What is BMI?
Body Mass Index (BMI) is a simple number based on your height and weight. It is used as a general screening tool to group people into weight categories such as underweight, healthy, overweight, and obese. Doctors and public health organizations around the world use it as a quick way to flag potential weight-related health risks and to track population trends.
BMI works because, for most people, weight scales with height in a predictable way. Instead of comparing weights directly across people of different heights, BMI adjusts for height so that the result is a fairer comparison. The number itself is not a direct measurement of body fat, but it correlates well enough with body fat for most adults to serve as a useful first screening step.
How to Use the BMI Calculator
Finding your BMI takes less than a minute. Choose your preferred units and enter your height and weight, and the calculator does the rest.
- Choose between metric (kg and cm) and imperial (lb and ft/in) units.
- Enter your weight and height in the fields shown.
- Click “Calculate BMI” to see your BMI value and category.
- Review the healthy weight range for your height, which is shown with the result.
BMI Categories
The World Health Organization defines four main BMI categories for adults:
- Below 18.5 – Underweight
- 18.5 to 24.9 – Normal weight
- 25 to 29.9 – Overweight
- 30 and above – Obese
Each category is associated with a different level of health risk. Both underweight and overweight categories carry elevated risks, while the normal range is associated with the lowest risk for most health conditions. The obese category is often split further into subcategories for more precise medical assessment.
How BMI Is Calculated
Metric Formula
In metric units, BMI equals your weight in kilograms divided by the square of your height in meters. If you are 70 kg and 1.75 m tall, your BMI is 70 divided by (1.75 × 1.75), which is about 22.9.
Imperial Formula
In imperial units, the formula adds a conversion factor. BMI equals 703 times your weight in pounds divided by the square of your height in inches. The calculator converts your feet and inches into inches automatically.
A Worked Example
Metric Example
Consider a woman who is 165 cm tall and weighs 62 kg. First, convert the height to meters: 1.65 m. Square that value: 1.65 × 1.65 = 2.7225. Then divide her weight by this number: 62 ÷ 2.7225 = 22.8. A BMI of 22.8 sits comfortably inside the normal weight category. Her healthy weight range for a BMI of 18.5 to 24.9 is roughly 50.4 kg to 67.8 kg, which is found by multiplying the lower and upper BMI cutoffs by the square of her height in meters.
Imperial Example
Now take a man who is 5 feet 10 inches tall (70 inches) and weighs 185 pounds. Square the height in inches: 70 × 70 = 4900. Apply the conversion factor: 703 × 185 = 130,055, then divide by 4900. That gives a BMI of about 26.5, placing him just inside the overweight category. His healthy weight range for a BMI of 18.5 to 24.9 is roughly 129 to 173 pounds. Working through the numbers yourself like this is a good way to check that a calculator is behaving correctly.
Understanding Your Healthy Weight Range
The calculator also shows the healthy weight range for your height, which corresponds to a BMI between 18.5 and 24.9. This range gives you a practical target: any weight inside it lands in the normal category. If you are outside the range, it gives you a clear idea of how much weight you would need to gain or lose to reach the healthy zone, although individual body composition means the ideal weight varies from person to person.
Important Limitations of BMI
BMI is a screening tool, not a diagnostic one, and it has well-known limitations. It does not measure body fat directly, so it can misclassify people with unusually high muscle mass, such as athletes and bodybuilders, who may show a high BMI despite being lean. It also tends to be less accurate for older adults and may not be appropriate for pregnant women or growing children, who are assessed with different charts.
Because BMI cannot tell you where fat is stored, it misses the added risk that comes from abdominal fat in particular.
For these reasons, treat BMI as a starting point for a conversation with a healthcare professional, not as a final verdict on your health. Waist circumference, body fat percentage, blood pressure, and blood tests all add valuable context that BMI alone cannot provide.
BMI and Health Risks
The relationship between BMI and health risk follows a J-shaped curve. Risk is lowest in the normal weight range and rises as you move away from it in either direction. Both underweight and overweight categories carry increased risks, though the conditions differ:
- Underweight BMI is linked to a weaker immune system, reduced bone density, anemia, and fertility problems.
- Overweight and obesity are associated with high blood pressure, type 2 diabetes, heart disease, stroke, and joint strain.
- Fat carried around the abdomen is especially associated with metabolic and cardiovascular risk.
These associations come from population-level data. Your personal risk depends on genetics, diet, physical activity, smoking, and blood markers as well, which is why BMI should be interpreted as one piece of a broader health assessment.
Beyond BMI: Other Measures Worth Knowing
Because BMI cannot show where fat is stored or how much of your weight is muscle, several complementary measures help fill in the picture:
- Waist circumference: a simple tape measure around the abdomen. Values above about 94 cm for men and 80 cm for women indicate increased health risk in many guidelines.
- Waist-to-hip ratio: divides waist by hip circumference to assess fat distribution.
- Waist-to-height ratio: keeping your waist below roughly half your height is a practical target many experts recommend.
- Body fat percentage: directly estimates the share of fat in your body using methods such as the US Navy formula, skinfold calipers, or DEXA scans.
Using BMI alongside one or two of these measures gives a far more complete picture than any single number, especially for people with high muscle mass or unusual body shapes.
Tips for Accurate Results
- Measure your weight in the morning on an empty stomach for consistency.
- Measure height without shoes for accuracy.
- Recheck your BMI only after meaningful weight changes, not daily fluctuations.
- Combine BMI with waist measurement for a fuller picture.
- Ask a doctor to interpret your BMI in the context of your overall health.
Why Use This BMI Calculator?
- Supports both metric and imperial units
- Shows your BMI category and healthy weight range
- Instant results with no signup
- Free and unlimited use
- No data is ever uploaded
- Works on any device with a browser