BMI Calculator: Calculate Your Body Mass Index and Understand Your Result

BMI (Body Mass Index) is the quickest way to check whether your weight is appropriate for your height. With the Befit BMI calculator, you enter your weight and height and find out in seconds which range you fall into: underweight, healthy weight, overweight or obesity.

Besides the number, the tool shows the healthy weight range for your height, which helps you set realistic goals to lose weight, build muscle or maintain your weight.

One warning up front: BMI doesn’t distinguish muscle from fat. An overweight result doesn’t always mean you need to lose weight, and this page explains how to read your result the right way.

What is BMI and what is it used for?

BMI is an index that relates a person’s weight to their height. It was created in the 19th century by Belgian mathematician Adolphe Quetelet and is now used by the World Health Organization (WHO) and health authorities around the world as a reference to classify nutritional status.

In practice, BMI works as a screening tool: it’s simple, free and quickly shows whether your weight is within, above or below what’s expected for your height. High values are associated with a greater risk of type 2 diabetes, high blood pressure and cardiovascular disease, while very low values may indicate malnutrition.

BMI, however, is not a diagnosis. It flags a point of attention that should be analyzed along with other measurements and, whenever possible, by a health professional.

How to use the Befit BMI calculator

Using the calculator takes less than a minute. Here’s how to fill in each field to get a reliable result:

1. Choose your unit system. Select metric (kg and cm) or imperial (pounds, feet and inches).
2. Enter your sex and age. Age matters because the reference ranges change for older adults and don’t apply the same way to children and teenagers.
3. Enter your current weight. If possible, weigh yourself in the morning, on an empty stomach and in light clothing, so you keep the same standard in future measurements.
4. Enter your height. Use a recent measurement, taken barefoot and standing straight.
5. Click “Calculate.” You’ll see your BMI, the corresponding classification and the healthy weight range for your height.

If you want to track your progress, repeat the calculation under the same conditions, at the same time of day and on the same scale. That’s the only way the comparison between results makes sense.

How to calculate BMI

The BMI formula is simple: divide your weight (in kilograms) by your height (in meters) multiplied by itself.

BMI = weight (kg) ÷ [height (m) × height (m)]

Here’s an example for a person who weighs 70 kg and is 1.75 m tall. First, multiply the height by itself: 1.75 × 1.75 = 3.06. Then divide the weight by the result: 70 ÷ 3.06 = 22.9.

If you use pounds and inches, the formula changes slightly: BMI = 703 × weight (lb) ÷ [height (in) × height (in)]. For someone who weighs 154 lb and is 5’9″ (69 inches) tall, the result is 703 × 154 ÷ 4,761 = 22.7.

Both results fall within the healthy weight range. Doing the math by hand is possible, but unit conversions and rounding often lead to mistakes. With the online BMI calculator, the result comes out ready and already classified.

BMI chart: what each range means

For adults aged 20 and over, the WHO classification is divided into six ranges:

BMI (kg/m²)	Classification
Below 18.5	Underweight
18.5 to 24.9	Healthy weight
25.0 to 29.9	Overweight
30.0 to 34.9	Class I obesity
35.0 to 39.9	Class II obesity
40.0 or higher	Class III obesity

Here’s what each range says about your health:

• Underweight: your weight is below what’s expected for your height. It may indicate insufficient food intake, loss of muscle mass or a health condition that deserves investigation.
• Healthy weight: this is the range associated with the lowest risk of weight-related diseases.
• Overweight: your weight is above what’s expected for your height. It’s a warning sign, but it needs to be interpreted together with body composition, as you’ll see in the next section.
• Class I obesity: indicates excess weight with an increased risk of metabolic and cardiovascular disease.
• Class II obesity: the health risk is considered high, and professional follow-up becomes even more important.
• Class III obesity: this is the highest-risk range, usually associated with health complications that require medical care.

These ranges apply equally to adult men and women. For children, teenagers and older adults, the interpretation changes, and you’ll find the details below.

A high BMI doesn’t always mean you need to lose weight

The main limitation of BMI is that it only considers total weight, without distinguishing muscle, fat, bone or water. Since muscle is denser than fat, people with a lot of muscle mass can have a high BMI even with little body fat.

Imagine a man who is 1.78 m (5’10”) tall, weighs 90 kg (198 lb) and has been lifting weights for years. His BMI is 28.4, classified as overweight. If much of that weight is muscle, he may have a healthy body fat percentage, and in that case, losing weight wouldn’t be the goal.

The opposite also happens: a sedentary person can have a BMI within the healthy range and still have little muscle mass and excess fat, especially around the abdomen.

Ideally, BMI should be combined with other measurements that show your body composition:

• Body fat percentage: shows how much of your weight is fat. It can be estimated with bioimpedance, skinfold measurements or scans such as DEXA.
• Waist circumference: indicates abdominal fat, which is more directly linked to cardiovascular risk. Under WHO criteria, risk increases from 94 cm (about 37 inches) in men and 80 cm (about 31.5 inches) in women.
• Waist-to-height ratio: your waist measurement divided by your height, both in the same unit. Values above 0.5 are commonly used as a warning sign.
• Changes in measurements and strength: if you’re getting stronger and losing inches, the number on the scale may go up without that being a problem.

If your BMI says overweight but you train and have good muscle mass, assess your body composition before deciding on your next step. Our article on bulking and cutting helps you understand whether it makes more sense to focus on losing fat or building muscle.

BMI for men and women: is there a difference?

Not in the formula or the ranges. For adults, the calculation and the classification chart are the same for men and women.

The difference lies in body composition. Women naturally have a higher body fat percentage than men due to hormonal and reproductive factors. A man and a woman with the same BMI can have very different amounts of fat and muscle.

Sex doesn’t change BMI, but it helps put the result into context. It’s essential in other assessments, such as body fat percentage and waist circumference, which have different reference values for each sex.

BMI for children: how it works for kids and teens

For anyone under 20, BMI is calculated with the same formula but can’t be interpreted using the adult chart. During growth, the ratio between weight and height changes a lot with age and differs between boys and girls.

Child BMI is assessed using growth charts, such as the WHO’s or, in the United States, the CDC’s. These charts compare the child’s or teen’s result with others of the same age and sex. The result is expressed as a percentile or z-score, not as a fixed range like 18.5 to 24.9.

This assessment should be done by a pediatrician, who tracks development over time. A single measurement says little about a child’s health.

BMI for seniors: why the ranges change after 60

With aging, the body goes through natural changes, such as loss of muscle and bone mass, reduced body water and a decrease in height. BMI reference ranges reflect these changes.

Some health authorities, such as Brazil’s Ministry of Health, use a specific classification for people aged 60 and over:

BMI (kg/m²)	Classification
Below 22	Underweight
22 to less than 27	Healthy weight
27 or higher	Overweight

The healthy range is higher than for younger adults because, at this stage of life, a slightly larger body reserve helps protect against malnutrition and muscle loss (sarcopenia). A BMI of 21, which would be healthy for a younger adult, already indicates underweight for an older adult.

For people over 60, maintaining muscle mass is just as important as controlling weight. Strength training and adequate protein intake make a real difference.

Why use the Befit BMI calculator?

Calculating BMI by hand requires converting units, squaring your height, dividing your weight and then checking the result against a chart that also changes with age. The Befit calculator does all of this in seconds and offers extra advantages:

• Instant result with no math errors: you don’t need to worry about conversions or rounding.
• Automatic classification: your result comes with the corresponding range, with no need to check charts.
• Healthy weight range for your height: it works as a BMI and ideal weight calculator at the same time, which helps you set realistic goals.
• Metric or imperial: you use the unit of measurement you prefer.
• Recalculate in seconds: just update your weight whenever you want to track your progress.

Once you know your BMI, the next step is to turn the result into a strategy. If your goal is to lose fat or build muscle, the right combination of training and nutrition is what gets you there.

Download the Befit app to track your weight, log your progress and follow a workout plan personalized for your goal. With training and nutrition aligned, the number on the scale is no longer the only measure of your results!