It is a calculation to determine the amount of fat in a person's body, and therefore their risk of weight-related diseases such as diabetes. The Body Mass Index was devised in 1840 by Adolphe Quetelet, a Belgian scientist, and has since been adopted worldwide as the standard way to measure obesity. To find your BMI, divide your weight in kilograms by the square of your height in metres.

Why is BMI in the news?

Authorities in Madrid have banned designers at the city's annual fashion week from using models who have a Body Mass Index of less than 18, to stop the promotion of super-skinny body shapes.

The new rule has made an immediate impact, with one designer, Antonio Pernas, having to change all 18 of his models for his catwalk show on Monday because they failed the BMI test.

Eating-disorder experts in Britain have also called for a law to be brought in that would make it illegal for agencies and bookers to employ models with a BMI of less than 18. The move has triggered a debate about what can and can't be considered a healthy size, with many in the fashion world insisting that some models are being scapegoated. On the other side, however, as many as 20% of our children are considered to be obese, a factor alarming many healthservice providers because obese children are much more likely to be overweight as adults, with all the attendant problems of diabetes and heart disease.

Here there is less pressure for Irish models to be so slim. Most shows here are organised by retailers and the sizes used are larger than the tiny garments used in the couture shows. Most models on Irish agencies' books are a size 10.

So what is a good BMI to have?

A BMI between 18 and 25 is considered healthy; less than 18 is classed as underweight, between 25 and 30 and you are overweight, while a score of more than 30 makes you obese.

People with a BMI of more than 40 are considered to be morbidly obese. For children, the rating is more complex. Juvenile BMI is calculated in the same way but is compared against the total range of scores for all children of the same age. A child whose BMI falls below the 85th centile of their age group is underweight, while those in the 85th to 95th centiles are classed as overweight and those in the 95th centile or above are obese.

How do we measure up?

Bodywhys, the Irish eating disorders association, estimates that as many as 200,000 Irish people suffer from an eating disorder such as anorexia or bulimia or binge eating.

Department of Health figures show that we have been piling on the pounds here. Two out of every five adults are overweight, and one in five is obese. About 300,000 of our children are overweight, a figure increasing by 10,000 every year.

Are there any problems with the Body Mass Index method of assessing a healthy weight?

Yes. First there is the problem of accuracy when assessing children. Three months ago, researchers at Loughborough University said that BMI was a "dreadfully imprecise measuring tool" for youngsters because they grow at different rates.

They warned that routine weight testing of children as young as four, something being debated in Britain at the moment, could lead to an increase in bullying and eating disorders. They said a "degree of panic" had set in about the perceived dangers of being overweight.

It is not just children; one flaw with the BMI system is that it assumes that everyone has the same ratio of muscle to fat.

Because muscle is more dense than fat, someone who is extremely fit and toned may have an 'unhealthy' BMI, while another person with little muscle tone but a lot of fat can be rated 'normal'.

This anomaly was highlighted by a study last year which showed that when American football players were rated on their BMIs, 50% fell into the 'obese' classification despite being highly physically fit and healthy. BMI also fails to take into account gender differences; the average man has 15 to 17% body fat, while the average woman has 18 to 22%.

Is a BMI test for models valid?

It is a difficult one. Experts say that with a BMI of 18, a woman is at risk of developing an eating disorder and any rating below that will mean she probably already has one.

Dee Dawson, who runs a London clinic for girls with eating disorders, says that it's a fallacy to say that some models are just naturally very skinny. "It's just not possible to be 6' 2" and to weigh eight and a half stone without it being about controlling your eating, " she said. Jennie O'Reilly, chief executive of Bodywhys, was highly critical last year of suggestions from Katie Ford of the New York-based Ford Model Agency when she suggested that Irish models are "too fat" and drink too much beer.

Promoting "unrealistic body types" such as 6ft tall women who are only a size six or eight in clothes leads to body dissatisfaction and could activiate an eating disorder in people who are already struggling with their self esteem, O'Reilly argued.

Most model agencies, fashion designers and magazine editors insist they will only work with healthy models and that an arbitrary BMI cut-off is unfair. And they point to the fact that the trend for super-slim, "size zero" models on the catwalk has hardly crossed over into 'real' high street life; the average Irish woman is a size 14-16 and growing.

Is there an alternative for measuring obesity?

Old-fashioned scales have been replaced by electronic machines that measure your exact fat content rather than simply your body mass.

German researchers found that the ratio of waist circumference to height is a more accurate way than BMI of indicating the risk of obesity-related heart disease.

The WTR (waist-to-tallness) measurement involves dividing your waist circumference by your height (both in inches).

Anything over 0.53 for women and 0.55 for men is considered unhealthy.

And the old adage about being able to "pinch an inch" may be the simplest health check; according to private healthcare providers Bupa, a waist measurement of more than 102 cm (40 inches) for a man and 88cm (35 inches) for a woman indicates that you are reaching the tipping point.

As dieticians never tire of telling us, assessing whether you are healthy comes down to counting the calories you are taking in and comparing them with the energy spent on exercise.