BMI Calculator

Please enter your weight and height and select the desired unit (metric or imperial). This BMI calculator tool will show you your Body Mass Index (BMI), corresponding weight category, healthy weight range for your height. It also shows two less common indices: BMI-Prime and Ponderal Index.

This tool allows both direct and indirect calculations. If you leave the BMI field blank, it will calculate your BMI. If you enter a target BMI and your height and leave the weight field blank, it will give you your current weight required to achieve that goal.

What is BMI?

BMI is a value derived from the ratio of body weight to height. It is calculated by dividing weight in kilograms by the square of height in meters. Because it can be determined using only two values that most people know, BMI is a standard screening tool for assessing weight in clinical and public health settings.

BMI does not directly measure body fat. It estimates whether a person's weight is higher or lower than typical values for the same height. At the population level, BMI generally correlates with body fat percentage. Doctors do not consider BMI to be diagnostic but rather as a screening tool. A result outside of healthy range indicates that further investigation is needed and no conclusions should be drawn based on this result alone.

How to use this BMI Calculator tool.

Please enter your weight in the "Weight" field and your height in the "Height" field and select the desired unit. It is also possible to combine different units. For example, you can enter your weight in pounds and your height in centimeters, and this tool will perform all conversions. The BMI will be displayed as the main result, and the corresponding category and healthy weight range for your height will be indicated with color indications.

You do not have to select which value you want to calculate. If you leave the BMI field blank, the tool will give values based on weight and height. If you want to know what weight you need to be at a particular BMI, enter that BMI and your height and leave the weight field blank. The corresponding weight will then be shown.

The formula for calculating your BMI is:

In the metric system, BMI is the value obtained by dividing a person's weight (in kilograms) by the square of their height (measured in meters).

Assume your weight is 80 kilograms and your height is 1.9 meters. The square of your height is 3.61, and the calculation would be as follows:

The BMI is 22.16 and falls within the healthy range. If you use pounds and inches, the same principle applies: weight in pounds divided by height in inches squared multiplied by 703.

Both methods will give the same value. The factor 703 is simply a conversion from pounds and inches to metric units. If you select the "yard-pound" system, then the tool will automatically do this conversion for you.

BMI categories

The World Health Organization classifies adult BMI into the following categories. The four main categories are underweight, healthy weight, overweight and obesity, with finer gradations indicating how far the result deviates from the healthy range.

Your healthy weight range

Since BMI is calculated using only body weight and height, it can be used to determine the corresponding healthy weight range for a given height. The healthy range is between a BMI of 18.5 and 25. So, for any given height, the healthy weight range would be the square of that height (in meters) multiplied by either 18.5 or 25.

For a person who is 1.75 meters tall, this range would be about 56.7 to 76.6 kilograms. The tool will show the individual's range in the same unit that was used when entering their weight, and it will also estimate how much the current weight falls above or below that range.

BMI Prime and the Ponderal Index

BMI Prime is a value calculated by dividing the BMI by a certain number (25, which is the upper limit of the healthy range). This can be converted into a simple ratio: 1.0 means that the BMI is exactly at the upper limit of the healthy range, values below 1.0 are within the healthy range and values above 1.0 show how much the upper limit is exceeded. For example, a BMI Prime of 1.2 means that the BMI is 20 percent higher than the upper limit of the healthy range.

The pondal index is calculated by dividing the weight by the cube of the height rather than the square. This extra power makes it more stable for very tall or short people who can be misclassified as overweight or underweight using normal BMI. Most adults have a value between 11 and 15. Do not use this as a replacement for BMI, but consider it another measure to take into account.

What BMI doesn't measure:

BMI is a rough screening method and has obvious limitations. As it cannot distinguish between muscle and fat, people with a muscular build or those who are athletes or labourers can have low body fat but still be in the overweight category. Conversely, people with little muscle mass may have high body fat even though they fall within the healthy range.

The significance of a particular BMI can vary by age, sex and ethnicity. Older people often lose muscle mass, and women generally have more body fat than men even with the same BMI. In some populations, including those from South Asian or East Asian backgrounds, health risks start at lower BMIs, which is why some guidelines lower the criteria for overweight. The BMI is also not applicable during pregnancy. Consider the result as a single data point and consult a doctor who can perform a comprehensive assessment that takes into account body composition, waist circumference and individual medical history.

BMI in children and adolescents

BMI is interpreted differently for individuals between the ages of 2 and 20. The calculation method is the same but it is compared to age- and sex-specific growth charts that are expressed in percentiles. This is because body fat percentage changes dramatically during a child's growth years. Children below the 5th percentile are considered underweight, children between the 5th and 85th percentile have healthy weight, children between the 85th and 95th percentile are overweight, and children above the 95th percentile are considered obese. This tool uses the adult categories. Therefore, you should use a BMI tool specific to age for children or consult with your pediatrician.

Health risks of high or low BMI:

A high BMI is associated with an increased risk of type 2 diabetes, hypertension, heart disease, various cancers, gallstones, sleep apnea and osteoarthritis. The higher the BMI and into the obese range, the greater the risk.

A low BMI can also pose risks such as malnutrition, weakened immune system, fragile bones, anaemia and reproductive problems. When the BMI is significantly outside of a healthy range, either too high or too low, it's usually more effective to make gradual changes under expert guidance rather than abrupt changes.

Tips for meaningful results:

Weigh yourself at the same time each day, ideally in the morning. Take off your shoes when measuring your height. Use the same tool and units of measurement every time to ensure comparable conditions. As weight naturally fluctuates due to food and fluid intake, don't overemphasize a single-day result but rather look for trends over several weeks. Also consider other indicators like how clothing fits, energy levels, and waist size when assessing yourself.