Is BMI the best body weight calculator?

How is BMI calculated?

BMI is calculated by dividing your weight in kilograms by your height in meters squared, according to the Centers for Disease Control and Prevention (CDC).

Don’t have a calculator handy? Luckily, many online tools can do the math for you – including the one on this page.

Here is how BMI results are categorized for people age 20 and older:

• Under 18.5: underweight
• 18.5-24.9: healthy weight
• 25-29.9: overweight
• 30 or higher: obese

BMI pros and cons

BMI can be a helpful tool due to how inexpensive and accessible it is. Aside from a scale, all you need to calculate your BMI is a calculator or online tool. Plus, the entire process can be done from the comfort of your home.

However, while BMI is a quick way to determine your weight classification, BMI might not be the best choice for everyone.

That’s because BMI doesn’t consider the following factors that may impact body weight.

Age: There is a separate BMI tool for people ages 2 to 19, but just one tool for anyone age 20 and over. However, BMI may not be the best tool for determining if an elderly person is at a healthy body weight. This is because people lose muscle mass as they age. Instead of BMI, using body fat distribution, or where on the body someone carries extra weight, can be a more reliable way for elderly people to determine if they are at a healthy weight, Poudyal says.

Ethnicity: BMI was originally created using data from non-Hispanic white males, Crowder notes. “There are variations among different ethnicities where BMI might classify someone as obese, but their body fat is within normal limits,” she says.

Gender: Adult BMI calculators don’t consider gender. This matters because men and women carry different amounts of fat — and for different reasons. “Men and women need different percentages of body fat for physiologic needs — mainly pregnancy and breastfeeding,” Crowder explains, noting that women generally carry more fat under the skin and in the hips and thighs, while men generally store more fat around the organs.

Muscle mass: BMI doesn't consider lean muscle mass. This means people with a high amount of muscle could be miscategorized as overweight. Lean muscle mass is important, not just in determining your body shape, but also for keeping your body strong during cancer treatment.

Body fat distribution: Where you hold fat matters when it comes to your health risks. Crowder notes that visceral fat, the fat surrounding your abdominal organs, is linked to a higher risk of conditions including:

• Metabolic disease
• Insulin resistance
• Diabetes
• Heart disease
• Certain cancers

Remember, a BMI calculator is just that: a calculator. While it can classify your weight, it can’t dig into the whys behind complex and personal subjects like weight and obesity.

“BMI doesn’t explain the variability of obesity and its underlying genetic, metabolic, physiological or psychological causes,” Poudyal says.

BMI alternatives

Want to determine your weight classification without BMI? You have plenty of options ranging from DIY to professional tools.

At-home options

Here, our experts explain several BMI alternatives you can perform yourself.

Waist circumference

Waist circumference simply involves measuring your waist.

To take this measurement, first, grab a tape measure. Next, wrap the tape measure around your waist between where your ribcage ends and your hip bones begin.

A few other tips to help you get the most accurate measurement:

• Take measurements while undressed.
• Make sure you are standing up straight and breathing normally.
• Resist the urge to suck in or flex your stomach.
• Make sure the tape measure isn’t too tight or too loose; aim for a happy medium.
• Take the measurement a few times to make sure your results are consistent.

While it is hard to make a list of BMI alternatives that skips waist circumference — especially because it’s a critical part of several other at-home body weight tools — there are several reasons why this tool isn’t our expert's favorite.

First, waist circumference results often don’t account for factors like genetics and ethnicity. Waist circumference also fails to consider body fat and body fat distribution, which can lead to misclassification, Crowder says.

Waist-to-hip ratio

To calculate your waist-to-hip ratio, measure your waist circumference using the steps above. Then, measure the circumference of the widest part of your hips. Divide your waist circumference by your hip circumference. Remember to take both measurements using the same metrics to avoid confusion!

According to the World Health Organization, a waist-to-hip ratio of over 0.9 for men and over 0.85 for women is considered abdominal obesity.

Waist-to-height ratio

Calculating your waist-to-height ratio starts with measuring your waist circumference. Next, measure your height using the same units you measured your waist in. Then, divide your waist measurement by your height. A result of 0.5 or less is considered healthy, Crowder says.

“Think: keep your waist half your height,” she says, noting that the waist-to-height ratio is sometimes used alongside BMI to calculate health risks.

Body roundness index (BRI)

The body roundness index is a calculator that can be found online. It predicts how much visceral fat your body has. It uses the following factors to calculate body shape:

• Gender
• Ethnicity
• Age
• Height
• Weight
• Waist circumference
• Hip circumference

“This is a more comprehensive measure; however, it still incorporates data tables from the early 1970s and still does not account for muscle,” Crowder says.

Professional tools

Health professionals have access to a variety of devices and methods that can help determine if you are at a healthy weight. These professionals can also help you interpret the results and understand what they mean for your health.

Ahead, our experts explain some professional options.

Dual-Energy X-ray Absorptiometry (DEXA)

In this method, X-rays are used to look at:

• Fat tissue
• Muscle mass
• Bone density

DEXA can also be used to screen for osteoporosis by examining hip and spine bone density.

Electric current measuring

In this method, electrodes that can send and receive electrical currents are attached to the skin. Currents are then sent through the body, and this information is used to analyze body composition.

• Bioelectrical impedance analysis (BIA): BIA, or InBody, is the most common type of electric current measuring. It uses your body’s resistance to an electrical current to estimate fat, muscle and fluid.
• Bioimpedance spectroscopy (BIS): This tool works similarly to BIA and allows for a closer look at fluid distribution.
• Electrical impedance myography (EIM): This form of electric current measuring can be used to examine muscle function, strength and fatigue.

Air displacement plethysmography

Air displacement plethysmography, which is sometimes called Bod Pod, measures how much airspace your body takes up inside a sealed chamber to determine body volume. Then, your weight is divided by your body volume to determine body density and estimate body fat percentage.

This is a good option if you have mobility issues, Crowder notes.

Underwater weighing

Underwater weighing, or hydrostatic weighing, is like the water version of Bod Pod. It involves measuring the amount of water that your body takes up while underwater.

“Calculations are made taking into consideration residual air volume. This is considered the gold standard, but it's very hard to do,” Crowder says.

3D body scans

This method uses 3D imaging to analyze body fat, muscle and posture. This method is on the pricier side, costing up to $1,000, depending on the scanner, Crowder says.

Which tool is right for you?

Not sure which tool to use? It can help to ask yourself the following questions:

• What is my budget for finding out my body fat composition?
• How often do I want to test my body fat composition?
• Why do I want to find out my body fat composition?

For example, if you want to get an idea of where your weight falls or track workout progress, the at-home options listed above are a good starting point. Plus, you can use them as often as you’d like, Poudyal notes.

However, if you'd like a more in-depth look at your body composition than DIY options offer, you might want to consider a professional tool.

“Though they are costly, methods like DEXA or BIA can be utilized with a more precise number when seeking greater accuracy,” she says.

No matter how you proceed, remember: your body composition is just one component of your overall health.

“Weight or BMI alone isn't always a reliable predictor of nutritional status. You shouldn't rely just on your BMI because maintaining a healthy body is determined by other factors, such as exercise, eating a diet rich in fresh produce and drinking plenty of water,” Poudyal says. “However, these tools can be very valuable in steering your body in a healthy direction.”

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