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Calculate your Body Mass Index (BMI) to check if your weight is in a healthy range. Free, instant, and accurate.
Body Mass Index (BMI) is a simple calculation using a person's height and weight. The BMI value is a moderately reliable indicator of body fatness for most people. It was devised in the 1830s by Belgian mathematician Adolphe Quetelet and remains one of the most widely used health screening metrics worldwide.
BMI is calculated by dividing your weight in kilograms by your height in meters squared (BMI = kg/m²). For example, a person weighing 70 kg with a height of 1.75 m would have a BMI of 70 ÷ (1.75 × 1.75) = 22.9. Our calculator handles both metric and imperial unit conversions automatically.
| Category | BMI Range |
|---|---|
| Underweight | < 18.5 |
| Normal Weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese (Class I) | 30.0 – 34.9 |
| Obese (Class II) | 35.0 – 39.9 |
| Obese (Class III) | ≥ 40.0 |
While BMI is a useful screening tool, it has important limitations. It doesn't distinguish between muscle and fat mass, meaning athletes may be classified as overweight despite having low body fat. BMI also doesn't account for age, sex, ethnicity, or body fat distribution. For a more comprehensive health assessment, consider combining BMI with other measurements like waist circumference, body fat percentage, and blood test results.
Body Mass Index was first introduced in the early 19th century by Belgian mathematician and statistician Lambert Adolphe Jacques Quetelet. Originally called the Quetelet Index, it was designed to provide a quick statistical measure of body weight relative to height across populations, not to diagnose individual health. The formula — weight in kilograms divided by height in meters squared (kg/m2) — gained widespread clinical adoption in the 1970s when researcher Ancel Keys published a landmark study comparing BMI to more direct measures of body fat. Keys found that BMI correlated reasonably well with body fat percentage across large groups, while being far simpler and cheaper to calculate than laboratory methods. The World Health Organization (WHO) officially adopted BMI as the standard for classifying overweight and obesity in 1995, establishing the thresholds still used today. Since then, epidemiological studies involving millions of participants have consistently demonstrated that BMI values outside the normal range (18.5-24.9) are associated with increased risks of cardiovascular disease, type 2 diabetes, certain cancers, and all-cause mortality. However, the relationship between BMI and health risk is not perfectly linear. Research published in the Journal of the American Medical Association has shown that individuals in the overweight category (25.0-29.9) sometimes have lower mortality rates than those in the normal category, a phenomenon called the 'obesity paradox.' This may be partly because BMI does not account for where fat is distributed in the body — visceral fat around the organs is far more dangerous than subcutaneous fat under the skin.
The BMI formula divides weight by the square of height: BMI = weight (kg) / [height (m)]2. For imperial measurements, the conversion formula is BMI = [weight (lbs) / height (in)2] x 703. The squaring of height is what makes BMI a useful ratio rather than a simple weight-to-height comparison. A taller person naturally weighs more, but their weight increases roughly with the cube of their height (since the body is three-dimensional), while BMI uses the square. This means BMI tends to slightly overestimate body fat in very tall people and underestimate it in very short people. To address this, some researchers have proposed the Body Mass Index Prime (BMI Prime), which expresses your BMI as a ratio to the upper limit of normal (25.0). A BMI Prime of 1.0 means you are exactly at the threshold. An alternative called the Ponderal Index uses height cubed instead of squared, which may be more accurate for individuals at the extremes of height. Our calculator applies the standard WHO formula and instantly categorizes your result into one of six classifications: Underweight (below 18.5), Normal Weight (18.5 to 24.9), Overweight (25.0 to 29.9), Obese Class I (30.0 to 34.9), Obese Class II (35.0 to 39.9), and Obese Class III (40.0 and above). Each category carries different statistical health risk profiles.
Understanding your BMI is just the first step. Here are evidence-based tips for each situation. If your BMI is in the underweight range, focus on nutrient-dense calorie sources like nuts, avocados, whole grains, and lean proteins rather than empty calories. Consult a dietitian to rule out underlying health issues such as thyroid disorders or malabsorption. If your BMI is in the normal range, maintain your healthy weight through a balanced diet emphasizing vegetables, fruits, whole grains, and lean proteins, combined with at least 150 minutes of moderate-intensity exercise per week. If your BMI indicates overweight, a modest calorie reduction of 500 calories per day can lead to roughly 0.5 kg of weight loss per week without extreme measures. Prioritize reducing processed foods, sugary beverages, and oversized portions. Strength training is particularly beneficial because it builds lean muscle mass, which increases your resting metabolic rate. If your BMI falls in the obese range, work with a healthcare team that may include a physician, dietitian, and exercise physiologist. Evidence shows that even a 5-10% reduction in body weight can significantly improve blood pressure, blood sugar, and cholesterol levels. For everyone, remember that BMI is a screening tool, not a verdict. Pair your BMI result with waist circumference measurement — a waist over 102 cm (40 inches) for men or 88 cm (35 inches) for women indicates elevated health risk regardless of BMI.
Extensive research has linked both very low and very high BMI values to increased health risks. A BMI below 18.5 is associated with weakened immune function, increased susceptibility to infections, osteoporosis and bone fractures, fertility problems, and nutritional deficiencies including anemia and vitamin deficiencies. Severe underweight can also signal or worsen eating disorders, which carry the highest mortality rate of any mental health condition. On the other end of the spectrum, a BMI of 25 or above progressively increases the risk of type 2 diabetes (the risk doubles at a BMI of 25 and increases fivefold at 30), cardiovascular disease including heart attack and stroke, hypertension, sleep apnea, osteoarthritis from excess joint stress, gallbladder disease, and several types of cancer including breast, colon, and kidney cancer. Importantly, different ethnic groups experience these risks at different BMI thresholds. The WHO recommends that Asian populations use lower BMI cutoffs — overweight begins at 23 rather than 25, and obesity at 27.5 rather than 30 — because research shows that Asian individuals tend to accumulate more visceral fat at lower BMI values compared to European populations. Similarly, certain Pacific Islander and Polynesian populations may have different risk profiles due to greater lean body mass. These ethnicity-specific thresholds are an important consideration when interpreting your BMI result.
Body Mass Index, commonly known as BMI, is one of the most widely used screening tools in public health. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, the index was originally intended as a population-level statistical measure rather than an individual diagnostic tool. Despite its simplicity, BMI remains a cornerstone of health assessments worldwide because it requires only two easily obtainable measurements: height and weight. The World Health Organization adopted BMI as its standard classification system for overweight and obesity in 1995, establishing the thresholds that clinicians still use today.
The WHO classification divides BMI into several categories: underweight (below 18.5), normal weight (18.5 to 24.9), overweight (25.0 to 29.9), and obese (30.0 and above), with obesity further subdivided into Class I (30.0-34.9), Class II (35.0-39.9), and Class III (40.0 and above). These thresholds were derived from large epidemiological studies linking BMI ranges to mortality and disease risk. However, they were primarily based on data from European and North American populations, which has led to important discussions about ethnic-specific cutoffs.
For Asian populations, research has demonstrated that health risks such as type 2 diabetes and cardiovascular disease begin to increase at lower BMI values compared to European populations. As a result, the WHO has recommended lowered thresholds for Asian individuals: overweight beginning at 23.0 and obesity at 27.5. This difference is attributed to the tendency of Asian populations to accumulate more visceral fat at lower overall body weights, a pattern linked to higher metabolic risk even at a seemingly normal BMI.
BMI has notable limitations for certain groups. Athletes and highly muscular individuals may be classified as overweight or obese despite having low body fat, because the formula does not distinguish between muscle and fat mass. Elderly individuals who have lost muscle mass may appear to have a normal BMI while actually carrying excess fat. Children and adolescents require age- and sex-specific BMI percentile charts rather than the standard adult categories. For a more complete health picture, BMI should be combined with other measurements such as waist circumference, body fat percentage, and metabolic blood markers.
The BMI formula divides your weight in kilograms by the square of your height in meters: BMI = weight (kg) / [height (m)]². For imperial measurements, the conversion formula is BMI = [weight (lbs) x 703] / [height (in)]². For example, a person weighing 70 kg with a height of 1.75 m would calculate their BMI as 70 / (1.75 x 1.75) = 22.9, placing them in the normal weight category.
The squaring of height in the denominator is a deliberate mathematical choice. Since the human body is three-dimensional, weight naturally increases roughly with the cube of height. Using the square rather than the cube provides a ratio that better normalizes weight across different heights for population-level comparisons, though it slightly overestimates body fatness in very tall individuals and underestimates it in very short individuals. Some researchers have proposed alternatives like the Ponderal Index (weight / height³) or a modified BMI using an exponent of 2.5 to address this, but the standard formula remains dominant in clinical practice due to its simplicity and extensive validation.