
• Obese: BMI 27.5+ (vs 30+)

These adjusted thresholds are important — someone of South Asian heritage with a BMI of 24 may be at similar health risk to a white European with a BMI of 26–27.

What BMI Doesn't Tell You

BMI is a useful screening tool but has significant limitations:

• It ignores muscle — a muscular athlete can have a BMI in the "overweight" or even "obese" range despite having very low body fat. Many professional rugby players fall into this category.
• It ignores fat distribution — where you carry fat matters. Visceral fat (around your organs) is more dangerous than subcutaneous fat (under the skin). Waist circumference is a better predictor of metabolic health than BMI alone.
• It treats men and women the same — women naturally carry more body fat than men at the same BMI, which isn't reflected in the standard ranges.
• It ignores age — older adults naturally carry more body fat and may have more muscle loss, meaning the same BMI represents different health risks at different ages.

Better Indicators to Use Alongside BMI

• Waist circumference — high risk: over 94cm (men) or 80cm (women)
• Waist-to-height ratio — aim for waist circumference less than half your height
• Body fat percentage — measured by DEXA scan, bioelectrical impedance, or calipers
• Blood pressure, cholesterol, and blood sugar — direct markers of metabolic health

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