BMI Calculator

Use this free BMI calculator to check your body mass index in seconds. Enter your height and weight — in metric or imperial units — and you will instantly see your BMI number, your BMI category, and the healthy weight range for your height. This bmi calculator works for adult men and women aged 20 and over, and it also includes a reverse BMI tool that tells you what weight you need to reach any target BMI you choose.

Here is a quick example before you start. An adult who is 180 cm tall and weighs 80 kg has a BMI of 24.7, which sits inside the healthy weight range of 59.9 to 80.7 kg for that height. Your own result may be higher or lower — this page gives you a clear, honest starting point in under a minute, free, with no sign-up. Simply check your numbers below, then read on to learn what your result really means.

What Is BMI (Body Mass Index)?

BMI, short for body mass index, is a simple ratio of your weight to your height. It takes your weight in kilograms and divides it by the square of your height in metres, which gives a single number expressed in kg/m². Health organisations use that number to estimate whether an adult’s weight is likely to be in a healthy range for their height.

The idea is much older than the modern calculator. In the 1800s, the Belgian mathematician Adolphe Quetelet developed the ratio while studying populations, and it was originally known as the Quetelet Index. It was designed as a population screening tool — a cheap, fast way to spot weight patterns across large groups — and health authorities still use it that way today.

One point matters from the start: BMI does NOT measure body fat directly. It cannot see fat, muscle, bone, or water, and it cannot tell where any of them sit on your body. It is a useful estimate based only on height and weight, nothing more. Read it as a screening signal, not as a complete picture of your health.

How BMI Is Calculated

The formula is short enough to do on paper. For metric measurements, BMI = weight (kg) ÷ height (m)². In words, you divide your weight in kilograms by your height in metres multiplied by itself. That is the same formula our bmi calculator uses above.

Here is the worked example, step by step. Take a weight of 80 kg and a height of 1.80 m. First square the height: 1.80 × 1.80 = 3.24. Then divide the weight by that answer: 80 kg ÷ (1.80 × 1.80) = 80 ÷ 3.24 = 24.7. So the BMI is 24.7, which falls inside the healthy weight range of 18.5 to 24.9 for adults.

If you prefer pounds and inches, the imperial formula is: BMI = 703 × weight (lb) ÷ height (inches)². The number 703 is simply the conversion factor that makes the pounds-and-inches version give the same result as the metric version. For example, 176 lb at 71 inches gives 703 × 176 ÷ (71 × 71), which is about 24.5, matching the metric example.

BMI Categories

Once you have your number, the next step is to see which category it falls into. Health authorities divide adult BMI results into six standard categories. These are the exact cut-offs our calculator applies, and they are the same for adult men and women aged 20 and over.

BMI	Category
Below 18.5	Underweight
18.5–24.9	Healthy weight
25.0–29.9	Overweight
30.0–34.9	Obese Class I
35.0–39.9	Obese Class II
40.0 and above	Obese Class III

Notice how wide the healthy band is. At most heights it covers a range of roughly 20 kg, so there is no single perfect weight you must hit. Anywhere inside 18.5 to 24.9 counts as the healthy weight range, and small movements inside that band are completely normal.

Healthy Weight Range by Height

Because BMI depends on height, the healthy weight range is different for every height. A weight that is healthy at 190 cm could be overweight at 160 cm. The table below shows the healthy weight range for nine adult heights, using the same 18.5 to 24.9 limits as the calculator above.

Height	Healthy weight range
150 cm	41.6–56.0 kg
155 cm	44.4–59.8 kg
160 cm	47.4–63.7 kg
165 cm	50.4–67.8 kg
170 cm	53.5–72.0 kg
175 cm	56.7–76.3 kg
180 cm	59.9–80.7 kg
185 cm	63.3–85.2 kg
190 cm	66.8–89.9 kg

To use the table, find the row closest to your height and compare your current weight with the range beside it. If you want the precise range for your exact height — or the exact weight for a specific target BMI — use the reverse tool in the calculator at the top of this page. It does the same maths instantly, for any height.

BMI for Men and Women — Is There a Difference?

A common question is whether men and women need separate tools. They do not. The formula and the adult cut-offs are exactly the same for both sexes, which is why one calculator serves searches for a bmi calculator for men and a women bmi calculator alike. There is no separate male or female chart for adults: 18.5 to 24.9 is the healthy range for everyone aged 20 and over.

The honest difference is body composition, not the number itself. At the same BMI, women naturally carry a higher body-fat percentage than men, while men typically carry more muscle mass. Two people can share a height, weight, and BMI of 24 and still have quite different bodies underneath — one leaner and more muscular, the other carrying more fat around the waist.

That is why the sensible advice is to use BMI as a screen, not a verdict. Pair your result with your waist size, how your clothes fit, your fitness, your blood pressure, and how you feel and perform day to day. If those other signals are good, a BMI near the edge of a band matters far less than the number alone suggests.

The Limits of BMI (Read This Before Worrying)

BMI is useful, but it has real limits, and knowing them will stop you worrying about a number that may not mean what it first seems to. This calculator is a screening tool for adults, not a medical diagnosis, and there are situations where you should read your result with care.

Muscular athletes are the classic example. Because muscle is dense and heavy, a rugby player, weightlifter, or dedicated gym-goer can easily score in the overweight or even obese range while carrying very little body fat, because BMI cannot tell a kilogram of muscle from a kilogram of fat.

The opposite problem can affect older adults. An older person can score inside the healthy range while having low muscle mass and relatively more fat. BMI also says nothing about where fat sits. Fat carried around the belly is linked with higher health risks than the same amount of fat carried on the hips and thighs, yet both give the same BMI.

Finally, BMI in this adult form is not for children and teenagers, whose bodies are still growing: they are assessed with age-and-sex percentiles instead of the adult cut-offs. It is also not a diagnostic tool in pregnancy, when weight naturally changes for other reasons. If your result surprises you, treat it as a prompt for a fuller check with a health professional, not as a label that defines you.

What To Do With Your Result

Your result is only useful if you act on it sensibly. Here is a calm, practical response for each of the main outcomes, based on the category your BMI falls into.

If you are underweight: aim to eat in a small, steady calorie surplus, and make protein a priority at every meal so the weight you gain is more likely to include muscle. Strength training helps that extra energy go to the right places. If you are losing weight without trying, speak to a doctor rather than relying on a calculator.

If you are in the healthy range: your job is simply to maintain it. Work out the calories that keep your weight steady with our Maintenance Calories Calculator, keep moving daily, and recheck your BMI every few months. Small, boring consistency beats any dramatic plan at this stage.

If you are overweight or in the obese range: the proven route is a moderate calorie deficit — eating a little less than you burn, consistently, rather than crash dieting. First, learn how many calories your body burns at rest with our BMR Calculator, then build your daily target from there. Aim for steady progress, prioritise protein and whole foods, and consider medical advice if you have other health conditions.

Frequently Asked Questions

In short, BMI is a quick, free first check — not a final judgement. Use the calculator at the top of this page to find your number, your category, and your healthy weight range, then use the reverse tool to set a target you can actually maintain. That turns one number into a practical plan.