Figuring out your ideal weight is trickier than it sounds. The NHS and World Health Organisation (WHO) still rely on BMI (Body Mass Index), but according to many medical professionals, this is an inadequate and often inaccurate method. BMI typically overestimates ideal body weight for shorter people with little muscle mass and underestimates ideal body weight for taller, fitter people. Many elite athletes would be classed as ‘obese' according to their BMI. Conversely, those with a dangerously high proportion of body fat may be lulled into a false sense of security by a ‘healthy' BMI result.
What is BMI?
BMI is calculated by dividing your weight (or ‘mass') in kilograms by your height in metres, squared.
i.e. BMI = mass in kg ÷ (height in metres x height in metres)
You can find a BMI calculator on the NHS website. According to WHO, a BMI between 18.5 and 25 falls within the healthy range. Those with a BMI of 18 or under are considered underweight, whilst those with a BMI over 25 are classed as overweight.
However, BMI doesn't take frame size or body composition into account. The ideal body weight for someone with a small frame is considerably lower than the ideal body weight for someone with a large frame, for example. Furthermore, muscle and bone weigh more than fat. If you have osteoporosis, your BMI will be lower than someone of the same height with the same amount of body fat, since your bones will be lighter. Equally, if you exercise regularly, you are likely to have a higher BMI than a sedentary person with the same measurements, as you are likely to have a higher ratio of muscle to fat.
How to Calculate Your Ideal Body Weight
So how can you calculate your ideal body weight? No formula is perfect, but the Hamwi formula takes body frame size into account, which enhances accuracy.
According to Dr G.J. Hamwi, who came up with the formula, the ideal weight for a woman who is 5ft tall is 100lbs (i.e. 7st 2lbs, or approx. 45kg). Add 5lbs (approx. 2.2kg) for every inch of height over 5ft. For example, a woman who is 5'4 would add 20lbs, making her ideal weight 120lbs, or 8st 8lbs (approx. 54.5kg). For a man, start at 106lbs, adding 6lbs for every inch over 5ft.
If you have a small frame, you should subtract 10% from this result. If you have a large frame, you should add 10%. After adjustment, the ideal body weight for a small-framed woman of 5'4 becomes 108lbs (7st 7lbs, or approx. 49kg). This weight would place her on the borderline between healthy and underweight according to her BMI calculation (18.5). The ideal body weight for a large-framed woman becomes 132lbs (9st 6lbs, or approx. 60kg). This weight would place her towards the upper end of the ‘healthy' BMI range, with a result of 22.7.
How to Calculate Frame Size
Measuring your wrist circumference with a tape measure is a simple and fairly reliable proxy for calculating frame size.
Women under 5'2
Small: wrist circumference less than 5.5"
Medium: wrist circumference between 5.5" and 5.75"
Large: wrist circumference over 5.75"
Women between 5'2 and 5'5
Small: wrist circumference less than 6"
Medium: wrist circumference between 6" and 6.25"
Large: wrist circumference over 6.25"
Women over 5'5
Small: wrist circumference under 6.25"
Medium: wrist circumference between 6.25" and 6.5"
Large: wrist circumference over 6.5"
Men over 5'5
Small: wrist circumference under 6.5"
Medium: wrist circumference between 6.5" and 7.5"
Large: wrist circumference over 7.5"
So, next time you step on the scales and try to work out your BMI, don't blanch in horror at the results. Try calculating your ideal body weight using the the Hamwi formula instead, you might be surprised at the results...