BMI Calculator

Free online BMI calculator

This BMI calculator works out your Body Mass Index from your height and weight and tells you which category you fall into — underweight, normal, overweight or obese. It supports both metric and imperial units.

What BMI tells you

Body Mass Index is a quick way to check whether your weight is in a healthy range for your height. It is widely used by doctors and health organisations as an initial screening measure. Remember that BMI does not account for muscle mass, so very muscular people may read as “overweight” despite low body fat.

The BMI formula

• Metric: BMI = weight (kg) ÷ height (m)².
• Imperial: BMI = 703 × weight (lb) ÷ height (in)².

This calculator runs entirely in your browser, so your measurements stay private and it works even without an internet connection. It is intended for general information and is not a substitute for professional medical advice.

What is BMI and how to calculate it (with its limits)

BMI — Body Mass Index — is the number your doctor mentions, your gym quotes, and health articles cite endlessly. It is genuinely useful as a rough screening tool, and it is also widely misunderstood and over-interpreted. This guide covers how to calculate it, what the categories actually mean, and — just as importantly — the situations where BMI gives a misleading answer.

What BMI actually measures

BMI is a simple ratio of your weight to your height. The idea is that taller people are naturally heavier, so raw weight tells you very little on its own. Dividing by height squared produces a number that is roughly comparable across people of different heights.

It is important to be clear about what it is not: BMI does not measure body fat, fitness, or health. It is a single number derived from two other numbers. Its value is that it is cheap, fast and needs nothing but a scale and a tape measure — which makes it excellent for screening large populations, and much blunter for judging any one individual.

The formula

Metric: BMI = weight (kg) ÷ height (m)²

Someone who is 70 kg and 1.75 m tall: 1.75² = 3.0625, and 70 ÷ 3.0625 = BMI 22.9.

Imperial: BMI = (weight in pounds ÷ height in inches²) × 703

The 703 is simply the conversion factor that makes pounds and inches produce the same number the metric formula would.

The most common arithmetic error is forgetting to square the height, or using centimetres instead of metres. Height must be in metres: 175 cm is 1.75 m, not 175.

The categories

For adults, the standard World Health Organization ranges are:

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

These boundaries are worth treating as soft. There is no meaningful health difference between a BMI of 24.9 and 25.1, despite them landing in different boxes. The categories describe a gradient, not a cliff edge.

Where BMI gets it wrong

This is the part most BMI pages skip, and it matters — because for a lot of people, BMI gives a genuinely misleading answer.

• It cannot tell muscle from fat.Muscle is considerably denser than fat, so a muscular person weighs more for the same body size. This is why many athletes — rugby players, sprinters, weightlifters — are formally classified “overweight” or even “obese” while carrying very little body fat. The formula sees weight; it cannot see what the weight is made of.
• It ignores where fat is stored. This is medically significant. Fat around the abdomen (visceral fat) carries far more health risk than fat on the hips and thighs. Two people with identical BMIs can have very different risk profiles. This is why waist circumference is often a better single predictor than BMI.
• It does not adjust for age. Older adults naturally lose muscle and bone density, so an identical BMI can mean more body fat at 70 than at 30.
• The thresholds are not universal. Health risk begins at a lower BMI for people of South Asian descent, and several countries use a lower overweight threshold accordingly. The standard cut-offs were derived largely from European populations.
• It does not apply to children as-is.Children's BMI must be compared against age- and sex-specific percentile charts, not adult categories.
• It is not valid during pregnancy.

So is BMI useless?

No — but it should be read as a starting point, not a verdict.

For most people who are not highly muscular, BMI does correlate reasonably well with body fat, and a BMI far outside the healthy range is a legitimate signal worth paying attention to. It is cheap enough to screen entire populations, which is exactly the job it was designed for.

Where it fails is being treated as a precise measure of an individual's health. A far better picture comes from combining it with waist measurement, blood pressure, blood tests, activity level and how you actually feel.

Better measures to consider alongside it

• Waist-to-height ratio. Keep your waist under half your height. Simple, and a better predictor of metabolic risk than BMI for many people.
• Waist circumference on its own, which directly captures abdominal fat.
• Body fat percentage, which measures what BMI can only guess at — though accurate measurement needs proper equipment.

This article is general information, not medical advice. Discuss any health concerns with a qualified professional.

Next steps

Pair your BMI calculation with the body fat calculator and unit converter for a complete picture.