BMI Calculator

What Is BMI?

BMI (Body Mass Index) is a measurement of body fat based on height and weight that applies to both men and women between the ages of 18 and 65 years.

BMI can be used to indicate if you are overweight, obese, underweight or normal. A healthy BMI score is between 18.5 and 25. A score below 18.5 indicates that you may be underweight; a value above 25 indicates that you may be overweight.

You can calculate your BMI by using our BMI Calculator above, or by using the BMI Formula.

BMI applies to adults only. When it comes to children and teenagers, things are a bit more complicated because their bodies change so much as they grow. For that reason we have a separate BMI calculator for children, that takes into account their age and sex.

Please remember, however, that this is only one of many possible ways to assess your weight. If you have any concerns about your weight, please discuss them with your physician, who is in a position, unlike this BMI calculator, to address your specific individual situation.

How Is BMI Calculated?

Calculating your BMI is pretty straightforward; you divide your weight by your height squared:

Metric: BMI = Weight (kg) / Height (m) x Height (m)
Imperial: BMI = (Weight (lbs) / Height (in) x Height (in)) x 703

For instance, if you weigh 70 kg and are 1.75 m tall, the calculation would be: 70 / (1.75 x 1.75) = 22.85. That puts you right in the normal weight range.

For a more detailed breakdown, check out our dedicated BMI formula page or take a peek at our BMI chart.

Limitations of BMI

BMI can be helpful as a quick screening tool, but shouldn't be used on its own due to some significant limitations:

It can't differentiate between muscle and fat. An athlete with a lot of muscle and an overweight person might have the same BMI. For example, a professional bodybuilder with 10% body fat may have a BMI that puts them in the obese category. That's misleading.

It doesn't consider fat distribution. Belly fat is more dangerous than fat stored on your hips and thighs. However, BMI doesn't account for this. It treats all fat equally, whilst the health risks are not.

It's less reliable as you become older. As we age, we lose muscle mass and gain fat. Someone with a normal BMI at 70 might actually have more body fat than they realize.

It wasn't meant for everyone. The cutoff values for the BMI categories were originally based on a data set from people from Europe. This leads to an underestimation of the health risks for Asian populations and an overestimation for Black populations. That's why places like Singapore and Hong Kong consider a BMI of 23 to already be overweight.

In 2023, the American Medical Association stated BMI alone shouldn't be used as a measurement of "clinical health" but rather used as "an imperfect clinical measure". They recommend doctors to use it alongside other metrics for a fuller health picture.

BMI Classification

BMI (Adult)	Classification	Treatment
18.5 or less	Underweight	Treatment
18.5 to 24.99	Normal Weight	Treatment
25 to 29.99	Overweight	Treatment
30 to 34.99	Obesity (Class 1)	Treatment
35 to 39.99	Obesity (Class 2)	Treatment
40 or greater	Morbid Obesity	Treatment

BMI is equal to or less than 18.5 (Underweight)

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A lean BMI can indicate that your weight may be too low. You should consult your physician to determine if you should gain weight, as low body mass can decrease your body's immune system, which could lead to illness such as disappearance of periods (women), bone loss, malnutrition and other conditions. The lower your BMI the greater these risks become.

BMI is between 18.5 and 24.9 (Normal Weight)

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People whose BMI is within 18.5 to 24.9 possess the ideal amount of body weight, associated with living longest, the lowest incidence of serious illness, as well as being perceived as more physically attractive than people with BMI in higher or lower ranges. However, it may be a good idea to check your Waist Circumference and keep it within the recommended limits.

BMI is between 25 and 29.9 (Overweight)

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People falling in this BMI range are considered overweight and would benefit from finding healthy ways to lower their weight, such as diet and exercise. Individuals who fall in this range are at increased risk for a variety of illnesses. If your BMI is 27-29.99 your risk of health problems becomes higher. In a recent study an increased rate of blood pressure, diabetes and heart disease was recorded at 27.3 for women and 27.8 for men. It may be a good idea to check your Waist Circumference and compare it with the recommended limits.

BMI is between 30-34.99 (Obese Class 1)

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Individuals with a BMI of 30-34.99 are in a physically unhealthy condition, which puts them at risk for serious illnesses such as heart disease, diabetes, high blood pressure, gall bladder disease, and some cancers. This holds especially true if you have a larger than recommended Waist Size. These people would benefit greatly by modifying their lifestyle. Ideally, see your doctor and consider reducing your weight by 5-10 percent. Such a weight reduction will result in considerable health improvements.

BMI is between 35-39.99 (Obese Class 2)

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If you have a BMI of 35-39.99 your risk of weight-related health problems and even death, is severe. See your doctor and reduce your weight to a lower BMI.

BMI is over 40 (Obese Class 3 : Morbid Obesity)

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With a BMI of 40+ you have an extremely high risk of weight-related disease and premature death. Indeed, you may have already been suffering from a weight-related condition. For the sake of your health it is very important to see your doctor and get specialists help for your condition.

Beyond BMI: Other Ways to Assess Your Health

BMI is only one metric to look at when it comes to health and wellness (and shouldn't be used on its own due to limitations described earlier). Other measurements include:

Waist-to-Hip Ratio This compares your waist and hip measurements. This is one of the strongest indicators of cardiovascular risk because it shows where your body stores your fat.

Waist-to-Height Ratio Even simpler! Just divide your waist measurement by your height. Many experts believe it's an even stronger predictor for heart disease and diabetes than BMI.

Body Fat Percentage This is the percentage of your total weight that consists of fat. Body fat percentage gives a clearer insight into your body composition than BMI.

Ideal Weight Various formulas exist to estimate a healthy weight range for a given height and frame.

Remember, no one measure can completely describe your health. Combining your BMI with one or 2 other measurements gives you a much better picture of your health than BMI alone.

BMI and Ethnicity

BMI cutoffs were developed based on data that came from a sample of (white) European populations, which unfortunately means that they do not apply equally to all people.

Studies show that BMI underestimates health risks for Asian populations and overestimates them for Black populations. That's why some Asian countries consider a BMI of 23 rather than 25 as being overweight.

On the other hand, people with a Black, African or Caribbean background tend to have more lean muscle and their fat may be overestimated by their BMIs. The NHS in the UK suggests these individuals should talk to their GP to discuss their results.

So, if you're not of white European descent, definitely consider these differences when interpreting your BMI.

Frequently Asked Questions

What does my BMI number mean?

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Your BMI is simply your weight-to-height ratio: Under 18.5 is underweight, from 18.5 to 24.9 is normal weight, from 25 to 29.9 is overweight and anything above 30 classifies as obese. These ranges are based on studies showing a correlation between a higher BMI and increased risks of health issues.

Is BMI accurate for athletes or muscular people?

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Not really! Since BMI can't distinguish between muscle and fat, athletes often get labeled as overweight or obese when that is clearly not the case! The best way to find out if you're in a healthy range is by measuring your body fat percentage.

Is BMI different for men and women?

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BMI is calculated the same way for men and women, but naturally women carry more body fat than men. A man and a woman who have the same BMI of 24 will have different body fat percentages. That is one of the limitations of BMI and why we recommend also doing other measurements such as the waist-to-height ratio.

What is a healthy BMI for someone over 65?

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Some studies suggest that people over 65 might actually do better with higher BMI than the general adult population. A BMI of 25 to 27 is considered overweight but it might offer protection against illness and fractures as we age. If you're over 65, talk with your GP to see if your weight is right for you.

Why is BMI still used if it has so many limitations?

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Well, it's free, quick and easy! All you need is a scale and a tape measure! More accurate methods like DEXA scans can be expensive or difficult to find. For large population studies BMI remains handy, but don't let it be the only metric you look at!

How often should I calculate my BMI?

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There's no strict rule. Measuring your BMI every few months should be enough for most people. However, if you're actively working toward losing or gaining weight, tracking monthly alongside other measurements such as waist circumference can really help to monitor your progress.

Should I lose weight if my BMI is over 25?

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Not necessarily. A person with a BMI between 25 and 30 who has a normal blood pressure, cholesterol and blood sugar level may not require weight loss, especially if they're already active. Fitness levels matter, so a person with a BMI of 26 can still be healthier than a sedentary person with a BMI of 23. Talk to your GP for advice specific to your situation.

References

World Health Organization. Body mass index - BMI. WHO Global Health Observatory.
Centers for Disease Control and Prevention. About Adult BMI. CDC, 2024.
Nuttall FQ. Body mass index: obesity, BMI, and health: a critical review. Nutr Today. 2015;50(3):117-128.
American Medical Association. AMA: Use of BMI alone is an imperfect clinical measure. AMA, 2023.
Wu Y, Li D, Vermund SH. Advantages and limitations of the body mass index (BMI) to assess adult obesity. Int J Environ Res Public Health. 2024;21(6):757.
Quetelet A. A treatise on man and the development of his faculties. Edinburgh: William and Robert Chambers, 1842.