Calculating Adult Healthy Weight

Our most healthy weight is that at which we carry some body fat stores, but not so much that the risks to our health rise.

The ideal percentage of body fat for one person may be different to that for another person. It will vary with age, gender and heredity.

For men, a level of up to about 20% body fat is generally healthy. For women, the level is up to about 25%.

Women need a minimum of about 10% body fat, and men, a minimum of about 3%.

The problem is that there is no easy and reliable way to measure body fat. When we stand on the scales we are only measuring our total body weight. We get no feedback on whether we are carrying too much fat.

We expect that taller people will weigh more than shorter people who are carrying the same percentage of fat in their bodies.

To account for the fact that taller people are expected to be heavier (to have a heavier lean body mass), scientists developed the Body Mass Index equation. This equation takes both weight and height into account.

In adulthood, a person's height is stable. With few exceptions, adults do not grow taller. This means that for any one person, if their weight increases for any reason, their Body Mass Index will increase.

The BMI formula tells us that our BMI number will go up by one point when our weight goes up by the equivalent of our height in metres², expressed as kilograms. Thus, a person who is 1.6 metres tall, will go up by one BMI point if their weight increases by 1.6² kg i.e. by 2.56kg.

Once we become adults, the weight of most of our body organs and tissues stays relatively stable.

How The BMI Recognises Weight Change

By far the most common cause of increasing weight in an adult is increasing body fat.

It is this basic assumption that underlies the use of the BMI number in detecting, researching and monitoring weight gain, weight loss and obesity.

As long as we know a person's height and weight, we have a pretty good indicator, through the BMI number, whether that person is carrying more fat than is optimal for their health. We can also use the number as a simple means to compare large numbers of people who may all be different heights, because the BMI accounts for this variability.

Also, because height in adults is stable, once a person's height is known, by measuring just their weight, we can repeatedly calculate their BMI and have ready data about the probable risks to their health at any particular weight.

Example: Different Heights, Same BMI Numbers
Jenny is 25. She is 1.7m tall and weighs 68kg. She believes she weighs too much.
Jenny really likes the look of Annie, a movie star she admires. She hears that Annie weighs 53kg and decides that this must be a good weight for her to aim for as well. What she does not know is that Annie is 1.5m tall.

Looking at the BMI numbers of these two women:

Jenny:  BMI = weight(kg)/height (m)²
                     = 68/1.7²
                     = 23.5

Annie:   BMI = weight(kg)/height (m)²
                     = 53/1.5²
                     = 23.5

Jenny and Annie have the same BMI. This means that Jenny's weight is just as right for her, as Annie's is for Annie. If Jenny attempted to lose weight so that she, too, could weigh 53kg, she would be far too thin. Her BMI number would be 18.3, which is officially classified as underweight.

BMI definitions: underweight, healthy weight, overweight, obese
Scientists have studied the risk of health problems associated with different BMI numbers.
The cut off points selected to define these groups were chosen because they were felt to reflect most accurately when the risk levels to health began to change significantly.
If we look at the 'most healthy' BMI range, (from 18.5 to 24.9) this represents a weight range of 18.5kg.

One would could conclude from this that a 1.7m tall adult could gain 18.5kg and not have a 'problem' with their weight.

In practice what this really means is that one person of 1.7m could have a BMI of 18.5 and be at their most healthy weight, while another person of 1.7m could have a BMI of 24.9 and be at their most healthy weight.

What is recognised as most important is that somewhere in this range, there is almost always a BMI which is optimally healthy for each person. 

Traditionally, the way to recognise the most healthy BMI is to look back on each person's weight when they were a young adult. 

This still holds true for most people, but with increasing numbers of young people entering adulthood already carrying more body fat than is healthy, it becomes more difficult to estimate from young adult weight alone. In these cases, more formal body fat measurements can help to define the healthiest weight.

The more the BMI increases, even from a BMI as low as 20, the more the risk to health from body fat, rises.

It is recommended that adults only allow their weight to rise by a maximum of 2kg on their healthy young adult weight, before taking action to stop the rise. This applies even for people who remain within the healthy BMI range.

Example: Different Heights, Same BMI Numbers

Jenny is 25. She is 1.7m tall and weighs 68kg. She believes she weighs too much.
Jenny really likes the look of Annie, a movie star she admires. She hears that Annie weighs 53kg and decides that this must be a good weight for her to aim for as well. What she does not know is that Annie is 1.5m tall.

Looking at the BMI numbers of these two women:

Jenny:  BMI = weight(kg)/height (m)²
                     = 68/1.7²
                     = 23.5

Annie:   BMI = weight(kg)/height (m)²
                     = 53/1.5²
                     = 23.5

Jenny and Annie have the same BMI. This means that Jenny's weight is just as right for her, as Annie's is for Annie. If Jenny attempted to lose weight so that she, too, could weigh 53kg, she would be far too thin. Her BMI number would be 18.3, which is officially classified as underweight.

BMI definitions: underweight, healthy weight, overweight, obese

Scientists have studied the risk of health problems associated with different BMI numbers.
The cut off points selected to define these groups were chosen because they were felt to reflect most accurately when the risk levels to health began to change significantly.
If we look at the 'most healthy' BMI range, (from 18.5 to 24.9) this represents a weight range of 18.5kg.

One would could conclude from this that a 1.7m tall adult could gain 18.5kg and not have a 'problem' with their weight.

In practice what this really means is that one person of 1.7m could have a BMI of 18.5 and be at their most healthy weight, while another person of 1.7m could have a BMI of 24.9 and be at their most healthy weight.

What is recognised as most important is that somewhere in this range, there is almost always a BMI which is optimally healthy for each person. 

Traditionally, the way to recognise the most healthy BMI is to look back on each person's weight when they were a young adult. 

This still holds true for most people, but with increasing numbers of young people entering adulthood already carrying more body fat than is healthy, it becomes more difficult to estimate from young adult weight alone. In these cases, more formal body fat measurements can help to define the healthiest weight.

The more the BMI increases, even from a BMI as low as 20, the more the risk to health from body fat, rises.

It is recommended that adults only allow their weight to rise by a maximum of 2kg on their healthy young adult weight, before taking action to stop the rise. This applies even for people who remain within the healthy BMI range.