BMI vs Body Composition: Everything You Need to Know

INSIGHT | January 2026

BMI, the abbreviation for Body Mass Index, has been the go-to measurement for what a “healthy” body weight is since the 19th century.

However, a person’s health goes beyond just their body weight. Fat distribution, water weight, and muscle mass are all important to consider for a more accurate gauge of someone’s physical condition. Together, these factors are known as body composition.

So – in the BMI vs body composition debate, which side should you be falling on when it comes to assessing progress against your fitness goals?

Below, we’re diving deeper into the BMI vs body composition distinction – unpacking how you can utilize your numbers to give a clearer picture of your health and fitness, and how F45 Training workouts can help you move closer to your health and fitness goals.

What is BMI?

Body Mass Index (BMI) was devised by a Belgian mathematician as a universal standard to compare body size across large populations. Initially, it was created to enable governmental bodies to evaluate public health¹. These days, BMI tends to be a go-to, cost-effective way for doctors to estimate if a patient’s weight is healthy compared to their height.

How BMI is measured

To calculate BMI in imperial units, divide a person’s weight in pounds (lbs) by the square of height in inches, which is multiplied by 703.

This number is then compared across a weight range to determine the weight class:

Underweight is classified as a BMI below 18.5.

A healthy weight falls between 18.5 and 24.9.

Overweight is categorized as 25 to 29.9.

Obesity begins at a BMI of 30 and above.

Seems straightforward, right? Well, remember that BMI was conceived in the 1800s – and, more recently, health experts advise that BMI is a poor metric for measuring people’s health³. This is because BMI fails to convey if someone is healthy – nor does it account for muscle mass and the fat distribution on the body.

For example, even someone training like an Olympian (and consuming the right mix of macronutrients and post-workout protein shakes) can be considered overweight – or even obese – based on their BMI calculation.

So – if BMI is a flawed metric, is there a better one?

What is body composition?

Body composition simply refers to all the elements that the body is composed of.

This encompasses the most obvious body parts, like your fat and muscles, but also considers the weight which is overlooked in the Body Mass Index – which is why water, bone, even blood and organs are included in overall body composition calculations. Because of this, body composition offers a more accurate barometer of an individual’s health and fitness levels.

How body composition is measured: 3 techniques

There are several ways to measure body composition, ranging from quick everyday methods to more accurate clinical scans.

Skin callipers

Skin callipers are a cost-efficient instrument that measures the thickness of subcutaneous fat at various areas on the body. From this, several equations are applied to figure out overall body fat percentage.

While this can provide a quick and accessible method that can be done anywhere, it has its limitations. You can’t, for example, measure levels of visceral fat or bones, which means the accuracy of skin callipers can vary – and it’s best to estimate under the guidance of a trained professional.

Bioelectrical Impedance Analysis (BIA)

Bioelectrical impedance analysis – also known by its simpler name, a ‘smart scale’ – works by sending safe, low-level electrical pulses that measure the speed of the currents moving through the water, fat, and muscle in your body.

This is a convenient way to measure body composition, but there are some limiting factors that can affect the accuracy of the measurements – for example, water retention at the time of weigh-in – making it less reliable for day-to-day measurement.

InBody Scan

An InBody scan is the gold standard for body composition measurement. The body composition analyzers deliver precise measurements of muscle, fat, and water to provide actionable insights for your health goals(5). Learn more about how and InBody scan can help track your training progress and check to see if the InBody scan is available at your F45 Training studio.

BMI vs body composition: what’s the difference?

As we’ve discussed, BMI would classify two people (both weighing 155lb and standing at 5’ 5” in height) as overweight. But their body compositions could be completely different.

Person A, for instance, might be a regular presence at F45 workouts – enabling them to build a larger amount of muscle and less body fat than person B.

Why body composition is a better measure for fitness

In the BMI vs body composition question, there are several other reasons why we recommend the latter as a unit of measurement for fitness.

Highlights actual progress

Whether you’re training for longevity and wellness or using strength training to build muscle, body composition enables you to gauge whether you’re moving in the right direction.

Supports personalized training and nutrition

Being able to accurately identify your body fat and muscle mass allows you to better tailor your training, nutrition, and recovery efforts to your overall goals.

Better predictor of health

Having higher levels of fat is associated with a wide range of health concerns⁴. Knowing your body composition, then, gives you a clearer picture of your health status and allows you to set realistic goals that you can achieve.

How to improve body composition

Now that there’s a clear winner in the BMI vs body composition debate, read on to see how you can improve your body composition at an F45 studio near you.

Exercise

It’s well-established that exercise is the key to boosting body composition. Make sure you’re implementing a hybrid approach that incorporates resistance training (to build muscle) and cardio (for its aerobic health benefits) to maximize your training output.

Want to learn more about how F45 group classes can work – and how they can propel you to your ideal body composition? Read more about what F45 is today.

Nutrition

Exercise will move the dial, but the right nutrition is key to shaking off any unwanted fat and building lean muscle. By knowing your body composition, you’ll have a better handle on how to reach your fitness targets through the right food: be that gut-healing smoothies, micronutrients, or by calculating the correct amount of protein you need per day.

Recovery

Recovery is the final piece of the body composition puzzle. The importance of implementing rest and recovery for improving body composition cannot be understated. This includes getting the right amount of sleep – so if you’re struggling to get the right amount, explore our top sleep hacks, or read up on the seven types of rest and how they can help.