BMI Calculator

How to Calculate BMI

Enter your height and weight in metric or imperial units and the BMI calculator works out your body mass index, your BMI category, the healthy weight range for your height and how far you are from it. If you add a waist measurement (and choose male or female) it also shows the commonly cited waist risk band and your waist-to-height ratio. It is an adult BMI calculator, designed for people aged 18 and over.

Whether you search for a BMI formula calculator, a BMI calculator for adults or just want to figure out your BMI, the maths is the same: one division. The rest of this page explains the formula, how to read the BMI table, and where BMI falls short.

The BMI Formula

Worked Example: 175 cm, 82 kg, Waist 96 cm (Male)

A man is 175 cm (1.75 m) tall and weighs 82 kg. Height squared is 1.75 × 1.75 = 3.0625, so BMI = 82 ÷ 3.0625 = 26.8, which falls in the overweight category (25.0 to 29.9). The healthy weight range for 175 cm is 56.7 to 76.3 kg, so he is about 5.7 kg above the top of that range. His BMI Prime is 1.07 and his ponderal index is 15.3 kg/m³.

His 96 cm waist is in the increased risk band for men (94 cm or more, with 102 cm or more being substantially increased), and his waist-to-height ratio is 96 ÷ 175 = 0.55, above the 0.5 flag. Together, these figures suggest it is worth talking to a GP about overall health, though they are not a diagnosis.

Adult BMI Categories

These are the World Health Organization adult categories. They are the same for men and women, but they are screening bands rather than a verdict on your health.

BMI by Height: Healthy Weight Range Table

This BMI table shows the weights that give a BMI between 18.5 and 24.9 at common adult heights.

Limits of BMI

BMI is a quick screening tool, not a measure of health. It cannot tell muscle from fat, so people with a lot of muscle can score in the overweight range while carrying little body fat, and people with little muscle can have a healthy BMI with a high body fat percentage. It also says nothing about where fat is stored, and fat around the abdomen is generally associated with higher health risk than fat elsewhere.

Age and background matter too. Research suggests that people from some ethnic backgrounds may have a higher risk of conditions such as type 2 diabetes at a lower BMI, so a GP may interpret your number differently. Pregnant women, children and teenagers, and very muscular or very frail people should not rely on this calculator at all.

BMI for Older Adults

For older adults, the standard BMI bands become harder to interpret. Height can shrink and muscle mass tends to fall with age, so the same BMI can hide different amounts of body fat. Some guidance suggests that a slightly higher healthy range can be appropriate for people over about 65, and unintentional weight loss can be a concern in its own right. If you are older, treat the result as a conversation starter and discuss what is right for you with your GP.

Waist Measurement

To measure your waist, stand relaxed, find the midpoint between your lowest rib and the top of your hip bone, and wrap a tape measure around your bare waist at that level. Breathe out normally and read the tape without pulling it tight. A waist of 94 cm or more (men) or 80 cm or more (women) is commonly cited in Australia as increased risk, with 102 cm and 88 cm marking substantially increased risk. A waist-to-height ratio of 0.5 or above is also often used as a flag, meaning your waist is half your height or more.

Frequently Asked Questions

What is the BMI formula?

BMI is your weight in kilograms divided by your height in metres squared (kg/m²). A person who is 1.75 m tall and weighs 82 kg has a BMI of 82 ÷ (1.75 × 1.75) = 82 ÷ 3.0625, which is about 26.8. A BMI formula calculator simply does this division for you, and converts feet, inches and pounds to metric first if you prefer imperial units.

How do I calculate BMI by hand?

Convert your height to metres (170 cm is 1.70 m), multiply it by itself, then divide your weight in kilograms by that number. If you only have pounds and inches, multiply pounds by 0.4536 to get kilograms and inches by 0.0254 to get metres first. Rounded to one decimal place, the result is your BMI.

How do I figure BMI if I only know my height in feet and inches?

Add up your height in inches (5 ft 9 in is 69 inches), multiply by 2.54 to get centimetres, then divide by 100 for metres. Convert your weight from pounds to kilograms by multiplying by 0.4536, and use the standard kg/m² formula. Or switch the calculator above to imperial units and it will do the conversions.

What BMI is considered healthy for adults?

For adults aged 18 and over, the World Health Organization categories are: under 18.5 underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, and 30 or above obese (class I 30 to 34.9, class II 35 to 39.9, class III 40 and above). These are general screening bands, not a diagnosis. Your GP can tell you what they mean for you.

Is there a different BMI calculator for adults and children?

Yes. The adult BMI calculator on this page is only for people aged 18 and over. Children and teenagers are still growing, so their BMI is compared with percentile charts for their age and sex rather than fixed cut-offs. If you are worried about a young person's weight, see a GP.

What is the healthy weight range for my height (BMI by height)?

Multiply 18.5 and 24.9 by your height in metres squared to get the lower and upper ends of the healthy range. At 170 cm that works out to roughly 53.5 to 72.0 kg, and at 180 cm roughly 59.9 to 80.7 kg. The BMI by height table on this page shows the range for heights from 150 cm to 195 cm.

What does a BMI table show, and how do I read one?

A BMI table lists weights against the BMI they produce at a particular height, with each row labelled underweight, healthy weight, overweight or obese. Find your height, then find the row nearest your weight. The calculator builds a BMI table for your own height in 5 kg steps, so you can see how far each category is from where you are now.

Is BMI accurate for older adults? Is there an elderly BMI calculator?

The BMI formula is the same at any adult age, but its meaning can change as we get older. People tend to lose muscle and height with age, and some guidance suggests that a slightly higher BMI can be appropriate in people over about 65. Because of this, it is best to discuss your result and your ideal weight with your GP rather than rely on the standard adult bands alone.

Why can BMI be misleading?

BMI uses only height and weight, so it cannot tell muscle from fat or show where fat is carried. A very muscular person can have a BMI in the overweight range with little body fat, while someone with low muscle can have a healthy BMI but a high proportion of body fat. Research also suggests that people from some backgrounds may have a higher health risk at a lower BMI, so waist measurement and a check-up with your GP give a fuller picture.

Why does the BMI calculator ask for my waist measurement?

Where fat is stored matters as well as how much weight you carry. In Australia, a waist of 94 cm or more for men and 80 cm or more for women is commonly cited as increased risk, and 102 cm or more for men and 88 cm or more for women as substantially increased risk. Many health professionals also look at waist-to-height ratio, where 0.5 or above is often used as a flag. These are guides only, so ask your GP how they apply to you.