What is BMI?
BMI (Body Mass Index) is a number that comes from your height and weight. Doctors and researchers have been using it since the 1800s to get a quick snapshot of whether someone's weight might be putting their health at risk.
Here's the deal: it's not perfect. BMI doesn't know the difference between muscle and fat, so a bodybuilder and someone with the same height/weight who never exercises will get identical scores. But for most people who aren't athletes or bodybuilders, it's a reasonable starting point to see where you stand.
Quick Health Check
A 2-second calculation that gives you a ballpark idea of where you are
Know the Limits
Doesn't account for muscle mass, bone density, or where fat is stored
Track Changes
Useful for monitoring trends over time as you work on fitness goals
Health Screening
Used by doctors as one of many tools to assess health risks
Your doctor looks at BMI alongside other factors like:
- Waist circumference — belly fat is riskier than fat elsewhere
- Blood pressure, cholesterol — actual health markers that matter
- Family history — genetics play a role in health risks
- Activity level — someone active at BMI 26 may be healthier than sedentary at 23
BMI Formula
The math is simple. You can do it yourself in 10 seconds with a calculator. There are two versions depending on whether you're using metric or imperial units.
Metric Formula
BMI = weight (kg) / height (m)²
Example: 70 kg, 1.75 m tall
BMI = 70 / (1.75 × 1.75) = 70 / 3.0625 = 22.9
Imperial Formula
BMI = (weight (lbs) / height (in)²) × 703
Example: 154 lbs, 5'9" (69 inches)
BMI = (154 / 4761) × 703 = 0.0323 × 703 = 22.7
Quick Conversion Tip
To convert height: multiply feet by 12 and add inches. So 5'9" = (5 × 12) + 9 = 69 inches. For weight: 1 kg = 2.2 lbs.
Why the Different Numbers?
The 703 in the imperial formula is just a conversion factor. It accounts for the different units—pounds vs kilograms, inches vs meters. Both formulas give you the same result for the same person.
The reason BMI uses height squared is because taller people naturally weigh more—not just because they're stretching out, but because they have more volume. A 6'2" person with the exact same body proportions as a 5'2" person will weigh considerably more. Squaring the height helps normalize this.
BMI Categories Explained
The World Health Organization breaks BMI into these ranges. Keep in mind these were created based on studies of primarily European populations, so they're not one-size-fits-all.
| Category | BMI Range | What It Means |
|---|---|---|
| Severely Underweight | < 16 | Significant health concerns. Talk to a doctor ASAP. |
| Underweight | 16 - 18.4 | May indicate nutritional deficiency or underlying condition. |
| Normal Weight | 18.5 - 24.9 | Associated with lowest health risks for most people. |
| Overweight | 25 - 29.9 | Slightly elevated risk. Many healthy people fall here. |
| Obese Class I | 30 - 34.9 | Moderate obesity. Increased risk for certain conditions. |
| Obese Class II | 35 - 39.9 | Severe obesity. Higher health risks. |
| Obese Class III | ≥ 40 | Very severe obesity. Serious health intervention recommended. |
Health Risks by Category
Underweight Risks
- Weakened immune system
- Osteoporosis
- Fertility issues
- Anemia
- Hair loss
Normal Weight Benefits
- Lower heart disease risk
- Better energy levels
- Healthy blood pressure
- Reduced diabetes risk
- Better sleep quality
Obesity Risks
- Type 2 diabetes
- Heart disease
- Sleep apnea
- Joint problems
- Certain cancers
Important Context
These are population-level guidelines, not individual diagnoses. Plenty of people at BMI 26-27 are healthier than some at BMI 23 because they exercise, eat well, and have good metabolic markers. Your doctor can give you a much better picture of your actual health than a single number ever could.
When BMI Gets It Wrong
BMI is a blunt instrument. It was designed in the 1830s by a Belgian mathematician who was trying to define the "average man"—not assess individual health. Here's where it falls short:
Athletes & Muscular People
Muscle weighs more than fat. An NFL linebacker or CrossFit athlete might have a "obese" BMI while being in peak physical condition with 10% body fat. The math just doesn't distinguish between muscle and fat.
Real example: Dwayne "The Rock" Johnson has a BMI around 30—technically "obese"—while being one of the fittest people in Hollywood.
Older Adults
As we age, we naturally lose muscle and bone density while often gaining fat. Someone at 70 with a "normal" BMI of 22 might actually have concerning levels of body fat and not enough muscle mass.
Better metrics: For seniors, waist circumference and muscle strength tests often tell a more useful story.
Children & Teenagers
Kids grow at wildly different rates. A chubby 10-year-old might thin out during a growth spurt, while a skinny kid might fill out later. Standard BMI categories don't account for age or development stage.
Use instead: CDC growth charts that compare children to others of the same age and sex.
Different Ethnicities
BMI categories were developed using data from European populations. Research shows that health risks differ by ethnicity—Asian populations tend to have higher health risks at lower BMIs, while some African and Pacific Islander populations may have lower risks at the same BMI.
WHO adjustment: For Asian populations, overweight starts at BMI 23, not 25.
Better Alternatives
Waist-to-Hip Ratio
Measures where you carry fat. Belly fat (apple shape) is riskier than hip fat (pear shape). Men should aim for under 0.9, women under 0.85.
Body Fat Percentage
Actually measures fat vs. muscle. Healthy range is 10-20% for men, 18-28% for women. Requires calipers, DEXA scan, or smart scale.
Waist Circumference
Simple tape measure test. Risk increases above 40 inches for men, 35 inches for women. Easy to track at home.
The Takeaway
BMI is useful for population studies and as one data point among many. But your actual health depends on way more—how you eat, how you move, your blood work, your sleep, stress levels, and genetics. Don't let a single number define you.