How to calculate BMI

Body Mass Index (BMI) is a useful benchmark for body weight and health. You might even have had yours measured by your GP.

But how can you calculate your BMI at home? And what does your final BMI number really mean?

What is BMI?

BMI is just one way of calculating whether your body mass is healthy for your height.

It’s more meaningful than just tracking your body weight, but it’s less accurate than calculating what your mass is made up of.

Your muscle mass, bone density, body fat and water weight are all important health markers. However, it’s not easy to know what they all are, so BMI is an easy calculation that acts as a useful benchmark for health.

This article explains how to work out your BMI and what your BMI numbers actually mean.

What does BMI measure?

BMI essentially tells you if you’re a healthy weight for your height based on general guidance (as listed below).¹

In terms of the specifics, BMI measures a person's weight in kilograms divided by their height in metres.

A high BMI score acts as an overall indicator of high body fatness (note the word ‘indicator’ - more on this at the end of this article). BMI can be used to screen for weight categories that may lead to health problems, but it does not calculate body fatness or how healthy you are.²

Summary

BMI essentially tells you if you’re a healthy weight for your height based on your overall body mass and how it then compares to general benchmark indicators.

How to measure BMI

Calculating your BMI simply involves measuring your height, weighing yourself and then doing this quick equation:

1. Divide your weight in kilograms by your height in metres.
2. Divide that answer by your height.

That’s your BMI!

For example:

If a person weighs 70kg and are 1.75m tall, they will have a BMI of 22.9 (rounded up).  This outcome is based on:

1. Dividing the weight (70) by the height (1.75), which equals 40.

2. Then taking the 40 figure and dividing it by the height, which comes to a BMI score of 22.857.

Summary

You can calculate your BMI manually by dividing your weight in kilograms by your height in metres and then dividing that answer by your height. Or, you can use a BMI calculator.

Is your BMI healthy?

Now that you know how to calculate your BMI manually (note – there are several online calculators you can use if you prefer) – what does it say about you?

Here’s some general guidance on BMI results:

But remember BMI can only measure your mass and doesn’t take muscle mass into account. This means if you’re slight with a small build, thin wrists and ankles, you may have an underweight BMI, but still be healthy.

And if you carry a lot of muscle mass and have an athletic build, you could have an overweight BMI, but actually be in good health. BMI is just one indication of health and should be considered alongside many other factors.

Summary

BMI is ranked according to four different levels – underweight, normal, overweight and obese. Your BMI calculation will fall into one of these different categories.

What is the correct BMI for my age?

Everybody’s BMI is unique to them, depending on their measurements, it’s not just based on age.³

It’s generally seen as a useful guideline and good estimator of body fat for the vast majority of adults aged between 19 and 70-years-old.

What’s more, there is no single ideal weight and height for men and women. Ideal weights vary from individual-to-individual and usually increase as people get older.

What is the best BMI for a woman?

The BMI readings apply to women, in the same way they would men. So, if you were to look at the table above, a good or ‘normal’ BMI reading for women is around 18.5 to 24.9. And a BMI of 30 or above may indicate obesity.⁴

Most women find that their body weight changes over time due to factors, such as pregnancy and the menopause, not just as a result of getting older. However, generally speaking, BMI is still used as a way of measuring overall body weight in women.

What is the best BMI for a man?

As with women, the same BMI guidance parameters apply to men too. Ideally, men’s BMI results should sit within the ‘normal’ bracket – between 18.5 and 24.9.⁵

It’s also worth noting here that muscle weighs more than fat so men with a high muscle bulk may have a high BMI without being unhealthily overweight. This is due to the fact muscle is contributing to their weight and not just fat.

Ethnicity is believed to play a part in BMI numbers too. Because of the increased risk of diabetes in Asian men, they’re advised to keep their BMI below 23.

Although the evidence is less clear-cut, black people and other minority groups are also advised to maintain a BMI below 25 to help reduce their risk of developing Type 2 diabetes.⁶

People of Asian origin are prone to accumulating intra-abdominal fat (fat deep inside their stomachs rather than under their skin) at a lower BMI than people of Caucasian origin.

People with this pattern of weight gain are described as ‘apples’ rather than ‘pears’ from their body outline. This means their health risks start to rise at a lower BMI, because intra-abdominal fat is directly linked to development of heart disease and Type 2 diabetes.⁷

Summary

The same four parameters are used for to determine both men and women’s BMI. It’s worth nothing men may have more muscle mass that may make them appear unhealthily overweight, when in actual fact, their weight is not due to excess fat. 

What is the best BMI for children?

When it comes to calculating BMI numbers for children and young people (from the age of 2 up to the age of 18), age and gender, as well as height and weight, are taken into consideration.⁸

Overweight children are thought to be at an increased risk of developing a variety of health conditions, and they're also more likely to be overweight as adults.

The BMI calculator works out if a child or young person is:

• Underweight – on the 2nd centile or below
• Healthy weight – between the 2nd and 91st centiles
• Overweight – 91st centile or above
• Very overweight – 98th centile or above

Children’s BMI scores are reported in centiles to show how their BMI compares with the national average.

For example, a girl on the 75th centile is heavier than 75 out of 100 other girls her age.

If you’re concerned about your child’s weight, speak to your GP about it.  They may be able to refer you to your local healthy lifestyle programme for children, young people and families.

Summary

When it comes to calculating BMI numbers for children and young people (from the age of 2 up to the age of 18), age and gender, as well as height and weight, are all taken into consideration.

Is BMI accurate?

On the whole, BMI is a useful measure for most adults. However, BMI results may be a less reliable indicator for some groups of people, including:^9,10

• Athletes with a lot of muscle – who may weigh more due to healthy muscle and not unhealthy fat
• Some ethnic groups – due to the fact their health risks start to rise at a lower BMI, as mentioned above
• Older people - because muscle mass tends to drop and body fat tends to rise in older people, BMI may not be an accurate reflector of body fat for people over the age of 60

Your BMI measures your weight compared to your height. It’s important to interpret your weight correctly because being overweight or obese can significantly increase your risk of a variety of health conditions, such as Type 2 diabetes and heart diseases.¹¹

However, some shortcomings in relation to BMI have been raised, mainly because it’s a standard tool that applies to all adults and is based on averages. What’s more, BMI only determines if somebody is a ‘normal’ weight without taking their age, sex, genetics, lifestyle, medical history or other factors into consideration.

As a result, working out BMI alone may result in other key health measurements, such as cholesterol, blood sugar, heart rate, blood pressure and inflammation levels, being overlooked.

Meanwhile, despite men and women having varying body compositions — with men having more muscle mass and less fat mass than women — BMI uses the same calculation for both groups.

Summary

As much as BMI is widely used, it’s not a clear measure for muscly athletes, some ethnic groups or older people. Plus, it doesn’t take wider factors, such as lifestyle and genetics, into account, as well as many other key health measurements.

Takeaway

Working out BMI is a useful way of calculating your weight based on your height that’s easy for everybody to use.

However, while it may be a useful indication of whether or not your weight is ideal, it doesn’t delve deeper and explore the factors that may have contributed to your final BMI score.

Given this, it’s best to assess your health, using your BMI score, alongside other health assessments to create more of a rounded picture that you can act on, if required.

If you have any concerns about your weight, speak to your GP.

Last updated: 11 June 2021