BMI Calculator

TL;DR: To calculate your BMI, divide your weight in kilograms by the square of your height in meters (BMI = kg/m²); for example, a 70 kg person who is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which falls in the healthy range of 18.5–25.

What Is the BMI Calculator?

The BMI Calculator is a screening tool that estimates whether your body weight is proportionate to your height. It produces a single numerical value—your Body Mass Index—which is then classified into categories such as underweight, healthy weight, overweight, or obesity. This calculator is intended for adults aged 18 and over, as the BMI categories for children and adolescents use age- and sex-specific percentiles rather than fixed ranges.

BMI is not a direct measure of body fat, but it correlates reasonably well with more direct measures in the general population. It is widely used by doctors, nutritionists, and public health organizations because it requires only two inputs—weight and height—and can be calculated in seconds. Understanding your BMI gives you a starting point for conversations about weight management, cardiovascular risk, and lifestyle changes.

You might use this calculator if you are beginning a fitness program, preparing for a medical consultation, or simply tracking changes in your body composition over time. It is also useful for employers running wellness programs or for individuals who want a quick, standardized reference point. However, you should always interpret the result alongside other metrics like waist circumference, muscle mass, and family history.

How to Use the Calculator

Using the calculator is straightforward. Follow these steps in order, and make sure you use the correct units for each input. The calculator requires your weight in kilograms and your height in centimeters; it handles the conversion from centimeters to meters automatically in the background. Here is the step-by-step process:

1. Enter your weight: Type your body weight in kilograms (kg) into the weight field. For example, if you weigh 70 kg, enter '70'. If your scale measures in pounds, please convert pounds to kilograms first (1 kg = 2.20462 lbs) to get an accurate result.
2. Enter your height: Type your height in centimeters (cm) into the height field. For example, if you are 175 cm tall, enter '175'. If your height is typically measured in feet and inches, convert to centimeters: 1 foot = 30.48 cm, and 1 inch = 2.54 cm.
3. Optional: review your inputs: Double-check that both numbers are correct. A common error is typing weight in pounds and height in inches, which will produce a wildly incorrect BMI value.
4. Click the calculate button: Press the 'Calculate' or 'Compute BMI' button. The calculator will square your height (converted to meters), divide your weight by that result, and display your BMI as a number rounded to one decimal place.
5. Read your category: The result will include a label such as 'Underweight', 'Normal weight', 'Overweight', or 'Obese', based on the standard World Health Organization (WHO) cut-offs for adults.

If you need to redo the calculation, simply change either input and press Calculate again. The calculator does not store your data; it works entirely in your browser, so your information remains private.

Formula and Calculation Method

The BMI formula is expressed as:

BMI = weight (kg) ÷ height² (m²)

In plain language, you take your weight in kilograms, then you divide it by the square of your height in meters. 'Squaring' means multiplying your height by itself. The formula has been in use since the 19th century when Adolphe Quetelet developed it, and it remains the standard metric for population-level weight classification.

The calculation happens in three steps. First, your height in centimeters is converted to meters by dividing by 100. For instance, 175 cm divided by 100 equals 1.75 m. Second, that height in meters is squared: 1.75 × 1.75 = 3.0625. Third, your weight in kilograms is divided by that squared value. This gives you a number that is independent of your height, allowing comparisons across people of different statures.

Let's walk through a worked example with real numbers to make it clear. Suppose you weigh 75 kg and are 175 cm tall. Step one: convert 175 cm to meters, which is 1.75 m. Step two: square this height: 1.75 × 1.75 = 3.0625 m². Step three: divide your weight by the squared height: 75 ÷ 3.0625 = 24.5. Your BMI is 24.5, which falls into the 'Normal weight' range (18.5–25). This number is meaningful because it shows you are within the healthy band for an adult, sitting comfortably below the overweight threshold of 25.

Practical Examples

To show how the BMI calculator works across different body types, here are three realistic scenarios with varied inputs and their corresponding results. Each example uses the exact method described above: weight in kg divided by height in meters squared.

The first scenario shows an athlete with a healthy BMI of 20.5, indicating a proportionate weight for her height. The second scenario, with a BMI of 28.4, falls into the overweight category (25–30); this result suggests the individual may want to monitor their cardiovascular health and consider diet or exercise adjustments. The third scenario produces a BMI of 17.9, which is below 18.5, categorizing her as underweight; this could warrant a conversation with a healthcare provider about nutritional intake or underlying medical conditions.

Notice how the same calculator can flag both ends of the spectrum. The result is always a single number between roughly 10 and 50, but the interpretation depends entirely on the category boundaries. The calculator also helps track changes over time—if the office worker loses 7 kg, his new BMI would be 85 ÷ 3.24 = 26.2, still overweight but trending in the right direction.

Tips for Accurate Results

Getting an accurate BMI requires attention to both the inputs and the interpretation. Here are the most important tips to follow to avoid common errors:

• Always use kilograms for weight: The formula is invalid if you enter pounds. To convert, divide your weight in pounds by 2.20462. For example, 154 lbs ÷ 2.20462 = 69.85 kg. Enter this value precisely instead of rounding to 70 kg to get a more exact BMI.
• Always use centimeters for height: The calculator expects centimeters, not meters and not inches. If you are 5 feet 9 inches tall, convert that to inches (5 × 12 + 9 = 69 inches), then multiply by 2.54 to get 175.26 cm. Enter 175.26.
• Do not forget the height conversion step: The most common mistake is entering height in centimeters and treating it as meters. If you incorrectly use 175 (instead of 1.75) in the squared height, your BMI would be 70 ÷ (175 × 175) = 0.0023, which is meaningless. The calculator handles this conversion, but you must not divide your weight by centimeters squared yourself.
• Weigh yourself under consistent conditions: For the most reliable reading, weigh yourself in the morning, after using the bathroom, and before eating or drinking. Your body weight can fluctuate by 1–2 kg throughout the day due to water retention and food intake, which will shift your BMI by up to 0.5 points.
• Be aware that BMI does not distinguish muscle from fat: Athletes and bodybuilders with high muscle mass may show a BMI in the overweight or obese range (25–30 or above), even though their body fat percentage is low. If you have a muscular build, consider additional measurements such as waist circumference or body fat percentage.
• Do not use adult BMI ranges for children or teens: For individuals under 18, BMI is interpreted using growth charts and percentile ranges that adjust for age and sex. A BMI of 22 might be normal for a 20-year-old but could indicate overweight for a 10-year-old.
• Interpret your result with context: BMI is a screening tool, not a diagnosis. A result in the 'overweight' range might be less concerning for an older adult with stable weight than for a younger person gaining weight rapidly. Always consult a healthcare professional for a full evaluation.

Frequently Asked Questions

Here are three specific questions people often ask when using a BMI calculator, answered in detail.

1. Is BMI calculated differently for men and women?

No, the BMI formula is identical for adult men and women. You use the same equation (BMI = weight in kg ÷ height² in m) regardless of gender. The reason is that BMI was designed as a population-level metric, not a gender-specific one. However, the interpretation can differ slightly in practice. Men tend to have more muscle mass than women at the same height, so a man and a woman with the same BMI may have different body fat percentages. For this reason, some healthcare providers use waist circumference or body fat percentage as a supplement to BMI when assessing women, who naturally have a higher percentage of essential body fat. But for the calculation itself, there is no gender adjustment in the standard adult formula.

2. What is a healthy BMI for a 65-year-old?

For adults over the age of 65, many clinicians use a slightly higher healthy BMI range than the standard 18.5–25. The common recommendation for older adults is to aim for a BMI between 24 and 27, rather than the lower end of the normal range. This is because a slightly higher BMI in older age is often associated with better survival outcomes, likely due to the protective effect of having more energy reserves and bone density. A BMI below 20 in an older adult can increase the risk of frailty, falls, and osteoporosis. Therefore, if your calculated BMI is 22, you are not underweight by the standard chart, but you might be encouraged by your doctor to maintain or slightly increase your weight if you are 65 or older. The calculator itself will still show the standard category, but you should interpret the 'Normal weight' band (18.5–25) with age-specific nuance.

3. Can I convert my BMI from kg/m² to lbs/in²?

You can, but it is not necessary because BMI is a universal index. The standard unit for BMI is kg/m², and all reference charts and categories use this unit. If you have your weight in pounds and height in inches, you can use the alternative formula: BMI = (weight in lbs ÷ (height in inches)²) × 703. For example, a person weighing 154 lbs and standing 69 inches tall would compute their BMI as follows: (154 ÷ (69 × 69)) × 703 = (154 ÷ 4761) × 703 = 0.0323 × 703 = 22.7. This value of 22.7 is directly comparable to the kg/m² value because the 703 factor accounts for the conversion between imperial and metric units. However, for simplicity and to avoid arithmetic errors, it is much better to convert your inputs to kilograms and centimeters and use the calculator on this page. The result will be identical, but the metric method involves fewer steps and less risk of misplacing a decimal point.