By Debra Waters
Wondering how much you should weigh and what your BMI should be? Of course, it's different for everyone - but here's our guide to figuring out you 'ideal' weight and the Hamwi Method.
Figuring out your ideal weight
Knowing how much should you should weigh is trickier than it sounds. The NHS and World Health Organisation (WHO) still use BMI (Body Mass Index) as a guide, but according to many medical professionals this is an inadequate and often inaccurate method. BMI typically overestimates ideal weight for shorter people with little muscle mass and underestimates ideal weight for taller, fitter people.
“BMI is not a great guide because it uses only height and weight without taking body composition into account,” says fitness trainer Julia Buckley.“A person could be muscular with very low body fat levels and be in fantastic shape but because muscle is heavy BMI would class them as obese. For example, many athletes like rugby players or boxers would be considered obese if you only looked at their BMI.”
Three-time Olympian and founder of Roar Fitness Sarah Lindsay, agrees,“BMI and fitness gauges can be flawed, as you need to take into consideration that there are many different factors at work, and exercise and weight training can make you heavier as you carry more muscle.”
However, a 2018 study conducted by researchers at the University of Bristol supported BMI as a helpful measure of health. According to Science Daily, Dr Joshua Bell, an epidemiologist who led the study, said, "BMI is often criticised. Our study asked how useful it really is for detecting the health effects of obesity by pitching it against more objective body scan measures. We found that trunk fat is the most damaging to health, but that simple BMI gives very similar answers to more detailed measures. This is good news since BMI is widely measured and costs virtually nothing."
If you're keeping an eye on your weight, it's recommended that you buy some good quality, accurate bathroom scales. Buy the excellent value Salter Ito Bluetooth Analyser Pro Scales(£59.99), or the highly accurate Tanita RD-545 Body Composition Monitor Scales(£399). Use the same scales and weigh yourself at the same time each day, such as first thing in the morning.
What should my BMI be?
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'll find a BMI calculatoron the NHS website. Current guidelines state that a BMI between 18.5 and 24.9 is considered healthy. Those with a BMI of 18 or under are considered underweight, whilst those with a BMI over 25 are classed as overweight.
What makes BMI questionable is that it 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're likely to have a higher ratio of muscle to fat.
Calculate your ideal body weight using the Hamwi Method
While no formula is perfect, the Hamwi Method takes body frame size into account, which may enhance accuracy.
Dr G.J. Hamwi states that 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.
Women with a small frame should subtract 10% from this result. If you have a large frame, you should add 10%. So, 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″
Next time you step on the scales and try to work out your BMI, don’t blanch in horror at the results. “I’d recommend using BMI and fitness gauges as a guide,” recommends Sarah. “Re-test to see if things have improved or got worse. And never compare yourself to someone else.”
“Numbers are not really important,” adds Julia. “We just need to be fit, then we know we’re doing something really powerful to increase our chances of living a long, healthy and active life.”
Waist to height ratio: another key measure
In one study by Leeds Beckett University waist-to-height ratio (WHtR) was found to be the a good proxy measure for 'central obesity' (or the fat in your abdominal area, also known as visceral fat) trumping the BMI and other measures.
While this won't tell you what you weigh, it can be an important additional measure used in tandem with the BMI.
This study looking at WHtR as an indicator of ‘early health risk’ found that the group with ‘healthy’ BMI, and WHtR above 0.5, had some significantly higher cardiometabolic risk factors (the risk of having cardiovascular event such as a heart attack or stroke) compared to the group with ‘healthy’ BMI but WHtR below 0.5.
Visceral fat: why it's so important
"Visceral or intra-abdominal fat sits under the abdominal wall, which means that it surrounds vital organs like our liver, stomach and intestines. It’s more dangerous than other types of fat because it produces inflammatory cytokines that can trigger low-level systemic inflammation," says nutritionist and weight loss specialist Kim Pearson. "This is linked to an increased risk of diseases and faster ageing."
Waist to height ratio calculation
You can find your weight-to-height ratio by dividing your waist measurement in centimetres by your height measurement in centimetres. So if you had a 38-inch (96cm) waist and were 5ft 5 inches (168cm), your WHtR would be 0.57.
This NHS chart also looks at optimum WHtR for men and women in more detail, but generally follows the less than 0.5 rule.
Waist to hip ratio
Waist-to-hip (WHR) ratio is also another measure of visceral fat. Find out why your waist-to-hip ratio is important and how to calculate it here.
"Regardless of your height or BMI, you should look to lose weight if your waist is greater than 94cm for men and 80cm for women," Kim suggests.
Debra Waters is an experienced online editor and lifestyle writer with a focus on health, wellbeing, beauty, food and parenting. Currently, she writes for the websites and Woman&Home and GoodtoKnow, as well as the Woman, Woman’s Own and Woman’s Weekly magazines.
Previously, Debra was digital food editor at delicious magazine and MSN. She’s written for M&S Food, Great British Chefs, loveFOOD, What to Expect, Everyday Health and Time Out, and has had articles published in The Telegraph and The Big Issue.
When she’s not parenting, cooking new dishes or trying (in vain) to make her cats Instagram stars, Debra writes fiction—she won the Bridport Short Story Prize in 2020, which led to an interview on R4’s Woman’s Hour, and her stories have been long- and short-listed in a number of writing competitions.
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