How to Calculate Your BMI — and What It Really Tells You

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.