Measuring your BMI, BMR and Waist-to-Height

There are several different measures out there, which are all designed to tell you if your weight falls within a healthy range. Personally, I found BMR the most useful, but this handy guide will hopefully help you decide for yourself. Let’s begin with BMI, as it’s the one you’re likely to be most familiar with.

Body Mass Index (BMI) is a very simple calculation using a person's height and weight. The formula is BMI = kg/m2, where kg is a person's weight in kilograms and m2 is their height in metres squared.

You’re considered normal weight if your BMI is between 18.5 and 25 or overweight if it’s between 25 and 30. Anybody with a BMI of 30 or more is considered obese. BMI is usually applied to adults aged 18-65 years.

Using myself as an example….10 stone 7 lbs is 66kgs, and 5’7” is 1.7m. So 68 divided by 1.7m2 = gives a BMI of 23.5, which puts me in the healthy range.

The problem with BMI is that the equation was designed to assess general population health trends, not individual health and so it doesn’t consider age, gender, fitness level, race or any other metric, because it wasn’t designed to do so. Nor does BMI take into account the distribution of fat around the body. Abdominal fat affects organs like the kidney, liver and heart much more severely than fat around the bottom or hips, for example.

But BMIs biggest flaw is that it doesn’t track body fat versus muscle (lean tissue). Muscle takes up less space than fat (it’s far denser) so your clothes might feel looser as you lose fat, but your weight may not actually change if you’re adding muscle. Therefore, BMI inevitably classes muscular, athletic people as fatter than they really are.

Here's one example. Wrestler Steve Austin, at the height of his career, weighed 114kg (252lbs). He was 6ft 2ins tall. His BMI was therefore 32.4, which would have classed him as obese. Which is a little ridiculous.

Using BMI, a 6ft-tall Olympic 100-meter sprinter weighing 90kg (200lbs) could have the same BMI (26) as someone who has a sedentary lifestyle of the same height and weight. A BMI calculation would class both of them as overweight. That calculation is probably right for one, but perhaps not for the athlete.

So is Basal Metabolic Rate (BMR) a better measure?

You may have noticed that most food labels and diets refer to the "percent of daily values”, which are based upon an ‘average’ person eating around 2000 calories per day. The problem is, you’re not average. And neither is anyone else.

As a result, and depending upon your height, weight, age, gender and level of physical activity, you may need more or less than 2000 calories in order to maintain your weight.

If you’re looking to shed a few pounds, BMR is good starting point, as it’ll give you a good guide on how much you need to be eating. But how do you calculate that specific daily calorific total?

The Basal Metabolic Rate (BMR) is the amount of energy (calories) your body needs while resting and accounts for around 60 to 70 percent of the calories you burn each day. One of the most accurate methods of estimating your basal metabolic rate is the Harris-Benedict formula:

• Adult male: 66 + (6.3 x weight in lbs.) + (12.9 x height in inches) - (6.8 x age in years) = BMR

• Adult female: 655 + (4.3 x weight in lbs.) + (4.7 x height in inches) - (4.7 x age in years) = BMR

To determine your daily calorie needs, you simply multiply your BMR score by the appropriate activity factor, as follows:

• If you are sedentary (little or no exercise) = BMR x 1.2

• If you are lightly active (light exercise/sports 1-3 days/week) = BMR x 1.375

• If you are moderately active (moderate exercise/sports 3-5 days/week) = BMR x 1.55

• If you are very active (hard exercise/sports 6-7 days a week) = BMR x 1.725

• If you are extra active (very hard exercise/sports & physical job or 2x training) = BMR x 1.9

Here’s my personal calculation. I weigh 150 lbs, I’m 5’7” tall (67 inches) and I’m 50 years old. So that gives me a BMR calculation as follows:

66 + (6.3 x 150) + (12.9 x 67) – 6.8 x 50) = 66 + 954 + 864.3 – 340 = 1544.3

I’m moderately active, so 1544.3 x 1.55 = 2,393

So I need 2393 calories per day in order to maintain my current weight. But notice I said, “maintain my current weight”.

If I wanted to lose weight, I’d need to reduce my caloric intake below that 2393 figure. That deficit can be achieved either by calorie reduction alone, or by a combination of eating less and doing more.

So what about Waist to Height ratio (WHtR)?

Waist to Height ratio (WHtR) is a better predictor of heart disease and diabetes risk than BMI, according to recent research, and keeping your waist circumference to less than half your height, can help increase life expectancy.

The advantage of WHtR is also the simplicity of the message. It’s a much easier thought to hold in mind than BMI, where not only do you have to work out the ratio of your weight in kilos to the square of your height in metres, but also remember what the healthy range is.

So a man who is 6ft or 72 inches tall (183 cm), should keep his waist under 36 inches (91 cm), and a woman who is 5ft 4 in or 64 inches tall (163 cm), should keep her waist measurement under 32 inches (81 cm).

To measure the waist circumference accurately, start by finding the bottom of your ribs and the top of your hips. Place a tape measure around your middle at a point halfway between them (roughly just above the belly button). Make sure it's pulled tight but isn't digging into your skin.

I wear waist 29 jeans, but my waist circumference using the method above is 32.5 inches. At a height of 67 inches (5’ 7”), I am just under the 50% WHtR (with a bit of room to improve).

So how does these measurments help?

Broadly speaking, BMI and WHtR will both tell you if you need to lose (or gain) weight. BMR will help you work out how many calories you need to eat to maintain your current weight, and therefore how many calories you need to cut in order to lose some.

Here’s a practical example.

There are approximately 3500 calories in one pound of stored body fat, and NHS guidance usually suggests that losing around 1-2lbs (0.5-1kg) a week is healthy and sustainable. In order to lose that 1lb a week, you’re going to need to cut 3500 calories each week (e.g. 500 calories per day) through diet and/or exercise. To lose 2lbs a week, you’ll need to cut 7000 calories a week (or 1000 per day).

Roughly speaking, 12 minutes of jogging or around 22 minutes of brisk walking will burn around 100 calories. That’s the equivalent of jogging or walking about 1 mile. So if you cut your calorific intake by 250 calories a day by eating less, then take some moderate exercise (such as going for a brisk walk for an hour each lunchtime) a 500 calorie a day reduction is a very achievable goal.