BMI Calculator
Body Mass Index from weight and height.
Body Mass Index from weight and height.
Body Mass Index, usually shortened to BMI, is a single number that compares your weight with your height. It was developed in the nineteenth century by the Belgian mathematician Adolphe Quetelet, and doctors and health organizations still use it today because it is quick, free and needs only a scale and a tape measure. This guide explains how the number is calculated, how to read your result, and, just as important, what BMI cannot tell you about your health.
The metric formula is weight in kilograms divided by height in meters squared. Take a person who weighs 70 kg and is 170 cm tall. Their height is 1.7 meters, and 1.7 multiplied by 1.7 is 2.89. Dividing 70 by 2.89 gives a BMI of about 24.2. If you prefer pounds and inches, multiply your weight in pounds by 703 and divide by your height in inches squared. The calculator above uses kilograms and centimeters, so you can type your height as 170 instead of converting it first.
A common mistake is entering height in meters in a field that expects centimeters, or the other way round. If your result is far outside the normal range, such as 0.02 or 240, check the units before you worry.
For adults, the World Health Organization uses these standard ranges.
These bands are population guides. They were built from studies that linked BMI with the risk of conditions such as type 2 diabetes, heart disease and some other illnesses. A higher BMI is associated with higher average risk across large groups, but it does not predict what will happen to any one person.
It can be more useful to flip the calculation and ask which weights fall in the normal range for your height. At 170 cm, the range of 18.5 to 24.9 corresponds to roughly 53.5 kg to 72 kg. At 160 cm it is about 47 kg to 64 kg, and at 180 cm it is about 60 kg to 81 kg. Notice how wide the range is. Two people of the same height can be at very different weights and both be in the normal band.
BMI measures size, not health. It cannot tell the difference between muscle and fat, and muscle is denser than fat. A strength athlete can have a high BMI and a low level of body fat. On the other hand, an older person who has lost muscle may have a normal BMI while carrying too much fat around the organs. BMI also does not show where fat is stored. Fat around the waist is linked with more health risk than fat stored on the hips and thighs.
Other things that BMI ignores include age, sex, bone structure, pregnancy and certain medical conditions. For these reasons a doctor will look at more than one measurement before drawing any conclusion.
Research has shown that the relationship between BMI and health risk is not identical in every population. For many people of Asian background, health risks such as diabetes begin to rise at lower BMI values. The WHO has suggested extra action points at 23 and 27.5 for these populations, and some countries use lower cut-offs for overweight and obesity. If this applies to you, discuss it with a doctor who knows the guidance for your region.
Children and teenagers are a special case. Their bodies are still growing, so the adult ranges do not apply. Doctors compare a young person's BMI with growth charts for their age and sex and describe the result as a percentile. This calculator is designed for adults only.
If your BMI is in the normal range, there is nothing special to do except keep up habits that support health, such as regular movement, balanced meals and enough sleep. If it is above or below the range, treat it as a reason to ask questions and not as a verdict. A doctor or registered dietitian can look at your history, measurements and lifestyle and suggest realistic steps.
Be careful with quick fixes. Extreme diets and fast weight loss plans can harm your health, and they often fail in the long run. Small changes that you can keep for years usually work better, such as adding a daily walk, reducing sugary drinks and eating more vegetables and whole foods. If you have a history of eating disorders, or if thinking about weight makes you anxious, talk to a health professional before you track numbers.
A single reading is a snapshot. If you do want to track it, measure under similar conditions, for example in the morning before eating, using the same scale. Look at the trend over months rather than daily changes, because water, food and hormones can move your weight by a kilogram or more in a day. Remember that a change in BMI could come from fat, muscle or both.
Enter your weight in kilograms and your height in centimeters. The tool shows your BMI and the standard adult category. Nothing you type is stored, because the calculation happens in your browser. Use the result as a conversation starter with a health professional, not as a diagnosis.
Is BMI accurate for athletes? Often not. Muscle adds weight, so athletes can be classed as overweight even with low body fat.
Is a BMI of 25 unhealthy? Not necessarily. It is the point where the overweight range begins, and other factors matter more for any individual.
Can I use BMI during pregnancy? No. Weight changes in pregnancy follow different guidance, so ask your doctor or midwife.
How often should I check it? Every few months is enough for most people.