What is BMI?
Body Mass Index, or BMI, is a number that compares your weight with your height. It is calculated by dividing your weight in kilograms by your height in metres squared. Doctors and health researchers use it as a quick first check, because it needs nothing more than a scale and a tape measure.
BMI is a screening tool, not a diagnosis. It cannot tell muscle from fat, and it says nothing about where fat is stored on the body. Treat the result as a conversation starter with a health professional, not as a verdict.
How BMI is calculated
In metric units the formula is weight in kg divided by height in metres squared. For example, a person who weighs 70 kg and is 170 cm tall has a BMI of 70 divided by 2.89, which is about 24.2. In pounds and inches the formula is 703 multiplied by weight in pounds, divided by height in inches squared.
The World Health Organization groups adult BMI like this: below 18.5 is underweight, 18.5 to 24.9 is a healthy range, 25 to 29.9 is overweight, and 30 or more is obesity.
What BMI does not tell you
Athletes and people with a lot of muscle can have a high BMI without carrying extra fat. Older adults may have a normal BMI but low muscle. Some health organisations suggest using lower cut-off points for people of South Asian descent, because health risks can appear at a lower BMI, so it is worth asking your doctor how to read your result. BMI is also not designed for children, teenagers or pregnant women, who need age-specific charts.
How to use the calculator
- Choose metric or imperial units.
- Enter your weight and your height.
- Press Calculate BMI.
- Read the number and the category shown beside it.
Practical tips
- Weigh yourself at a similar time of day, ideally in the morning, so readings can be compared fairly.
- Combine BMI with other signs such as waist size, fitness level, diet and blood tests.
- If your result is outside the healthy range, speak to a doctor before changing your diet or exercise routine.
- Focus on long-term habits instead of chasing a single number.
More features
Besides your BMI, the tool shows a colour scale with your position, the healthy weight range for your height and how far you are from it. If you enter your age, sex and activity level it also estimates your daily calories using the Mifflin-St Jeor formula.
Frequently asked questions
What is a healthy BMI?
For most adults, 18.5 to 24.9 is considered the healthy range.
Is BMI accurate for athletes?
Not always, because muscle weighs more than fat, so athletes can score high while being fit.
How do I calculate BMI in pounds and inches?
Multiply your weight in pounds by 703 and divide by your height in inches squared.
Does age or gender change the formula?
No, the formula is the same, but doctors may interpret the number differently by age, sex and background.
Is my data stored?
No. The calculation runs in your browser and nothing is sent anywhere.