How to Calculate BMI: Formula, Chart & What Your Number Means
Body Mass Index (BMI) is one of the most widely used screening tools in medicine and public health. A single number derived from your height and weight can tell doctors, researchers, and individuals a great deal about potential health risks. In this guide, you'll learn exactly how to calculate BMI, what the results mean, and when BMI falls short as a health indicator.
What Is BMI?
BMI stands for Body Mass Index. It's a numerical value calculated from a person's weight and height. It was developed by Belgian mathematician Adolphe Quetelet in the 19th century and has been used by health organizations worldwide ever since. The World Health Organization (WHO) and the CDC both use BMI as a standard screening tool for weight categories.
While BMI is not a direct measure of body fat, it correlates reasonably well with more direct measures of body fat for most people. Its main advantage is simplicity — you need only two measurements.
The BMI Formula
There are two versions of the BMI formula depending on whether you use metric or imperial units.
Metric Formula (kg and meters)
Example: A person weighing 70 kg and standing 1.75 m tall has a BMI of:
Imperial Formula (lbs and inches)
Example: A person weighing 154 lbs and standing 69 inches tall (5'9"):
Skip the math and use our ⚡ BMI Calculator to get your result instantly.
The BMI Chart: Understanding Your Number
The WHO defines four standard BMI categories for adults:
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency, osteoporosis risk |
| 18.5 – 24.9 | Normal weight | Lowest risk for most people |
| 25.0 – 29.9 | Overweight | Moderate increased risk |
| 30.0 – 34.9 | Obese (Class I) | High risk |
| 35.0 – 39.9 | Obese (Class II) | Very high risk |
| 40.0 and above | Obese (Class III) | Extremely high risk |
BMI for Children and Teens
For children and teenagers aged 2–19, BMI is interpreted differently. Instead of fixed categories, BMI is plotted on age- and sex-specific growth charts as a percentile:
- Below 5th percentile: Underweight
- 5th to 84th percentile: Healthy weight
- 85th to 94th percentile: Overweight
- 95th percentile and above: Obese
How to Calculate BMI Step by Step
Here's a simple walkthrough for calculating BMI manually:
- Measure your weight in kilograms or pounds.
- Measure your height in meters (or inches if using imperial).
- Square your height: multiply height × height.
- Divide weight by height squared.
- If using imperial, multiply the result by 703.
- Compare your result to the BMI chart above.
What Affects BMI Accuracy?
BMI has well-known limitations. Here's when it may not give you the full picture:
Muscle Mass
BMI doesn't distinguish between fat and muscle. A highly muscular athlete may have a BMI in the "overweight" range despite having very low body fat. For example, many professional athletes and bodybuilders have BMIs above 25 due to muscle, not fat.
Age and Sex
Older adults tend to have more body fat than younger people at the same BMI. Women generally carry more fat than men at the same BMI. A BMI of 24 may represent a different health profile at age 20 versus age 65.
Ethnicity
Studies suggest that people of Asian descent face higher health risks at lower BMI thresholds. Some health organizations recommend lower cutoffs (e.g., 23 instead of 25 for overweight) for Asian populations.
Body Fat Distribution
Where fat is stored matters more than total fat. Abdominal fat (apple-shaped bodies) is associated with higher cardiovascular risk than fat stored in the hips and thighs (pear-shaped bodies). BMI cannot tell you where fat is distributed.
Healthy BMI for Adults: What to Aim For
For most adults, a BMI between 18.5 and 24.9 is considered healthy. However, "healthy" is context-dependent. A person at BMI 24 who exercises regularly, eats well, and has normal blood pressure and cholesterol is in better health than someone at BMI 22 who is sedentary.
Your goal should not be to achieve a specific BMI number, but to maintain a healthy lifestyle that supports your weight, body composition, and overall wellbeing.
When to Use Other Body Composition Metrics
BMI is a good starting point, but consider supplementing it with:
- Waist-to-height ratio: Simple and highly predictive of cardiovascular risk.
- Body fat percentage: Measured by DEXA, hydrostatic weighing, or skinfold calipers.
- Lean body mass: Particularly useful for athletes tracking muscle gain.
Try our ⚡ Body Fat Calculator or ⚡ Ideal Weight Calculator for deeper insights beyond BMI.
Frequently Asked Questions
Is BMI 30 always obese?
A BMI of 30 or above falls in the Obese Class I category by standard WHO definitions. However, context matters — a very muscular person may hit BMI 30 without having excess fat. Always consult a healthcare provider before drawing health conclusions from BMI alone.
What is a good BMI for women vs men?
The BMI scale is the same for men and women: 18.5–24.9 is healthy. However, at the same BMI, women typically have a higher percentage of body fat. Health professionals sometimes use sex-specific body fat percentage targets alongside BMI.
Can I lose weight by targeting a specific BMI?
BMI can be a useful tracking metric but it's better to focus on behavior changes (diet, exercise, sleep) rather than a specific number. Track trends over time rather than a single data point.
Conclusion
BMI is a simple, accessible way to screen for potential weight-related health issues. Calculated with a straightforward formula — weight divided by height squared — it provides a useful starting point for health discussions. However, it should always be considered alongside other health indicators and lifestyle factors.
Use the RunCalc BMI Calculator to get your number in seconds, then consider pairing it with our Body Fat Calculator for a more complete picture of your body composition.