Our personal trainer, Jess, explains how to calculate your BMI (body mass index) and what a healthy BMI is. Jess also discusses other measures to use alongside BMI when setting fitness and weight loss goals.
Your BMI is calculated by dividing your weight in kilograms by your height in metres squared. The result indicates where you sit on the BMI scale: underweight, healthy, overweight, or obese. This helps you assess if you may need to lose or gain weight for health reasons.
For most adults, a healthy BMI is between 18.5 and 24.9 range. Here are the different ranges on the scale:
The NHS has a handy BMI calculator that you can use to calculate where you are on the scale. Or, if you’d like to calculate yours manually, you’ll find the BMI formula at the end of this post.
Although it’s a widely used metric, the BMI calculation is often disputed. Many health professionals don't believe it provides a clear enough picture of someone’s health.
As a Personal Trainer I have to agree. There are a host of other factors to take into account when determining whether someone’s weight is healthy. For example, their age, gender, body type, activity levels, and muscle to fat ratio.
I’ve had female clients whose weight was distributed mainly on their hips, legs and breasts. They’ve been regularly active and eaten a healthy diet. But genetically, they are predisposed to carry more weight in those areas.
For this reason, their BMI may be outside the healthy range. Yet, they are a healthy weight for their body type.
In contrast, I have had clients with a lower BMI who still smoke, and don’t yet do any regular exercise. They have incredibly slim arms, legs and minimal breast tissue. But the weight they do have sits around their stomach.
In terms of overall health, I would class them as having a fair amount of work to do. Yet their BMI is within the healthy range.
My concern is where someone should be improving their health – but their BMI brings up no red flags. It’s important to look at where the fat is carried. Whatever your BMI, too much around the midsection is cause for concern.
A study quoted in The Independent, gives the example of a man with a healthy BMI of 22 who carries most of that weight around his midsection. According to his waist to hip ratio, he had an 87% higher mortality risk. This is as compared with a man with the same BMI but a healthy waist to hip ratio.
As a metric, BMI doesn’t take into account muscle mass, which is 18 times denser than fat.
BMI can tell if you are carrying too much weight – but not how much of that weight is fat. The measurement doesn’t take into account the breakdown of muscle mass, bone or fat.
As muscle mass is heavy, this can mean that someone has a high BMI but relatively little body fat. Their muscle mass skews their BMI result.
For example, I have a BMI of 24 – which although not in the overweight range still puts me above average. But my body fat percentage of 17.4% puts me in the athlete category. I have just always been heavy boned and muscly (not a bad problem to have, I know!).
This is really common for people in the fitness industry. Countless athletes and weight lifters are considered overweight or obese if you look at their BMI alone.
Despite the exceptions I’ve outlined, BMI can be useful when you are setting fitness and weight loss goals. But, I’d suggest using it in combination with other measures of health. For example, hip to waist ratio, blood pressure, cholesterol, and overall fitness level.
Along with these measures, use common sense. Are you taking care of yourself through regular exercise? Are you eating healthy foods and not overeating? Do you have weight on your stomach?
As a general guide, here’s what to do based on your BMI result:
If your BMI result is within the underweight range, it is a good idea to check-in with your GP.
You may not be consuming enough calories for the amount of activity you are doing. Or you could have an underlying health condition.
If your BMI result is within the healthy weight range, keep up your current diet and exercise routine.
That said, this advice doesn’t apply if you know that you aren’t exercising regularly, and your diet isn’t great. Healthy eating and exercise is important, whatever your BMI result.
It is possible to have a healthy BMI but still need to address health issues that are normally related to being overweight. Having a slim build and a low weight does not necessarily mean your internal system is healthy, even if your BMI is in the healthy range.
It is important to check your cholesterol, muscle mass and blood pressure too. You may have high cholesterol or blood pressure and low muscle mass, despite a healthy BMI.
To double check your health, it is a good idea to check your hip to waist ratio. This gives you an idea of weight carried around your midsection. Try using this online calculator.
You could also measure yourself using a bio-impedance scale. Also known as smart scales, these can provide you with metrics on your body composition.
There are many on the market and some link to fitness trackers. I own this smart scale and think it’s great. The reviews suggest it is just as accurate as high end models.
If your BMI is within the overweight range, you may need to lose weight. Try checking your waist to hip ratio and body composition to make sure.
If you decide you do need to lose weight, keep a log of your diet, and your current exercise levels. This will help you work out how many calories you are consuming and expending.
If you are consuming more calories than you are burning, you may need to eat less and move more. By eating a healthy, calorie-controlled diet and exercising, you can reduce your BMI and improve your health.
If your BMI is within the obese range, it is a good idea to check in with your GP to get an overall picture of your health. Your doctor can advise you on the best ways to increase your activity levels and improve your diet.
If you love a bit of maths and fancy calculating your BMI manually, here’s the formula in imperial and metric units:
Weight in lbs x 703 / divided by height in inches squared.
For example, an adult weighing 10st 4lbs (144lbs) and measuring 5ft 10inches (70 inches):
144 x 703 = 101,232
70 x 70 = 4,900
101,232 / 4,900 = BMI of 20.6
Weight in kg / divided by height in metres squared.
For example, an adult weighing 60.3kg and measuring 163cms (1.63m):
1.63 x 1.63 = 2.6
60.3 / 2.6 = BMI of 23.1--
We hope this post has helped you to understand what a healthy BMI is. And how to use BMI alongside other measures of health. If you have any concerns about your weight, it is a good idea to discuss these with your doctor.
Looking for ways to become more active? Check out our library of home workouts and fitness challenges: