BMI Calculator

1. Choose metric or imperial units.
2. Enter your weight (kilograms or pounds).
3. Enter your height (centimetres, or total inches for imperial).
4. Read your BMI, your WHO weight category and the healthy weight range for your height.

Here is what each result means:

Body mass index, or BMI, is a simple ratio of your weight to your height that estimates whether you are a healthy weight. It was designed as a population screening tool, giving a quick, low-cost signal that is useful across large groups of people.

It helps flag whether weight might be a health risk and is a common starting point in check-ups. Because it uses only height and weight, it cannot tell muscle from fat, so a very muscular person can score as overweight. It is not valid for children, pregnancy or athletes, where other measures are better.

It divides your weight by the square of your height, using a units constant so metric and imperial give the same result.

1. Convert height to the right unit (metres for metric, inches for imperial).
2. Square the height.
3. Divide weight by that value, multiplying by 703 for imperial units.

The result is compared to the WHO categories to give your weight band.

Suppose you weigh 70 kg and are 175 cm tall.

Calculation: height in metres = 1.75, squared = 3.0625. BMI = 70 / 3.0625 = 22.9, which is in the normal-weight range.

In imperial, someone weighing 154 lb at 69 in gets BMI = 703 x 154 / (69 x 69) = about 22.7, essentially the same person and the same result.

The World Health Organization uses these adult cut-offs, which this calculator applies.

These bands are for adults aged 20 and over. Some populations, including many of Asian descent, carry higher risk at lower BMI values, so local guidance may use adjusted thresholds.

BMI is only weight and height, so several things can make it misleading.

Muscle Mass

Muscle is denser than fat, so athletes and heavy trainers can register as overweight despite low body fat.

Age and Sex

Older adults lose muscle, and body-fat levels differ between men and women at the same BMI, so the number means slightly different things.

Body Frame and Ethnicity

Frame size and ethnic differences in fat distribution affect health risk at a given BMI, which is why some guidelines adjust the cut-offs.

BMI is quick but blunt. Other measures add detail about where and how much fat you carry.

For fat distribution, pair BMI with the waist-to-hip ratio calculator.

A Quick Self-check

Use it as a fast, no-equipment screen of whether your weight sits in a healthy range for your height.

Tracking a Trend

Watch BMI move over months as you change habits, since a single reading matters less than the direction of travel.

Talking to a Clinician

Bring it to a check-up as one data point alongside blood pressure, waist size and blood tests, rather than reading it in isolation.

1. Mixing Units

Entering pounds with the metric toggle, or centimetres with imperial, gives a wrong BMI. Match the toggle to your numbers.

2. Treating BMI as Body Fat

BMI cannot see body composition. A muscular person is not unhealthy just because BMI says overweight.

3. Using Adult BMI for Children

Children and teens are read against age-and-sex percentile charts. For young ages, check growth with an age-aware chart instead.

4. Using It During Pregnancy

BMI is not valid in pregnancy. Follow your pregnancy care team guidance on weight.

5. Fixating on One Reading

Daily weight swings from food and water move BMI slightly. Track the trend, not a single day.

The arithmetic is exact, but BMI is a screening estimate, not a measure of health or body fat.

The broad categories split further, and clinicians often use these finer bands because health risk rises across the obesity range rather than switching on at a single point.

These bands come from WHO and are widely used in clinical guidance. A very low BMI carries its own risks, including weakened immunity, bone loss and nutrient deficiency, which is why the underweight range is split too. The point of the finer bands is not to label people but to match the level of follow-up to the level of risk.

The same BMI number can mean different things depending on who you are, because BMI ignores body composition and fat distribution.

Men and Women

At an identical BMI, women on average carry a higher percentage of body fat than men, while men carry more muscle. The WHO cut-offs are the same for both sexes, so read BMI alongside a body-fat or waist measure for a fairer picture of health.

Older Adults

Muscle mass falls with age, a process called sarcopenia, so an older person can sit in the normal BMI range while carrying more fat and less muscle than the number suggests. Some research finds that a slightly higher BMI is protective in older age, so the healthy range is often read more loosely after about 65.

Athletes and Heavy Trainers

Muscle is denser than fat, so people with a lot of muscle frequently score as overweight or even obese on BMI while having low body fat. A rugby forward or a competitive lifter is the classic example. For these groups BMI is close to meaningless, and body-fat testing is the better tool.

Adult BMI categories do not apply to anyone under 20. Children and teenagers grow at different rates, and their healthy body-fat level changes with age and sex, so a fixed cut-off would misclassify them.

Instead, a child BMI is calculated the same way but then plotted on an age-and-sex growth chart and expressed as a percentile. A child on the 60th percentile has a higher BMI than 60 percent of children of the same age and sex. Broadly, the 5th to 85th percentile is considered healthy, the 85th to 95th is overweight and above the 95th is obesity, though a paediatric provider should interpret the result.

If you are checking a child, use a paediatric BMI-for-age tool rather than this adult calculator, and treat any concern as a prompt to speak to a health professional rather than a diagnosis.

BMI is used for screening because, across large groups, it correlates with the risk of several conditions. It does not predict any individual outcome, but the associations are well established.

Risks Associated with a High BMI

A higher BMI is linked to a greater risk of type 2 diabetes, high blood pressure, heart disease and stroke, several cancers, sleep apnoea, and joint problems such as osteoarthritis. Risk generally climbs as BMI rises through the overweight and obesity bands.

Risks Associated with a Low BMI

Being underweight carries risks too, including a weakened immune system, brittle bones and osteoporosis, anaemia, fertility problems and, in older adults, a higher risk of complications after illness or surgery.

Because BMI cannot see where fat is stored, someone in the normal range can still carry risky abdominal fat. That is why a waist measure adds so much, and why no single number should drive a health decision on its own.

If your BMI sits outside the healthy range, small sustainable changes usually beat drastic ones. The aim is a steady trend, not a fast number.

If Your BMI Is High

A modest, consistent calorie deficit is the core driver of fat loss. Combine it with regular activity that mixes cardio and some resistance training to protect muscle, prioritise protein and fibre to stay full, and get enough sleep, since short sleep is linked to weight gain. A loss of about 0.5 to 1 percent of body weight per week is a common, sustainable pace.

If Your BMI Is Low

Gaining weight healthily means a modest calorie surplus built from nutrient-dense foods rather than empty calories, paired with resistance training so the gain is muscle rather than only fat. If you are underweight without trying, that is worth discussing with a clinician to rule out an underlying cause.

Whichever direction you are heading, track the trend over weeks and pair BMI with how you feel, your energy and your measurements, not the scale alone.

BMI was devised in the 1830s by the Belgian mathematician Adolphe Quetelet, who was studying the characteristics of the average person rather than individual health. For that reason it was originally called the Quetelet Index.

It was renamed the body mass index in 1972 by the physiologist Ancel Keys, who showed it was a reasonable population-level proxy for body fat and, crucially, was cheap and easy to measure. That practicality is why it spread through medicine and public health and why it persists today, despite well-known limitations. Understanding its origin as a population statistic, not a personal diagnostic, is the key to using it sensibly.

BMI is most useful as one line in a fuller report, not as a headline on its own. Because it cannot see body composition or where fat is stored, a sensible check reads it next to a small set of other measures that fill those gaps.

Waist circumference and waist-to-hip ratio show whether fat is concentrated around the abdomen, which is the pattern most strongly linked to metabolic risk. A body-fat estimate separates muscle from fat, which matters for anyone athletic. And the numbers from a routine check-up, such as blood pressure, fasting blood sugar and cholesterol, describe what is happening inside rather than on the scale.

When these agree, the picture is clear. When they disagree, for example a normal BMI with a large waist or raised blood sugar, the other measures usually carry more weight than BMI, and that disagreement is itself a useful prompt to look closer.

In practice, certain BMI thresholds tend to change what a clinician suggests, which is part of why the number remains so widely used despite its bluntness.

Around a BMI of 25, guidance often shifts toward lifestyle changes to prevent further gain. At 30 and above, the conversation may include structured weight-management programmes and, for some people, medication. At higher thresholds, often 35 with a weight-related condition or 40 on its own, options such as bariatric surgery may be discussed. On the other side, a BMI below 18.5, especially if it is falling without intention, can prompt tests to look for an underlying cause.

These thresholds are general and vary by country and by individual circumstances. The point is not that a number dictates treatment, but that BMI is a convenient trigger for a more detailed assessment, which is exactly the role a screening tool should play.

Because BMI depends entirely on two measurements, small errors in either can nudge you into a different band. A little care makes the result more reliable and more comparable over time.

Weight

Weigh yourself at the same time of day, ideally in the morning after using the bathroom and before eating, wearing little or consistent clothing. Body weight can swing a kilogram or two across a day from food and water, so a fixed routine removes most of that noise.

Height

Stand against a wall without shoes, heels together, looking straight ahead, and mark the top of your head with a flat object held level. Height shrinks slightly through the day as the spine compresses, so morning is again the most consistent time.

Record both in the same units you enter here, and repeat the measurement the same way each time so that any change you see reflects your body rather than your method.

Myth: a Normal BMI Means You Are Healthy

Not necessarily. It means your weight is proportional to your height, but it says nothing about fat distribution, fitness, diet or blood markers. Health is broader than any single ratio.

Myth: BMI Works for Everyone

It does not. It is unreliable for athletes, pregnant people, children, the very elderly and some ethnic groups, which is why guidance layers other measures on top for those cases.

Myth: a Higher BMI Always Means More Fat

Muscle weighs more than fat for its size, so a muscular person can have a high BMI and low body fat. The number cannot tell the two apart.

Myth: You Should Aim for the Lowest BMI Possible

No. Very low BMI carries real risks of its own. The goal is the healthy range, not the smallest number.

Divide your weight in kilograms by your height in metres squared. In imperial units, multiply your weight in pounds by 703 and divide by your height in inches squared. This calculator does both automatically from the unit toggle.

The World Health Organization considers a BMI of 18.5 to 24.9 healthy for most adults. Below 18.5 is underweight, 25 to 29.9 is overweight and 30 or more is obesity.

Not always. BMI cannot tell muscle from fat, so athletes and people with high muscle mass can score as overweight despite low body fat. Pair BMI with a body-fat or waist measure for a fuller picture.

No. Children and teenagers are assessed with age-and-sex-specific BMI percentile charts, not the adult categories. Ask a paediatric provider to interpret a child growth chart.

No. Weight gain in pregnancy is expected and BMI categories do not apply. Follow the weight guidance from your midwife or doctor instead.

Because the imperial formula includes a 703 constant that converts pounds and inches to the same scale as kilograms and metres. The underlying ratio of weight to height squared is identical.

BMI is weight relative to height, while body-fat percentage is the share of your weight that is fat. Two people with the same BMI can have very different body compositions.

Some do. Many people of Asian descent face higher health risk at lower BMI values, so certain guidelines lower the overweight and obesity thresholds. Check local guidance for your group.

Yes. BMI misses body composition and fat distribution, so a person in the normal range can still carry excess abdominal fat, sometimes called normal-weight obesity. Blood pressure, blood sugar and waist size give a fuller picture.

For adults, the healthy range of 18.5 to 24.9 applies across ages, though muscle loss in older adults means the number can understate body fat. For anyone under 20, BMI is read as an age-and-sex percentile rather than against the adult bands.

BMI moves as your weight changes, which for healthy fat loss or gain is gradual, often around 0.5 to 1 percent of body weight per week. Day-to-day scale swings are mostly water and food, so judge BMI over weeks, not days.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere unless you choose to Save a result, which stays only in this browser.

• Body mass index (BMI) classification (World Health Organization).
• Assessing your weight and health risk (NIH National Heart, Lung, and Blood Institute).
• BMI healthy weight ranges (UK NHS).