⚖️ BMI Calculator

How it works

Body Mass Index is a simple ratio of your weight to your height, used worldwide as a quick screening tool for weight-related health risk. Doctors, insurers, and researchers use it because it is cheap, consistent, and works reasonably well across large populations — it flags who might need a closer look, not who is definitively healthy or unhealthy.

This calculator works in both metric (kilograms and centimetres) and imperial (pounds and inches), and places your result in the standard categories: under 18.5 is underweight, 18.5–24.9 is the healthy range, 25–29.9 is overweight, and 30 and above is obese. Remember that BMI cannot see the difference between muscle and fat, so treat it as a starting point for a conversation with your doctor, not a diagnosis.

The formula

Metric: BMI = weight in kilograms / (height in metres)^2. Imperial: BMI = 703 × weight in pounds / (height in inches)^2. Both formulas give the same number — the 703 is just a conversion factor so the imperial units line up with the metric scale.

Worked example

Someone weighs 80 kg and stands 180 cm (1.80 m) tall. Square the height: 1.80 × 1.80 = 3.24. Divide the weight: 80 / 3.24 ≈ 24.7. That lands inside the 18.5–24.9 healthy range. In imperial terms, the same person weighs about 176 lb and is about 71 inches tall: 703 × 176 / (71 × 71) = 703 × 176 / 5041 ≈ 24.5 — the same answer, give or take rounding.

Frequently asked questions

I lift weights and my BMI says overweight. Is that wrong?
Possibly. Muscle is denser than fat, so very muscular people can score as overweight or even obese while being lean. Athletes and regular lifters should pair BMI with waist measurement or body-fat estimates.

Does BMI work for kids or older adults?
Not directly. Children are assessed against age-and-sex percentile charts, not adult cutoffs, and for older adults a slightly higher BMI is often fine — muscle loss with age shifts what the number means.

What should I do with my result?
Use it as a prompt, not a verdict. A result outside the healthy range is a good reason to check blood pressure, blood sugar, and cholesterol with your doctor and talk about sustainable habits — not a reason to panic or crash-diet.

How to read your BMI result

BMI places adults into four bands: underweight below 18.5, healthy weight from 18.5 to 24.9, overweight from 25 to 29.9, and obese at 30 or above. Treat the band as a screening signal, not a verdict. The number knows only your height and weight, so it cannot tell fat from muscle, it cannot see where fat is carried, and it applies the same cutoffs to a 25 year old and a 70 year old. Fat around the waist raises health risk more than fat on the hips, which is why waist size adds useful context: risk climbs above roughly 80 cm for women and 94 cm for men. Muscular people are routinely pushed into the overweight band while carrying little fat, and some people with a healthy BMI carry high body fat with low muscle, especially as they age. Health risk also rises at lower BMI values in many South Asian and East Asian populations than the standard cutoffs suggest. For children and teenagers the adult bands do not apply at all; their BMI is read against age and sex percentiles.

A worked example

Take a woman of 34 who is 168 cm tall and weighs 74 kg. Her BMI is 74 divided by 1.68 squared, which comes to 26.2. That puts her in the overweight band, near its lower end. The healthy band for her height runs from about 52 kg to about 70 kg, so the top of the band sits roughly 4 kg below her current weight. That framing is more useful than the label: she is not being told she is far from healthy, she is being shown the size of the gap. If she also measured her waist and it came to 88 cm, the two numbers together would say more than either alone, because both point the same way. If instead she lifted weights four times a week and her waist measured 74 cm, the same BMI of 26.2 would deserve a very different reading.

Common mistakes

• Guessing your height. Because height is squared in the formula, a 2 cm error at 168 cm moves the result by more than half a point. Measure against a wall, without shoes.
• Weighing at random times of day and reacting to the noise. Food, water and salt can swing body weight by 1 to 2 kg in a day without any change in body fat.
• Reading BMI as a diagnosis. It says nothing about blood pressure, blood sugar, fitness or family history, all of which a clinician weighs alongside it.
• Applying the adult cutoffs to children, teenagers or during pregnancy, where the bands simply do not apply.
• Assuming a healthy BMI guarantees good health, or that an overweight BMI guarantees poor health. Both shortcuts fail regularly.

Frequently asked questions

Is a BMI of 27 dangerous?

Not by itself. A BMI of 27 sits in the overweight band, which flags higher average risk across populations, but your personal risk depends on waist size, fitness, blood pressure, blood sugar and family history. Plenty of people at 27 are metabolically healthy, and some at 23 are not.

Why does BMI say athletes are overweight?

Because the formula sees only height and weight, and muscle weighs more than the same volume of fat. A rugby player at 105 kg and 180 cm scores around 32, deep into the obese band, while carrying very little fat. Waist size and body fat tests give a fairer picture for muscular builds.

How often should I check my BMI?

Monthly is plenty. Body weight naturally moves 1 to 2 kg across a day and across a week with water, salt and food, so daily BMI changes are noise. Weigh under the same conditions, same time, same scale, and judge the direction over a month or more.

Do men and women use the same BMI scale?

Yes, the formula and the adult cutoffs are identical for men and women. The catch is body composition: at the same BMI, women naturally carry a higher share of body fat than men, so the same number does not always mean the same thing for health.