How to Calculate BMI: Formula, Ranges & What It Means

· 7 min read

Body Mass Index — better known as BMI — is one of the most widely used health screening tools in the world. Doctors, insurance companies, and public health agencies rely on it to quickly assess whether a person's weight falls within a healthy range for their height. But what exactly does BMI measure, how is it calculated, and when should you look beyond it?

In this guide, we will walk through the BMI formula step by step, explain what each BMI category means, highlight the well-known limitations of BMI, and show you when body fat percentage or other metrics are a better choice. You can also skip straight to our free BMI calculator for instant results.

The BMI Formula

BMI is a simple ratio that compares your weight to your height. The formula uses either metric or imperial units:

Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = [weight (lbs) ÷ height (in)²] × 703

The multiplication factor of 703 in the imperial formula converts the result to match the metric scale. Both formulas produce the same BMI value.

Worked Example: Metric

A person weighs 75 kg and stands 1.78 m tall.

  • Height squared: 1.78 × 1.78 = 3.1684
  • BMI: 75 ÷ 3.1684 = 23.7

A BMI of 23.7 falls in the "normal weight" category.

Worked Example: Imperial

A person weighs 170 lbs and is 5 feet 10 inches tall (70 inches total).

  • Height squared: 70 × 70 = 4,900
  • Weight divided by height squared: 170 ÷ 4,900 = 0.03469
  • BMI: 0.03469 × 703 = 24.4

This also falls within the normal weight range, just under the overweight threshold of 25.

BMI Categories: What the Numbers Mean

The World Health Organization (WHO) defines four standard BMI categories for adults aged 20 and older:

BMI Range Category Health Risk
Below 18.5 Underweight Increased risk of nutritional deficiency, osteoporosis, weakened immune system
18.5 – 24.9 Normal weight Lowest overall health risk for most adults
25.0 – 29.9 Overweight Moderately increased risk of heart disease, type 2 diabetes, high blood pressure
30.0 and above Obese Significantly increased risk of cardiovascular disease, diabetes, joint problems, certain cancers

Obesity is further divided into three classes: Class I (30–34.9), Class II (35–39.9), and Class III (40+), with health risks increasing at each level. You can check your category instantly with our BMI calculator.

The Limitations of BMI

BMI was invented in the 1830s by Belgian mathematician Adolphe Quetelet as a population-level statistical tool — not as a diagnostic measure for individuals. While it works reasonably well as a quick screening tool, it has several well-documented blind spots:

1. It Cannot Distinguish Muscle from Fat

This is the most commonly cited limitation. A 6-foot-tall athlete weighing 220 lbs with 12% body fat has a BMI of 29.8 — technically "overweight" — despite being in excellent physical condition. Muscle is about 18% denser than fat, so muscular individuals will almost always have inflated BMI scores.

Professional athletes routinely fall into the "overweight" or even "obese" BMI category. For example, many NFL running backs have BMIs above 30 while maintaining body fat percentages below 15%.

2. It Misses Body Fat Distribution

Where you carry fat matters as much as how much you carry. Visceral fat around the abdomen is far more dangerous than subcutaneous fat on the hips and thighs. Two people with identical BMIs of 27 can have vastly different health risks depending on their waist-to-hip ratio. Waist circumference (above 40 inches for men, above 35 inches for women) is a better predictor of metabolic disease risk than BMI alone.

3. It Varies by Age, Sex, and Ethnicity

BMI does not account for the fact that women naturally carry more body fat than men, that older adults lose muscle mass (sarcopenia), or that health risks associated with certain BMI levels differ across ethnic groups. For example, research shows that Asian populations face increased metabolic risk at BMIs as low as 23, while some Pacific Islander populations may have lower risk at BMIs above 25.

4. It Ignores Overall Fitness

A person with a BMI of 26 who exercises five days a week, has normal blood pressure, and maintains healthy cholesterol levels is almost certainly healthier than a sedentary person with a "normal" BMI of 22 who has prediabetes. Cardiorespiratory fitness is a stronger predictor of mortality than BMI.

When to Use Body Fat Percentage Instead

If you fall into any of the following groups, body fat percentage gives a more accurate picture of your health than BMI:

  • Athletes and regular weightlifters — muscle mass skews BMI upward
  • Adults over 65 — age-related muscle loss means BMI may underestimate fat
  • Anyone with a BMI between 25 and 30 — the "gray zone" where body composition matters most
  • People concerned about visceral fat — combine body fat percentage with waist circumference

Healthy body fat ranges are typically 10–20% for men and 18–28% for women. You can estimate yours using our body fat calculator, which uses the U.S. Navy method based on neck, waist, and hip measurements.

Practical BMI Examples

To illustrate how BMI works across different body sizes, here are several real-world examples:

Height Weight BMI Category
5'4" (163 cm) 110 lbs (50 kg) 18.9 Normal
5'7" (170 cm) 155 lbs (70 kg) 24.3 Normal
5'10" (178 cm) 190 lbs (86 kg) 27.3 Overweight
6'0" (183 cm) 220 lbs (100 kg) 29.8 Overweight
6'2" (188 cm) 260 lbs (118 kg) 33.4 Obese (Class I)

Beyond BMI: A Broader Health Picture

BMI is best used as a starting point, not a final verdict. For a more complete understanding of your health and body composition, consider these complementary tools:

Calculate Your BMI Now

Ready to check where you stand? Our BMI calculator instantly computes your BMI from your height and weight, shows your WHO category, and provides context about what the number means for your health. It works in both metric and imperial units, right in your browser with no data sent to any server.

Related Calculators

Frequently Asked Questions

A healthy BMI falls between 18.5 and 24.9, which the World Health Organization classifies as "normal weight." However, BMI is only one indicator of health. A person with a BMI of 25 who exercises regularly and has normal blood pressure may be healthier than someone with a BMI of 22 who is sedentary. Always consider BMI alongside other metrics like waist circumference, body fat percentage, and blood work.
BMI is calculated by dividing your weight in kilograms by your height in meters squared: BMI = weight (kg) / height (m)². In imperial units, the formula is BMI = [weight (lbs) / height (in)²] × 703. For example, a person weighing 170 lbs who is 5'10" (70 inches) tall has a BMI of 170 / (70 × 70) × 703 = 24.4.
BMI is often inaccurate for athletes and muscular individuals because it cannot distinguish between muscle mass and fat mass. Muscle is denser than fat, so a lean athlete with significant muscle may have a BMI in the "overweight" range despite having a low body fat percentage. For athletes, body fat percentage measured via calipers, DEXA scan, or bioelectrical impedance provides a more accurate picture of body composition.
BMI is a simple ratio of weight to height and does not measure fat directly. Body fat percentage measures the actual proportion of your body weight that comes from fat tissue. A healthy body fat range is 10–20% for men and 18–28% for women. BMI is a quick screening tool, while body fat percentage gives a more precise assessment of body composition and health risk.
Yes, BMI can change with age as body composition shifts. Adults tend to lose muscle mass and gain fat as they age, even if their weight stays the same. Research suggests that slightly higher BMIs (25–27) may be associated with lower mortality risk in adults over 65. The standard BMI categories were designed primarily for adults aged 20–65, which is why many health professionals recommend different assessment tools for older adults.