How to Calculate Your BMI (Formula, Chart and What It Means)

Body Mass Index, or BMI, is probably the most quoted health number in the world. Doctors use it, insurance forms ask for it, and fitness apps display it — yet most people have never actually calculated it themselves or stopped to ask what it really measures. This guide walks through the formula, the categories, and (just as important) what BMI can't tell you.

The BMI formula

BMI is simply your weight divided by the square of your height. In metric units:

BMI = weight (kg) ÷ height² (m²)

For example, someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.

If you think in pounds and inches, the imperial version adds a conversion factor:

BMI = 703 × weight (lb) ÷ height² (in²)

You can skip the arithmetic entirely with our free BMI calculator, which supports both unit systems and shows your category instantly.

What the BMI categories mean

The World Health Organization groups adult BMI values into four broad bands:

• Below 18.5 — underweight
• 18.5 to 24.9 — healthy weight
• 25.0 to 29.9 — overweight
• 30.0 and above — obesity

These bands exist because, across large populations, the risk of conditions such as type 2 diabetes, high blood pressure and heart disease rises as BMI moves above the healthy range. That's why BMI is called a screening tool: it flags when a closer look might be worthwhile.

Where BMI falls short

BMI has one big blind spot: it only knows your height and weight. It cannot tell the difference between muscle and fat. That leads to a few well-known limitations:

1. Athletes score "overweight." Muscle is denser than fat, so a lean rugby player can easily land in the overweight band while carrying very little body fat.
2. It ignores where fat is stored. Fat around the abdomen carries more health risk than fat on the hips — waist circumference captures this, BMI doesn't.
3. Age and ethnicity matter. Older adults naturally lose muscle, and some health bodies use lower cut-offs for people of South Asian descent because risk rises at lower BMI values.

Better numbers to track alongside BMI

Rather than fixating on a single figure, pair BMI with one or two of these:

• Waist circumference — a tape measure around your middle is one of the cheapest, most predictive health checks available.
• Waist-to-height ratio — keeping your waist under half your height is a widely used rule of thumb.
• Trends over time — a single measurement is noise; the direction over months is signal.

The bottom line

BMI is a quick, free, population-level screening number — genuinely useful, as long as you treat it as a starting point rather than a verdict. Calculate yours, note the category, and if it sits outside the healthy band (or your waist measurement is creeping up), that's a good prompt for a conversation with a healthcare professional — not a reason to panic.

This article is for general information only and is not medical advice. Always consult a qualified professional about your individual health.

Working it out step by step

The arithmetic is short, and nearly every mistake happens in one of two places: squaring the wrong thing, or mixing units.

Metric, with a worked example at 1.68 m and 62 kg:

1. Take your height in metres, not centimetres: 168 cm becomes 1.68 m.
2. Square it: 1.68 × 1.68 = 2.8224.
3. Divide your weight in kilograms by that: 62 ÷ 2.8224 = 21.97.

Imperial, at 5 ft 4 in and 137 lb:

1. Convert height entirely to inches: (5 × 12) + 4 = 64 in.
2. Square it: 64 × 64 = 4,096.
3. Divide weight in pounds and multiply by 703: (137 ÷ 4,096) × 703 = 23.51.

Where the 703 comes from

It is not arbitrary. It converts pounds-per-square-inch into kilograms-per-square-metre so the imperial result lands on the same scale as the metric one. A pound is 0.45359237 kg and an inch is 0.0254 m, so the factor is 0.45359237 ÷ 0.0254², which works out at 703.07. Rounding it to 703 changes a typical BMI by less than 0.01.

The measurement matters more than the maths

BMI is sensitive to height, because height is squared. An error of 2 cm in height shifts a typical adult BMI by roughly 0.5 — more than most people expect, and enough to cross a category boundary near a threshold.

• Measure height without shoes, standing straight, heels together, looking forward.
• Weigh yourself at a consistent time, ideally morning, before eating. Body weight fluctuates by 1–2 kg across a day from food and fluid alone.
• Use the same scale. Two scales can disagree by a kilogram, which matters more than the third decimal place of your calculation.

The practical consequence: treat a BMI near a boundary as "around the boundary" rather than definitively one side of it. The measurement error is comparable to the width of the gap.

What to do with the number

A BMI figure is a prompt for a question, not an answer. It is a population screening indicator, and it cannot distinguish muscle from fat, tell you where fat is carried, or account for age, ethnicity or pregnancy — our full BMI guide covers those limits and what measures work better alongside it.

If your figure sits outside the healthy range, or has moved noticeably, that is worth discussing with a doctor who can interpret it with everything BMI leaves out. This page is educational and is not medical advice.

Sources and limitations

• WHO — Obesity and overweight: BMI classification.
• CDC — Adult BMI calculator and categories.
• NHS — BMI healthy weight calculator.

BMI is a population screening measure. It does not distinguish fat from muscle, and thresholds differ for children, older adults, athletes and some ethnic groups (WHO suggests lower cut-offs for many Asian populations). It is not a diagnosis — discuss results with a health professional.

For the full picture, our complete BMI guide covers the ranges and the limitations. Working in imperial? Kilograms to pounds and metres to feet convert your measurements first.