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Look, I’m going to level with you. Last month I was filling out forms at a new gym, and they asked for my BMI. I had no idea what mine was. Had to pull out my phone right there in the lobby and figure it out like everyone else.
Turns out I’m not alone in this. Most people know BMI exists but couldn’t tell you their number off the top of their head. And honestly? That’s probably fine.
But if you’re here because your doctor mentioned it, or you’re just curious where you stand, let’s walk through this together.
Body mass index is just a ratio. Your weight compared to your height. That’s literally all it is.
Some mathematician in Belgium invented it way back in the 1830s. Not a doctor—a mathematician. Keep that in mind because it matters later.
Doctors started using it because it’s easy. No blood tests, no expensive equipment. Just a scale and a tape measure. Two minutes and you’ve got a number that supposedly says something about whether you’re at a healthy weight.
Except… well, we’ll get to the problems in a minute.
You need two things: your weight and your height. Then you pick which formula matches how you measure stuff.
The metric version:
Weight in kilograms ÷ (height in meters × height in meters)
The imperial version:
(Weight in pounds ÷ (height in inches × height in inches)) × 703
Yeah, that 703 is random. Nobody knows where it came from. It’s just the conversion factor that makes the numbers work out.
Here’s a real example because abstract math is useless. Say you’re 5’6″ and weigh 140 pounds.
5’6″ = 66 inches 66 × 66 = 4,356 140 ÷ 4,356 = 0.032 0.032 × 703 = 22.5
Your BMI would be 22.5.
Or you could just Google “BMI calculator” and avoid the math entirely. That’s what most people do.
Healthcare folks put BMI into buckets:
Below 18.5 = underweight 18.5 to 24.9 = normal weight
25 to 29.9 = overweight 30+ = obese
Pretty straightforward, right?
Except my buddy Dave is 6’2″ and lifts weights four times a week. His BMI is 27. According to these categories, he’s overweight. In reality, the guy is in better shape than he was in college.
That’s problem number one with these ranges. They don’t account for what your body is actually made of.
Ever been to a doctor’s office where they have this big poster on the wall? Usually it’s a grid with height going one way, weight going the other. Different colored zones show you where you land.
Green zone = you’re good Yellow/orange = getting into risky territory
Red = definitely talk to your doctor
They’re helpful for a quick glance. My doctor’s office has one right next to where they take your blood pressure. But treating it like some absolute truth is where people get into trouble.
The formula doesn’t care if you’re male or female. Same calculation for everybody.
But bodies work differently. Women are supposed to have more body fat—that’s normal biology, not a problem. Yet the same BMI standards apply.
My wife and I can have the exact same BMI and our bodies look completely different. She’ll be at a healthy weight and I might need to lose a few pounds, or vice versa. The system just doesn’t capture that.
Should there be separate standards? Probably. Will there be? Who knows. For now, we’re stuck with one-size-fits-all numbers that don’t really fit all.
You can’t use regular BMI for children. Their bodies are growing and changing too fast.
Doctors use percentile charts instead. They compare your kid to thousands of other kids the same age and gender. Somewhere between the 5th and 85th percentile is usually the goal.
My friend’s daughter jumped from the 40th to the 75th percentile in six months and they freaked out. Pediatrician said it was just a normal growth pattern. Kids shoot up, fill out, lean out again. A snapshot in time doesn’t tell you much.
If you’re checking your kid’s BMI, use a calculator made for children and talk to their doctor. This isn’t a DIY situation.
Alright, time for some honesty. BMI has major limitations.
It can’t tell fat from muscle. Professional athletes often have “unhealthy” BMIs because muscle weighs more than fat. Meanwhile someone who never exercises might have a perfect BMI but actually be in terrible shape.
Where you carry weight matters a lot. Belly fat is way riskier than fat on your butt or thighs. BMI doesn’t distinguish between the two.
It ignores your age even though bodies change as we get older. A 25-year-old and a 65-year-old with the same BMI are in completely different situations.
And here’s a big one—it doesn’t work equally for all ethnicities. Asian populations tend to have health issues at lower BMIs than white populations. Pacific Islanders might be totally healthy at higher BMIs. Using the same cutoffs for everyone is kind of absurd when you think about it.
BMI gives you one piece of information. That’s it. One piece.
Your waist circumference is actually more predictive of health problems than BMI. Too much weight around your middle? That’s when doctors start worrying about heart disease and diabetes.
How much muscle you have versus fat—that’s your body composition. You can get this measured with special scales or scans. Way more useful than BMI.
How do you feel day to day? Can you do the physical activities you want to do? Are you sleeping well? Those things matter more than any number.
Blood pressure, cholesterol, blood sugar—these are actual health markers. BMI is just a screening tool that might point toward checking these other things.
Don’t lose your mind over it. Seriously.
And if your BMI is perfect, don’t assume you’re automatically healthy.
I knew someone who had a BMI of 23, right in the sweet spot. Ate terribly, never exercised, had high blood pressure. The BMI told her everything was fine when it definitely wasn’t.
On the flip side, I’ve met people with BMIs of 27 or 28 who run marathons and have perfect bloodwork. The number didn’t tell their story at all.
Use it as a reason to check in with your doctor if you want. But it’s not a verdict on your health or your worth as a person.
Forget everything the internet tells you about rapid weight loss or gain. That stuff doesn’t stick.
Just eat more real food and less processed junk. That’s it. You don’t need to go keto or vegan or paleo or whatever’s trending. More vegetables, decent protein, less stuff from boxes and bags.
Move around in ways that don’t make you miserable. Hate the gym? Don’t go. Like hiking? Do that. Swimming? Awesome. Dancing in your living room? Counts. Just move.
Get actual sleep. Not five hours. Not six. Like seven or eight real hours. Your body needs it to regulate hunger and make decent decisions.
Stress will sabotage everything. Find something that helps you chill out. Could be yoga, could be video games, could be calling your best friend. Whatever works.
Type “BMI calculator” into Google. You’ll find a million options.
Pick Tooldeck.pro that lets you use pounds and inches or kilograms and centimeters—whichever you’re comfortable with. Some people still think in feet and pounds. Others went metric years ago. Shouldn’t matter.
The better calculators also show you what weight range would put you in the “healthy” BMI zone. That’s more helpful than just seeing a single number.
Your BMI changed significantly without you trying? Yeah, mention that.
You’re way outside the normal range and stuff hurts or you can’t breathe right or you’re always tired? Definitely talk to them.
You want professional advice on getting to a healthier weight instead of following some influencer’s plan? Smart move.
Doctors have access to way better tools than BMI. They can check your body fat percentage, run blood tests, assess your actual health instead of relying on a calculation from the 1830s.
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