BMI Calculator
Calculate your BMI from height and weight using metric or imperial units. See your WHO classification and the healthy weight range for your height.
Key features
- Quick BMI calculation for both metric and imperial
- Clear WHO classification and status breakdown
- Calculation of the ideal weight range for your height
- Simple, easy-to-understand health visualization
Guide
Body Mass Index is a numerical value derived from your weight and height that provides a standardized way to categorize body weight. Developed by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the World Health Organization as a global screening tool, BMI remains the most widely used population-level indicator of weight status despite its well-documented limitations. Understanding what BMI measures, what it does not measure, and how to interpret your result in context is essential for using this metric appropriately. The BMI formula is simple. In metric units: BMI = weight in kilograms divided by (height in meters squared). In imperial units: BMI = (weight in pounds multiplied by 703) divided by (height in inches squared). A person weighing 75 kg at 175 cm (1.75 m) tall has a BMI of 75 / (1.75 x 1.75) = 75 / 3.0625 = 24.5. A person weighing 165 pounds at 5 feet 9 inches (69 inches) has a BMI of (165 x 703) / (69 x 69) = 115,995 / 4,761 = 24.4. The WebRecast BMI calculator accepts both metric and imperial inputs and displays the result instantly. The WHO classifies BMI into four primary categories. Underweight: below 18.5. Normal weight: 18.5 to 24.9. Overweight: 25.0 to 29.9. Obese: 30.0 and above, with obesity further divided into Class I (30.0-34.9), Class II (35.0-39.9), and Class III (40.0 and above). These thresholds are based on epidemiological data showing that health risks for conditions like type 2 diabetes, cardiovascular disease, hypertension, and certain cancers increase statistically as BMI moves above 25, with significant increases above 30. The BMI calculator also shows the healthy weight range for your height. For someone 175 cm tall, the normal BMI range of 18.5 to 24.9 corresponds to a weight range of 56.7 kg to 76.3 kg. This range gives you a concrete target if you are working toward a healthier weight. It is important to understand that this range is wide because healthy body composition varies significantly between individuals based on muscle mass, bone structure, age, and other factors. BMI has several important limitations that affect its interpretation for individuals. The most significant limitation is that BMI does not distinguish between lean mass (muscle, bone, organs) and fat mass. A person with substantial muscle mass may have an "overweight" BMI while actually having low body fat and excellent metabolic health. This is common among athletes, regular weightlifters, and individuals with naturally heavy bone structures. Conversely, a person with a "normal" BMI may have a high percentage of body fat and low muscle mass (a condition sometimes called "skinny fat" or normal-weight obesity), which carries metabolic risks similar to those associated with higher BMIs. Age affects BMI interpretation. Older adults tend to lose muscle mass (sarcopenia) and gain fat mass even if their weight remains stable. A 70-year-old with a BMI of 24 may have significantly more body fat than a 30-year-old with the same BMI. Research published in the Journal of the American Geriatrics Society suggests that for older adults, the optimal BMI range may be slightly higher (23 to 27) because some additional weight is protective against the risks of falls, illness, and age-related muscle loss. Sex differences also matter. Women naturally carry a higher percentage of body fat than men at any given BMI (approximately 10 to 15 percentage points more). A BMI of 25 in a woman represents a different body composition than a BMI of 25 in a man. Despite this, the WHO categories do not distinguish between sexes, which is a known limitation of the metric. Ethnicity influences the relationship between BMI and health risk. Research has consistently shown that South Asian and East Asian populations experience higher rates of type 2 diabetes and cardiovascular disease at lower BMI values compared to European populations. The WHO has suggested lower BMI thresholds for Asian populations: overweight at 23 (instead of 25) and obese at 27.5 (instead of 30). If you are of Asian descent, discuss appropriate BMI thresholds with your healthcare provider. Given these limitations, several complementary measurements provide a more complete picture of body composition and health risk. Waist circumference is the most practical additional measurement. The WHO defines elevated health risk as a waist circumference above 94 cm for men and above 80 cm for women. Substantially elevated risk is above 102 cm for men and above 88 cm for women. Abdominal fat (visceral fat) is more metabolically active and more strongly associated with health risks than fat stored in the hips, thighs, or subcutaneously. The waist-to-height ratio (WHtR) is an even simpler screening tool. Divide your waist circumference by your height. A ratio below 0.5 indicates healthy abdominal fat levels. Above 0.5 suggests elevated risk. Above 0.6 indicates substantially elevated risk. This ratio works across different heights, ages, and ethnicities, making it a useful complement to BMI. A person with a normal BMI but a WHtR above 0.5 should pay attention to their visceral fat, even though their weight appears healthy. Body fat percentage provides the most direct measurement of what BMI is trying to approximate. Healthy body fat ranges are approximately 10 to 20% for men and 18 to 28% for women, though these vary by age and fitness level. Methods for measuring body fat include dual-energy X-ray absorptiometry (DEXA, highly accurate, available at medical facilities), bioelectrical impedance analysis (BIA, available in smart scales, moderately accurate), skinfold calipers (requires trained measurer, inexpensive), and hydrostatic weighing (very accurate, limited availability). If you want a more precise understanding of your body composition than BMI provides, a DEXA scan is the gold standard. For most people, BMI serves its intended purpose as a quick screening tool. If your BMI falls within the normal range (18.5 to 24.9) and you are not an athlete or heavily muscled, your weight is likely within a healthy range. If your BMI falls outside the normal range, it is a signal to investigate further, not a diagnosis. An "overweight" BMI in someone who strength trains regularly and has a waist circumference well under 94 cm (men) or 80 cm (women) is a very different situation from an "overweight" BMI in someone who is sedentary with a large waist circumference. BMI trends over time are more useful than a single measurement. If your BMI has been gradually increasing over the past several years, that trend indicates increasing fat accumulation regardless of where you currently fall in the categories. A BMI of 24.5 that was 22.0 five years ago tells a story that a single measurement does not. Track your BMI at regular intervals (monthly or quarterly) alongside waist circumference to monitor trends. The BMI calculator is also useful for setting concrete goals. If your BMI is 28 and you want to reach the normal range (below 25), the calculator tells you exactly what weight corresponds to a BMI of 24.9 at your height. For a person 170 cm tall, a BMI of 24.9 corresponds to 71.9 kg, and a BMI of 28 corresponds to 80.9 kg. The goal is to lose approximately 9 kg, which at a safe rate of 0.5 kg per week would take about 18 weeks. Having specific numbers makes the goal tangible and trackable. Children and adolescents use a different BMI interpretation system called BMI-for-age percentiles, which accounts for normal growth patterns. A child's BMI is plotted against age-and-sex-specific growth charts, and the percentile ranking determines weight status. Below the 5th percentile is underweight, 5th to 84th percentile is healthy weight, 85th to 94th percentile is overweight, and 95th percentile and above is obese. The adult BMI categories do not apply to anyone under 18. If you are calculating BMI for a child, consult the growth charts from the CDC or WHO. For clinical use, BMI is one data point among many that physicians consider. Blood pressure, fasting blood glucose, HbA1c, lipid panel (cholesterol, triglycerides), waist circumference, family medical history, physical activity level, and dietary patterns all contribute to an overall health assessment. A person with a BMI of 27 who exercises regularly, eats a balanced diet, has normal blood pressure and blood glucose, and has no family history of metabolic disease is in a very different risk category than a person with a BMI of 27 who is sedentary with elevated blood pressure and blood sugar. BMI opens the conversation about weight and health, but it does not conclude it. The relationship between BMI and specific health conditions has been extensively studied. For type 2 diabetes, the risk begins increasing at a BMI of 23 and roughly doubles at BMI 30 compared to BMI 22. For cardiovascular disease, a 2016 meta-analysis of 3.9 million individuals published in The Lancet found that the lowest mortality risk occurred at BMI 22 to 23, with progressive increases in both directions. Underweight (BMI below 18.5) was associated with increased mortality primarily from respiratory disease and lung cancer, while overweight and obesity increased mortality from cardiovascular disease, diabetes, and certain cancers. For hypertension, each 5-unit increase in BMI is associated with a 30 to 40% increase in risk. Sleep apnea has one of the strongest associations with elevated BMI. Approximately 60 to 70% of individuals with obstructive sleep apnea are obese, and BMI is the single strongest predictor of the condition. A BMI above 30 increases the risk of sleep apnea by 10 to 14 times compared to normal weight. Weight loss of even 10% of body weight can significantly reduce the severity of sleep apnea, making BMI a relevant tracking metric for patients with this condition. Joint health is directly affected by BMI because excess weight increases the mechanical load on weight-bearing joints. Each kilogram of excess body weight adds approximately 4 kg of pressure to the knee joint during walking. A person carrying 15 kg of excess weight places an additional 60 kg of force on their knees with every step. Over years, this accelerates cartilage degradation and increases the risk of osteoarthritis. Studies show that losing just 5 kg reduces knee arthritis symptoms by 50% in many patients. The BMI calculator helps you quantify how much weight reduction you need to reach a target that would meaningfully reduce joint stress. Mental health and BMI have a bidirectional relationship. Depression and anxiety are more common at both extremes of the BMI spectrum (underweight and obese) compared to normal weight. This relationship is complex because mental health conditions can both cause weight changes (through appetite effects, medication side effects, and reduced physical activity) and be affected by weight status (through body image, social stigma, and inflammatory pathways). Using BMI as one metric in a holistic health assessment, rather than as the sole indicator of health, is the appropriate approach. Pregnancy and BMI interact in several important ways. Pre-pregnancy BMI affects fertility, pregnancy complications, and delivery outcomes. A BMI below 18.5 is associated with increased risk of preterm birth and low birth weight. A BMI above 30 is associated with increased risk of gestational diabetes, preeclampsia, cesarean delivery, and macrosomia (large birth weight). The recommended weight gain during pregnancy varies by pre-pregnancy BMI: 12.5 to 18 kg for underweight women, 11.5 to 16 kg for normal weight, 7 to 11.5 kg for overweight, and 5 to 9 kg for obese. Knowing your pre-pregnancy BMI helps you and your healthcare provider set appropriate weight gain targets. The BMI calculator provides your number in seconds. Enter your height and weight, see your BMI, check your WHO category, and note the healthy weight range for your height. Use it as a starting point. If the number concerns you, measure your waist circumference for additional context, and discuss the results with a healthcare professional who can assess your complete health picture. A single BMI reading is a data point, but a series of BMI readings over time tells the story of your metabolic trajectory. The New BMI formula, proposed by mathematician Nick Trefethen at the University of Oxford, attempts to correct a known mathematical bias in the traditional formula. The classic BMI formula overestimates BMI for tall people and underestimates it for short people because it divides weight by the square of height, while body volume scales more closely with the cube of height. The New BMI formula is: 1.3 x weight(kg) / height(m)^2.5. For a person 1.80 m tall and 80 kg, the classic BMI is 24.7 and the New BMI is 24.3. For a person 1.55 m tall and 65 kg, the classic BMI is 27.1 and the New BMI is 27.9. The difference is small but systematic, and it means that shorter individuals may be slightly underclassified by the standard formula. The WebRecast BMI calculator uses the standard WHO formula because it is the internationally recognized standard, but awareness of this bias is useful context when interpreting results at the extremes of height. BMI and surgical risk have a well-documented relationship. Anesthesiologists use BMI as part of their pre-operative assessment because obesity increases the risk of airway management difficulties, respiratory complications, wound infections, and venous thromboembolism after surgery. Many surgical centers require a BMI below 40 (or sometimes 35) for elective procedures like joint replacement or bariatric referral. Knowing your BMI before a surgical consultation helps you understand whether weight management will be a prerequisite for the procedure you need. Insurance applications in many countries use BMI as a factor in premium calculations. Life insurance underwriters categorize applicants based on BMI ranges, with preferred rates for BMI 18.5 to 25, standard rates for 25 to 30, and higher premiums or potential decline for BMI above 30 or below 17. Health insurance in countries without universal coverage may also adjust premiums based on BMI. While this practice is debated on fairness grounds (given the known limitations of BMI as an individual health metric), it is a practical reality that makes knowing your BMI relevant beyond its medical implications. Military and law enforcement fitness standards often include BMI or body composition requirements. The Turkish Armed Forces, US military branches, and many police departments set maximum BMI thresholds for recruitment and continued service. Candidates who exceed the BMI threshold may be given an opportunity to pass a body fat percentage measurement instead, acknowledging that muscular individuals can have elevated BMIs without excess fat. If you are preparing for such an assessment, the BMI calculator tells you where you stand relative to the threshold and how much weight change is needed to comply. The obesity paradox is a counterintuitive finding in medical research that deserves mention. Several large studies have found that among patients with certain chronic conditions (heart failure, chronic kidney disease, and some cancers), those classified as overweight by BMI (25 to 30) have better survival outcomes than those at normal weight or underweight. This finding does not mean that being overweight is protective in healthy populations. Rather, it likely reflects the role of metabolic reserves during serious illness and the limitations of BMI as a measure of health. For healthy individuals, the conventional BMI ranges remain the appropriate targets. BMI interacts with bone density in ways that affect interpretation. Individuals with higher bone mineral density (which can be genetic or the result of long-term weight-bearing exercise) will have higher BMIs at the same body fat percentage compared to those with lower bone density. A DEXA scan, which measures both bone density and body fat percentage, reveals this distinction clearly. If your BMI is in the overweight range but your DEXA scan shows normal body fat and above-average bone density, your weight is not a health concern. For athletes and fitness enthusiasts, tracking body weight alongside performance metrics provides more useful information than BMI alone. A runner whose BMI increases from 22 to 24 over a year while improving their 5K time by 2 minutes has likely gained functional muscle mass, not concerning fat. A weightlifter whose BMI is 29 but who can deadlift twice their bodyweight is operating at a level of fitness that makes the BMI classification meaningless. Performance-based metrics (strength, endurance, flexibility, cardiovascular fitness) combined with body composition measurements paint a far more accurate picture of health than BMI alone. The global prevalence of obesity has tripled since 1975 according to WHO data, with over 1 billion people worldwide now classified as obese (BMI 30+). In Turkey, the obesity rate among adults exceeds 30%, with higher rates among women and in eastern regions. These population-level statistics are where BMI is most useful: tracking trends across millions of people over decades to inform public health policy. At the individual level, your BMI is a starting point for a conversation with yourself and your healthcare provider about whether your current weight supports your health goals. The BMI calculator puts that starting number at your fingertips. Regular monitoring builds awareness. Check your BMI quarterly, record it alongside your waist circumference, and note your activity level and general dietary patterns. Over a year, these four quarterly snapshots tell you whether you are moving toward or away from your health goals. The BMI calculator is free, instant, and private. Use it as part of a simple self-assessment routine that takes less than a minute but provides lasting value for your health awareness.
Frequently asked questions
What is a healthy BMI?
The WHO says 18.5 to 24.9 is the sweet spot. I'll show you exactly where you stand!
Is BMI everything?
It's a great general guide, but it doesn't know about your muscle or bone density. Always consult a pro for a full health check!
