The BMI calculation was invented in the 1840s and as used currently gives results which may not be very useful for people who are some way above or below average height.

If you scale an object linearly in all dimensions to double its height, its mass is cubed, not squared.

The BMI calculation only relies on a squaring of weight for a doubling of height rather than cubing although it should be noted that human growth isn’t linear in three dimensions.

The effect is that for those of below average height, the BMI is probably less than it should be for a given height and weight and for those taller, greater.

A new BMI calculation has been prpopsed by Professor Nick Trefethen of the University of Oxford which addresses some of the limitations of the current BMI calculation.

You can see what effect the new calculation has here;

http://www.bbc.co.uk/news/magazine-21229387

BMI is not a precise measure as it really doesn’t give the actual body fat percentage. Because muscle is denser than fat, athletes may appear to be overweight according to BMI inspite of being very lean and fit.

You can read about other ways to measure body fat here;

http://www.builtlean.com/2010/07/13/5-ways-to-measure-body-fat-percentage/

Medical imaging with CT or MRI scanners is used to determine where the fat is distributed and can reveal whether someone is ‘TOFI’ – thin on the outside and fat on the inside;

http://www.theguardian.com/science/2006/dec/10/medicineandhealth.health

This has health implications as well as the body fat percentage.

Sarah