BMI Calculator
Calculate your Body Mass Index (BMI) based on weight and height to assess whether you are underweight, normal weight, overweight, or obese.
Results
Visualization
How It Works
Body Mass Index divides weight (kg) by height squared (m^2) to produce a single number that correlates with body fatness at the population level. WHO categories (Technical Report Series 894): under 18.5 underweight, 18.5-24.9 normal, 25.0-29.9 overweight, 30.0-34.9 Class I obesity, 35.0-39.9 Class II, 40.0+ Class III. The metric was created by Adolphe Quetelet in 1832 as a population statistic and only became a clinical screening tool in the 1970s after Ancel Keys validated its correlation with skinfold and densitometry data. BMI ignores muscle mass, bone density, and fat distribution, which is why a 95 kg NFL running back and a 95 kg sedentary adult of the same height land in the same category despite radically different metabolic profiles. The WHO Asia-Pacific guidelines apply lower cutoffs for South and East Asian populations: overweight at 23.0, obesity at 27.5.
The Formula
Variables
- weight — Body weight in kilograms
- height — Height in meters (centimeters / 100)
- BMI — Body Mass Index value (kg/m^2)
Worked Example
Maya, 32, weighs 68 kg and stands 165 cm. Convert height: 165 / 100 = 1.65 m. Square it: 1.65 x 1.65 = 2.7225 m^2. Divide weight by that: 68 / 2.7225 = 24.98 kg/m^2. Maya's BMI sits at the top edge of the normal category (18.5-24.9). Her healthy weight range works out to 18.5 x 2.7225 = 50.4 kg on the low end and 24.9 x 2.7225 = 67.8 kg on the high end. If Maya identifies as South Asian, the WHO Asia-Pacific cutoffs reclassify her as overweight (23.0+). The 1 kg difference between her current weight and the upper Asia-Pacific threshold of 67.8 kg illustrates how ethnicity-specific cutoffs change clinical interpretation without changing the math.
Methodology
Quetelet derived the formula in 1832 as a population descriptor, calling it the Quetelet Index. The term Body Mass Index was coined by Ancel Keys in his 1972 paper validating the metric against skinfold thickness in 7,402 men across five countries. The WHO codified the modern cutoffs (18.5, 25.0, 30.0) in Technical Report Series 854 (1995) and refined them in TRS 894 (2000). The Asia-Pacific cutoffs (23.0, 27.5) were added in 2004 after the WHO Expert Consultation reviewed evidence from 16 Asian cohorts showing diabetes and cardiovascular disease incidence rising at lower BMI values. BMI correlates with DEXA-measured body fat at r = 0.7-0.8 in general populations but drops to r = 0.3-0.5 in athletic samples. The American Medical Association acknowledged BMI's limitations in a 2023 policy statement recommending it be used alongside waist circumference, body composition, and metabolic markers rather than as a standalone diagnostic.
When to Use This Calculator
Primary care physicians use BMI as the first screen during annual physicals, typically combining it with blood pressure, fasting glucose, and a lipid panel to estimate cardiometabolic risk. Bariatric surgery programs use BMI thresholds (35 with comorbidity, 40 without) to determine procedure eligibility per the 1991 NIH Consensus Conference criteria, updated by the ASMBS in 2022 to lower the threshold to BMI 30 for patients with type 2 diabetes. Anesthesiologists adjust drug dosing for patients with BMI over 35, since lipophilic drugs distribute differently in adipose tissue. Public health agencies including the CDC and NCD Risk Factor Collaboration track population BMI distributions to monitor obesity prevalence; US adult obesity prevalence rose from 13.4% in 1962 to 41.9% by 2020 per NHANES.
Common Mistakes to Avoid
Treating BMI as a diagnosis rather than a screen leads providers to dismiss metabolic disease in normal-BMI patients (the normal-weight obesity blind spot) and over-pathologize muscular patients. Using imperial units without conversion (lbs / inches squared) gives values that don't match WHO categories; the formula requires kg / m^2 or the imperial conversion factor (lbs * 703 / inches^2). Measuring height in shoes adds 1-3 cm and shifts BMI downward by 0.3-0.7 points. Applying adult cutoffs to anyone under 20 ignores the age-specific percentile system that pediatrics uses. Ignoring ethnicity when WHO Asia-Pacific cutoffs apply underestimates diabetes and cardiovascular risk by roughly 50% in South and East Asian patients.
Practical Tips
- WHO categories use exact thresholds: 18.5, 25.0, 30.0, 35.0, 40.0. A BMI of 24.95 is normal, 25.05 is overweight. Track to one decimal place to avoid category mismatches between visits.
- BMI overestimates body fat in muscular athletes. A 1996 NHANES analysis found that 13.5% of men classified as overweight by BMI had body fat percentages in the athletic range. If you lift heavy 4+ days per week, get a DEXA scan or use the Navy circumference method instead.
- Asia-Pacific cutoffs (overweight 23.0+, obesity 27.5+) apply to people of South Asian, East Asian, and Southeast Asian descent. The WHO recommends these because cardiometabolic risk in these populations rises at lower BMI values than the standard cutoffs predict.
- Children and adolescents under 20 use BMI-for-age percentile charts from the CDC, not adult categories. Underweight is below the 5th percentile, healthy weight 5th-85th, overweight 85th-95th, obesity above the 95th.
- Pair BMI with waist circumference for a sharper risk picture. Men with BMI 25-29.9 and waist over 102 cm, or women with BMI 25-29.9 and waist over 88 cm, face the same cardiovascular risk as people in the obese BMI range.
- Weigh yourself in the morning after using the bathroom and before eating, in light clothing, on the same scale. Day-to-day weight swings of 1-2 kg from food, water, and glycogen are normal and can shift BMI by 0.4-0.7 points.
Frequently Asked Questions
Why does my BMI say overweight when I'm muscular?
BMI counts every kilogram identically, whether it sits on your femoral biceps or your abdomen. A bodybuilder with 8% body fat at 95 kg and 178 cm has a BMI of 30 (Class I obesity by the formula) but is metabolically healthier than most people in the normal BMI range. Use a Navy circumference body fat estimate, skinfold calipers, or DEXA scan if you train seriously.
How accurate is BMI for athletes?
Poor. A 2007 study in Medicine and Science in Sports and Exercise found BMI misclassified 28-44% of male athletes as overweight or obese depending on the sport. American football linemen, rugby players, throwers in track and field, and Olympic weightlifters routinely fall in the obese BMI category despite single-digit body fat. For these populations, body fat percentage via DEXA, hydrostatic weighing, or air displacement plethysmography (Bod Pod) gives a real answer.
Should pregnant or lactating women use BMI?
No. Standard BMI categories don't apply during pregnancy because weight gain is expected and includes amniotic fluid, placental tissue, increased blood volume, and fetal mass. The Institute of Medicine publishes pregnancy-specific weight gain ranges based on pre-pregnancy BMI: 12.5-18 kg if pre-pregnancy BMI was under 18.5, 11.5-16 kg if 18.5-24.9, 7-11.5 kg if 25-29.9, and 5-9 kg if 30+. Lactating women should consult their provider rather than relying on standard BMI.
What's the difference between Class I, II, and III obesity?
Class I (BMI 30.0-34.9) carries roughly 2-3x the risk of type 2 diabetes versus normal weight. Class II (35.0-39.9) raises cardiovascular mortality 2.5x. Class III (40.0+, sometimes called severe or morbid obesity) raises all-cause mortality 6-13x in long-term cohort studies. Bariatric surgery is typically considered at BMI 40+ or 35+ with comorbidities like sleep apnea or type 2 diabetes.
Does BMI work for older adults?
Less reliably. After age 65, the BMI range associated with the lowest mortality shifts upward to roughly 25-32. This is the obesity paradox: mild overweight in older adults correlates with better outcomes, possibly because BMI doesn't distinguish protective lean mass from sarcopenic muscle loss. The American Geriatrics Society recommends combining BMI with grip strength and gait speed for adults over 65.
What's a good BMI to aim for?
The lowest mortality risk in large prospective cohorts (Prospective Studies Collaboration, 2009, 894,576 participants) sits at BMI 22.5-25.0 for adults under 65. Outside that range, all-cause mortality rises about 30% per 5-point BMI increment in either direction. Aim for the middle of the normal range if you're under 50, and be more flexible if you're over 65 or carry significant muscle mass.
Can I have a normal BMI and still be unhealthy?
Yes. Normal-weight obesity (NWO) describes people with BMI under 25 but body fat over 30% (women) or 23% (men). NHANES data show 30 million Americans fit this profile and face elevated cardiometabolic risk despite a normal BMI. Visceral adiposity, not subcutaneous fat, drives the harm. A waist-to-height ratio over 0.5 is a useful supplementary screen.