Body Mass Index - What is it?

Body Mass Index is a measurement used to determine broadly how overweight or obese an individual or group of people is. It is used by scientists to compare how large a group of people are compared with another, for example BMI would be used to see whether the population of one state is more or less overweight than another state.

As you can read here, it's not an accurate measurement for judging the risk factor caused by excess weight, and can't actually tell you whether you're carrying too much body fat at all. This is a problem when it is used by doctors and insurance companies to make judgements about your body or health.

How is your BMI calculated?
Here's how you calculate your Body Mass Index:

Weight in kilograms / height in metres2

For the less "formula minded", to calculate your BMI, follow these three steps.

1. Work out your height in metres and multiply the figure by itself
2. Measure your weight in kilograms
3. Divide the weight by the height squared (the answer to Q1)

For example, you might be 1.6m (5ft 3in) tall and weigh 65kg (143lbs). The calculation would then be:

1.6 x 1.6 = 2.56. BMI would be 65 divided by 2.56 = 25.39.

How is your BMI interpreted?
The following divisions are used to determine whether you fall into the category of underweight, normal, overweight or obese based on your Body Mass Index:

     Less than 18.5 - Underweight
     18.5 - 25 - Normal
     25-30 - Overweight
     30+ - Obese
     40+ - Morbidly obese
(if you suffer with diabetes, high blood pressure or other "co-morbid conditions", you are considered morbidly obese at a BMI of 35 or higher)

Is it always an accurate judge of how overweight I am?
No. Body Mass Index is a crude measurement which plots height against weight. It takes no account of body shape or muscle weight.

For example if you have a large frame or you carry a lot of muscle in your body, you would have a very high Body Mass Index. Neither of these things is detrimental to your health, but you still weigh a lot for your height.

A better system of using BMI is to interpret them using a table which cross references them with build-type. These effectively move the boundaries between underweight, normal, overweight and obese depending on whether you have a light, medium or heavy build. This at least gives you an idea of whether you are "too heavy for your height and build", but again its effectiveness is limited - after all who can tell you whether you are small, medium or heavy build, and what if you're on the borderline?

It also does NOT resolve the problem of whether you carry your weight int erms of excess body fat (which is potentially problematic for your health), in extra muscle weight (say, if you're very sporty, or have a physically demanding job) or a combination of both.

The worst situation is to suspect that you're carrying too much weight, but also to be muscular and large framed. You'll find out very little from your Body Mass Index reading - all that will do is tell you that you're very heavy for your weight.

The best judge of whether you're overweight or obese:
There's no doubt that a body fat analysis is the best way of telling you whether you're carrying too much weight in body fat and if so by how much. Since muscle is more dense than fat, if you have a larger than average amount of muscle in your body, BMI will not be an accurate judge of how much fat you carry. A body fat analysis is far more accurate.

The best way to tell whether your weight is a risk to your health:
Hip to waist ratio is the best way of working out whether your size is going to pose a risk to your health. It is a simple measurement that you can calculate yourself with a tape measure and a calculator. Click here for more information about hip to waist ratio.

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