BMI Calculator

Understanding your BMI result

BMI is one of the quickest ways to see whether your weight is roughly in line with your height. It is a screening number, not a diagnosis, and it is most useful when you read it alongside everything else you know about your body.

What BMI actually measures

Body mass index compares your weight to the square of your height. That single ratio was designed in the nineteenth century by Adolphe Quetelet as a way of describing populations, and it stuck because it is cheap, fast and needs no equipment beyond a scale and a tape measure. It tells you how heavy you are for your height. It does not know whether that weight is muscle, bone, fluid or fat, and it does not know where on your body the weight sits.

That limitation matters, but so does the convenience. For most adults who are not athletes, BMI tracks reasonably well with body fat percentage, which is why clinics still use it as a first look before ordering anything more detailed.

How BMI is calculated

The metric formula is weight in kilograms divided by height in metres squared. Someone who is 1.75 m tall and weighs 70 kg has a BMI of 70 / (1.75 x 1.75) = 22.9. The imperial formula multiplies the same ratio by 703: weight in pounds divided by height in inches squared, times 703. This calculator handles the conversion for you, so you can switch units without redoing any arithmetic.

How to use this calculator

1. 1Pick the unit system you actually know your measurements in - guessing a conversion is the most common source of a wrong result.
2. 2Enter your height. In imperial mode, feet and inches are separate fields so you do not have to convert to inches yourself.
3. 3Enter your current weight, ideally measured in the morning before eating.
4. 4Read the BMI number, the category it falls into, and the healthy weight range shown for your height.

Reading the categories

The World Health Organization category boundaries for adults are the ones this tool uses. They are the same bands you will see on a clinic wall chart.

A single reading is far less interesting than a trend. If you check every few months under the same conditions, the direction of travel tells you more than the exact decimal ever will.

Where BMI gets it wrong

• Muscular bodies. Muscle is denser than fat, so rugby players, weightlifters and many manual workers land in the overweight band while carrying very little fat.
• Older adults. Muscle mass falls with age, so a "healthy" BMI can hide a low muscle, higher fat composition.
• Very tall or very short people. Squaring height exaggerates the number at the extremes of the range.
• Children and teenagers. They need age and sex specific percentile charts, not adult categories.
• Pregnancy. BMI is not a meaningful measure during pregnancy.
• Ethnic differences. Several health bodies use a lower overweight threshold for people of South Asian descent because health risks appear at a lower BMI.

Measurements that fill the gaps

Waist circumference is the most useful companion number because abdominal fat carries most of the metabolic risk. Waist-to-height ratio is even simpler: keeping your waist under half your height is a rule of thumb that works in any unit system. Resting heart rate, blood pressure and how you feel on a flight of stairs all add context that a weight-to-height ratio cannot.

Why the number is still worth checking

Large population studies keep finding the same U-shaped curve: health risk rises at both ends of the BMI range and is lowest through the middle. Because it is so easy to calculate, BMI remains a practical way to notice a drift you might otherwise miss. Weight changes slowly enough that you rarely feel it happening, and a number checked twice a year makes the change visible.

Related calculations

If you are tracking changes over time, the Percentage Calculator will tell you exactly how much your weight has moved in percentage terms. The Age Calculator is useful when you need your precise age for a health form, and the Unit Converter handles kilograms to pounds or centimetres to feet when a form asks for units you do not use day to day.