BMI Calculator
The Body Mass Index (BMI) Calculator helps you quickly determine whether your weight falls within a healthy range for your height. It is one of the most widely used screening tools in healthcare, fitness, and insurance assessments worldwide, offering a fast, standardized way to estimate whether a person may be underweight, at a healthy weight, overweight, or obese.
BMI + BMR Calculator
Table of Contents
What is BMI?
Body Mass Index (BMI) is a numerical value calculated from a person’s height and weight. It was developed in the 19th century as a simple method for estimating body fat levels across large populations, and it remains the standard first-step screening measure used by organizations such as the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC). BMI does not measure body fat directly, but it correlates closely enough with it to serve as a useful general indicator.​
How to Use This Calculator
- Select your preferred unit system — Metric (kilograms/centimeters) or Imperial (pounds/feet-inches).
- Enter your height.
- Enter your current weight.
- Click “Calculate.”
- Review your BMI value along with its corresponding weight category.
- Optionally, compare your result against the BMI category table below to understand what it means for your health.
The BMI Formula
Metric formula: BMI = weight (kg) ÷ [height (m)]²
Imperial formula: BMI = [weight (lb) ÷ height (in)²] × 703
Where:
- Weight is your body mass in kilograms or pounds
- Height is your height in meters (metric) or inches (imperial)
- 703 is a conversion constant used to adjust the imperial formula to match the metric result
Worked Example
Suppose a person weighs 68 kg and is 1.70 m tall.
BMI = 68 ÷ (1.70 × 1.70) BMI = 68 ÷ 2.89 BMI = 23.5
A BMI of 23.5 falls within the “Normal weight” category (18.5–24.9).
Imperial example: A person weighing 150 lb at a height of 66 inches: BMI = (150 ÷ 66²) × 703 = (150 ÷ 4356) × 703 = 24.2
BMI Categories (WHO Standard)
Individuals are classified in several BMI categories (classes) according to their mortality risk. There are different approaches to categorization, but the widely accepted WHO categorization is shown below.
WHO BMI categories and corresponding BMI prime and risk levels Category BMI range (kg/m2) BMI Prime Risk level Severely underweight less than 16.5 less than 0.66 Moderate Underweight 16.5 – 18.4 0.66 – 0.74 Moderate Normal (healthy) 18.5 – 24.9 0.74 – 0.99 Very low Overweight 25 – 29.9 1.0 – 1.2 Low Obesity 30 or greater 1.2+ Moderate to Very High     Obesity Class I 30 – 34.9 1.2 – 1.4 Moderate     Obesity Class II 35 – 39.9 1.4 – 1.6 High     Obesity Class III 40+ 1.6+ Very high As you can see, there is a moderate risk in being underweight, but obesity is certainly a primary risk factor for different sorts of causes for mortality. Sticking to a healthy BMI of between 18.5 and 24.9 (normal weight / healthy weight range) is therefore recommended by health authorities such as the US CDC (Center for Disease Control and Prevention) in order to minimize the risk of health problems. With our calculator you will also get a percentage over/under the normal limit as an output.
Factors That Affect Your Result
- Age — Body composition shifts with age; older adults tend to carry more body fat at the same BMI as younger adults.
- Sex — Women generally have a higher body fat percentage than men at an equivalent BMI.
- Muscle mass — Athletes and highly trained individuals often have a higher BMI due to muscle weight, despite low body fat.
- Ethnicity — Health risk at a given BMI can vary by ethnic background, which is why some countries use different reference ranges.
- Pregnancy — BMI calculations are not designed for use during pregnancy, as weight gain during this period is expected and healthy.
Common Mistakes to Avoid
- Mixing unit systems — Entering height in centimeters while selecting the imperial formula (or vice versa) will produce an inaccurate result.
- Using BMI for children without adjustment — Children and teenagers require age- and sex-specific percentile charts, not the standard adult ranges.
- Treating BMI as a diagnosis — A single BMI reading is a screening indicator, not a medical diagnosis of any condition.
Limitations of BMI
BMI is a useful starting point, but it has clear limitations. It cannot distinguish between muscle mass and fat mass, so muscular individuals may be classified as “overweight” despite having low body fat. It also does not indicate where fat is distributed in the body — visceral fat around the abdomen carries greater health risk than fat stored elsewhere, and BMI cannot detect this difference. For a more complete picture of health, BMI results are best interpreted alongside other measurements such as waist circumference, body fat percentage, and an overall clinical assessment from a healthcare professional.
Related Calculators
- Body Fat Calculator — Provides a more detailed estimate of body composition using waist, neck, and hip measurements, useful alongside your BMI result.
- Calorie Calculator — Once you know your BMI category, use this to estimate your daily calorie needs for maintaining, losing, or gaining weight.
- Ideal Weight Calculator — Offers a target weight range based on your height, age, and frame size, complementing your BMI result.
Frequently Asked Questions
For most adults, a BMI between 18.5 and 24.9 is considered within the normal, healthy range. However, healthy ranges can vary slightly based on age, sex, and ethnicity.
Not always. Because muscle weighs more than fat, athletes and bodybuilders often register a higher BMI despite having low body fat percentages. Body fat measurement is more accurate for this group.
No, not using the standard adult formula's categories. Children and teenagers should be assessed using age- and sex-specific BMI percentile charts, since body composition changes significantly during growth.
The calculation formula is identical, but interpretation can differ slightly, since women typically have a higher percentage of body fat than men at the same BMI value.
BMI tends to underestimate body fat in older adults, who often experience muscle loss (sarcopenia) with age even if their weight remains stable.
No. BMI calculations are not designed for pregnancy, since healthy weight gain during pregnancy is expected. Prenatal weight guidance should come from a healthcare provider.
No. Being significantly underweight (BMI below 18.5) carries its own health risks, including nutrient deficiencies and weakened immunity. The goal is a balanced, healthy range — not simply the lowest possible number.
There's no fixed rule, but checking every few months — or whenever your weight changes noticeably — is generally sufficient for tracking trends over time.
Not by default. However, several health authorities, including those in parts of Asia, recommend lower BMI cutoffs for overweight and obesity classifications due to differing health risk profiles at the same BMI.
No. BMI is calculated purely from height and weight, while body fat percentage measures the actual proportion of fat in your body. They are related but not identical, which is why BMI is best used alongside other health measurements.
Summary
BMI offers a quick, standardized way to check whether your weight is in a healthy range relative to your height. It’s a useful first step for tracking your health over time, but it works best when combined with other measurements like body fat percentage and waist circumference. Use the calculator above to get your BMI instantly, and explore the related tools to build a fuller picture of your overall health.