How to Count BMI: Understanding Body Mass Index

Have you ever wondered how to figure out your Body Mass Index, or BMI? It’s simpler than you might think! You just take your weight in kilograms and divide it by your height in meters, squared. Or, if you use pounds and inches, you divide your weight by your height squared, then multiply that number by 703.

BMI is a common tool. It gives you a simple number to see if your weight is healthy for your height. It’s easy to calculate. Doctors use it to spot possible health risks. These risks can come from being underweight, normal weight, overweight, or obese. BMI doesn’t directly measure body fat. But it’s a good, quick guide for checking general health or looking at health trends in large groups of people.

How to Calculate BMI: The Formulas

The way you calculate BMI changes based on the units you use. Let’s look at both ways.

Metric Formula:

If you use kilograms and meters, the formula is very direct:

For a deeper look at this topic, our guide What Is Khums and How to Calculate It (Shia Guide) covers it step by step.

BMI = weight (kg) / (height (m))^2

Here’s what those parts mean:

• weight (kg) is how much you weigh in kilograms.
• height (m) is how tall you are in meters.

Imperial Formula:

If you use pounds and inches, you’ll need an extra step. We add a conversion number:

BMI = (weight (lbs) / (height (in))^2) * 703

Here’s what these parts stand for:

• weight (lbs) is your weight in pounds.
• height (in) is your height in inches.

Let’s Do an Example Together

I’ll show you how to count BMI with an example. We’ll use both metric and imperial units. This way, you can see how it works no matter which system you prefer.

Our Example Person:

Imagine someone who weighs 150 pounds and is 5 feet 5 inches tall.

Metric Calculation Steps:

1. First, change weight to kilograms:

   150 lbs multiplied by 0.453592 kg/lb equals 68.0388 kg.

2. Next, change height to meters:
   • Start by turning feet into inches: 5 feet times 12 inches per foot is 60 inches.
   • Add the extra inches: 60 inches plus 5 inches gives us 65 inches total.
   • Now, change those inches to meters: 65 inches times 0.0254 meters per inch is 1.651 meters.
3. Square the height in meters:

   (1.651 m) squared is 2.725801 m squared.

4. Now, use the BMI formula:

   BMI = 68.0388 kg divided by 2.725801 m squared. This gives us 24.96.

So, our example person’s BMI is about 25.0.

Imperial Calculation Steps:

1. Find the weight in pounds:

   Our person weighs 150 lbs.

2. Find the height in inches:
   • 5 feet times 12 inches per foot is 60 inches.
   • Add the extra 5 inches: 60 inches plus 5 inches makes 65 inches.
3. Square the height in inches:

   (65 inches) squared is 4225 inches squared.

4. Apply the BMI formula:

   BMI = (150 lbs divided by 4225 inches squared) times 703.

   BMI = 0.0355027 times 703.

   BMI = 24.96.

You can see we get the same BMI, about 25.0!

Want to dig deeper into the math or use a super easy online calculator? Check out my guide on How BMI Is Calculated: The Simple Math Behind the Score.

What Your BMI Number Means: The Chart

Once you have your BMI number, you can use this chart. It comes from health experts like the CDC and WHO. It helps you understand what your weight status is. I’ve also added some examples to show how BMI works for different people.

Why BMI Isn’t Perfect

BMI is helpful, but it’s not the whole story. It has some limits you should know about:

• It doesn’t tell fat from muscle: Think about athletes. They often have lots of muscle. Muscle is heavy. So, an athlete might have a high BMI. The chart might say they are overweight or obese. But their body fat percentage could be very low, and they could be super healthy.
• It can change with age, sex, and background: How we read BMI can be different. For example, older people might have a higher BMI because their bodies change naturally. Also, what’s a healthy BMI range can differ for certain ethnic groups.
• It doesn’t care where fat is stored: Where your body stores fat matters more than just how much fat you have. For instance, fat around your waist can be riskier than fat on your hips. BMI doesn’t show this.
• It’s not a full health check: BMI is a starting point. It’s a screening tool. It can’t tell you if you’re truly healthy or sick all by itself. Doctors use it along with other checks. These include measuring your waist, checking body fat, looking at your diet, how active you are, and your family health history.

Even with these limits, BMI is still a practical and accepted way to get a first idea of health for many people. It helps identify those who might need a closer look at their weight and health.

Common Questions About BMI

What’s a healthy BMI?

According to health organizations like the CDC and WHO, a healthy BMI is usually between 18.5 and 24.9. This range means your weight is normal for your height.

How do I change my height for the BMI calculation?

First, measure your height in feet and inches. If you’re using the imperial formula, change your total height to inches. For example, 5 feet 7 inches becomes 67 inches. If you need meters for the metric formula, take your total inches (like 67 inches) and multiply by 0.0254. That gives you meters (about 1.7018 meters).

Is BMI correct for everyone?

No, BMI isn’t perfect for everyone. It can give a wrong idea for athletes with lots of muscle, pregnant women, and older people. This is because BMI doesn’t tell the difference between muscle and fat. It also doesn’t consider how a person’s body changes over time.