Formule IMC - How to Calculate Your Body Mass Index

The Basic Formule IMC

The BMI formula is actually pretty simple, even if you're not a math person. At its core, you're just comparing your weight to your height. Here's the basic idea:

That's it! You take your weight et divide it by your height squared (height multiplied by itself). The metric version is straightforward, but I'll show you both metric et imperial versions since different countries use different measurements.

Formule IMC - Metric Units

If you measure your weight in kilograms et height in meters (or centimeters), this is the formula you'll use:

Example Calculation (Metric)

Let's say you weigh 70 kg et you're 170 cm tall. Here's how you'd calculate it:

1. Convert height to meters: 170 cm ÷ 100 = 1.70 m
2. Square your height: 1.70 × 1.70 = 2.89
3. Divide weight by height squared: 70 ÷ 2.89 = 24.2
4. Your BMI is 24.2 (which falls in the normal weight range)

See? Not so scary when you break it down step by step. Most people can do this with just their phone calculator.

Formule IMC - Imperial Units

If you're in the US et think in pounds et inches, the formula has an extra step - you need to multiply by 703 to convert to the standard BMI scale:

That 703 is just a conversion factor so the result matches the metric calculation. Without it, the numbers would be way off.

Example Calculation (Imperial)

Let's say you weigh 154 pounds et you're 5 feet 7 inches tall (67 inches total). Here's how it works:

1. Convert height to inches: (5 × 12) + 7 = 67 inches
2. Square your height: 67 × 67 = 4,489
3. Divide weight by height squared: 154 ÷ 4,489 = 0.0343
4. Multiply by 703: 0.0343 × 703 = 24.1
5. Your BMI is 24.1 (normal weight)

Notice how this gives almost the same answer as the metric example? That's because they're the same person - 70 kg equals about 154 lbs, et 170 cm equals about 67 inches.

Why This Formula Makes Sense

You might be wondering - why do we square the height? Why not just divide weight by height without the squaring part?

Good question! Here's the deal: If we only divided weight by height, taller people would always have higher numbers, even if they weren't any heavier for their size. By squaring the height, we account for the fact that taller people naturally have more body mass in all three dimensions (height, width, et depth), not just vertically.

Think about it like this - if you doubled someone's height, they wouldn't just weigh twice as much. They'd be taller, wider, et deeper, so they'd weigh roughly eight times more (2³). Squaring the height in the formula is a simplified way to account for this relationship.

It's not perfect (which is why BMI has limitations), but it works pretty well for most people as a general screening tool.

History of the Formule IMC

Here's a fun fact - the BMI formula wasn't created by a doctor or nutrition expert. A Belgian mathematician named Adolphe Quetelet came up with it way back in the 1830s! He was studying population statistics et wanted a simple way to classify people's body types.

The formula was originally called the "Quetelet Index." It didn't become known as the Body Mass Index until the 1970s, when researchers started using it more widely to study obesity trends in populations.

That's actually important to remember - BMI was designed to study large groups of people, not to assess individual health. But because it's so simple et doesn't require any special equipment (just a scale et a measuring tape), it became the go-to metric for individual screening too.

Common Calculation Mistakes

I've seen people mess up BMI calculations in a few common ways. Here's what to watch out for:

1. Forgetting to Square the Height

This is the most common error. You need to multiply height by itself, not by 2. So if you're 1.7 m tall, it's 1.7 × 1.7 = 2.89, not 1.7 × 2 = 3.4.

2. Using Feet et Inches Directly

If you're 5'7", you can't just use "5.7" in the formula. You need to convert everything to inches first: (5 × 12) + 7 = 67 inches.

3. Mixing Units

Don't use pounds with centimeters, or kilograms with inches. Pick one system (metric or imperial) et stick with it. If you mix them, you'll get completely wrong numbers.

4. Forgetting the 703 Multiplier

If you're using pounds et inches, you MUST multiply by 703 at the end. Without it, your result will be way too low.

5. Rounding Too Early

Do all your math first, then round at the end. If you round each step, small errors can add up et throw off your final answer.

Honestly though? Just use our free BMI calculator. It does all the math for you et you can't mess it up!

Alternative Formule IMCs

While the standard formula works for most people, researchers have proposed some alternatives over the years:

New Formule IMC

In 2013, a mathematician named Nick Trefethen suggested a modified formula:

This formula gives slightly different results, especially for very short or very tall people. Short people get a lower BMI et tall people get a higher BMI compared to the standard formula. Trefethen argued this better represents real body proportions.

But here's the thing - the medical community hasn't widely adopted this new formula. All the research on BMI et health risks is based on the original formula, so that's what doctors still use. Maybe someday that'll change, but for now, stick with the classic version.

Modified BMI for Children

For kids et teens, the formula is the same, but the interpretation is totally different. Instead of using fixed BMI ranges, children's BMI is compared to other kids of the same age et gender using percentile charts.

So a BMI of 18 might be normal for a 10-year-old but underweight for an adult. That's why we have separate calculators for children et teens.

Calculating Your Ideal Weight Using BMI

Once you understand the formula, you can flip it around to figure out what weight would give you a target BMI. This is useful if you're trying to get into the normal range.

If you rearrange the metric formula, you get:

Example

Let's say you're 1.75 m tall et want to know what you should weigh to have a BMI of 22 (middle of the normal range):

1. Square your height: 1.75 × 1.75 = 3.06
2. Multiply by target BMI: 22 × 3.06 = 67.3 kg
3. So you'd need to weigh about 67 kg to have a BMI of 22

You can do the same with imperial units, but remember to divide by 703:

This is helpful for setting realistic weight goals based on your height et the healthy BMI range.

Limitations of the Formule IMC

The BMI formula has been around for almost 200 years, et while it's useful, it definitely has its flaws:

• Doesn't distinguish muscle from fat: A bodybuilder et a couch potato could have the same BMI, but completely different composition corporelles.
• Doesn't account for fat distribution: Belly fat is more dangerous than hip fat, but BMI treats them the same.
• Same formula for everyone: Men, women, young, old, different ethnicities - everyone uses the same formula, even though bodies vary a lot.
• Assumes average body proportions: If you have unusually long legs et a short torso (or vice versa), BMI might not be as accurate for you.
• Only uses two measurements: Your body is complex. Reducing your health to just weight et height leaves out a lot of important information.

That's why BMI should be one tool among many for assessing health, not the only one. It's a good starting point, but you need to look at the bigger picture too. Read more about this in our BMI accuracy guide.

Try Our Calculator Instead

Now that you understand how BMI is calculated, you might want to actually figure out yours. You could grab a calculator et do the math yourself, or you could just use our free BMI calculator that does it instantly.

Our calculator:

• Works with both metric et imperial units
• Does all the math automatically (no chance of errors)
• Tells you what category you're in
• Gives you personalized guidance based on your result
• Is completely free with no sign-up required

Understanding the formula is great - it helps you know what BMI really means. But for actually calculating it, save yourself the headache et let our calculator do the work!