How to Calculate BMI

BMI, or body mass index, is a measurement generally used to gauge the healthiness of a person's weight. Invented by a physician called Quetelet in the early 1900s, BMI is widely used as a rough indicator of body mass health.

So, how is BMI calculated? You need two measurements: your height and your weight. Then, simply follow the formula below to get a reading (or use our BMI calculating graph), and compare your result to the healthy BMI limits recommended by the NHS.

If you've been thinking about weight management and the health of your heart, there's no harm in finding out how to calculate your BMI. Just remember to always get the opinion of a healthcare professional before making key decisions about weight.

Why should I be calculating BMI?

BMI can indicate whether a person’s weight is healthy, taking into account their height as well. Being overweight can mean that you’re more likely to have elevated cholesterol levels, so keeping an eye on your BMI (and managing it) is one way you can help to maintain heart health.

However, you should be aware that BMI does not factor in physical fitness – your BMI could be high if you’ve built up a lot of muscle, rather than fat. If you’re looking at your weight with a healthcare professional, they might wish to also measure your waist circumference (just above the top of your pelvis), as this measurement alongside a BMI reading can give them a better picture of your health. They’re also likely to have a chat with you about your overall diet and lifestyle.

How to calculate BMI: The formula

If you want to know how to calculate BMI manually, the formula is as follows:

KG/M2 or, weight in kilograms ÷ (height in meters x height in meters)

So, to enter this on a normal calculator, follow these steps:

  1. Height x height = height squared
  2. Kilograms ÷ height squared = BMI

Or you could use the graph below.


When calculating BMI, you'll need your weight in kilograms, and your height in meters. When you have your reading, read our guide to healthy BMI readings here to find out where you stand.

How can I lower my BMI?

You might like to try the following:

  • Schedule regular exercise, at least three times a week
  • Reduce your portion sizes
  • Cut down your energy intake by eating less high sugar and high saturated fat foods
  • Choose active options every day – stairs instead of lifts, walk instead of getting the bus, and so on

Things to consider when calculating BMI

BMI isn't strictly accurate, as a range of contributing factors can affect your weight, from muscle, to age, to conditions such as an overactive thyroid.

BMI also rarely has a lower weight bracket, which could lead very underweight and elderly people to believe they are a safe weight.

For these reasons, if you want to know how to calculate your BMI manually, never take your result as fact, and always ask a health professional for their assessment.

Remember – BMI is used as an indicator alongside other measurements, not a rule by itself.