How do you calculate BMI?
BMI is calculated by dividing your weight in kilograms by your height in meters squared: BMI = weight(kg) / height(m)². For example, a person who is 5'10" (1.78m) and weighs 180 lbs (81.6 kg) has a BMI of 25.8, which falls in the overweight category.
What is BMI and how is it calculated?
BMI (Body Mass Index) is calculated as weight in kilograms divided by the square of height in meters (kg/m²). For imperial units, the formula is (weight in lbs × 703) / (height in inches)². It was developed by Adolphe Quetelet in the 19th century as a population-level metric, not an individual diagnostic tool.
What are the BMI categories?
Standard WHO categories: Underweight (<18.5), Normal weight (18.5–24.9), Overweight (25–29.9), Obese Class I (30–34.9), Obese Class II (35–39.9), Obese Class III (≥40). Asia-Pacific cutoffs are lower: Overweight ≥23, Obese ≥27.5.
Why is BMI not a perfect measure of health?
BMI doesn't distinguish between muscle and fat mass. Muscular athletes can have high BMI while having low body fat, and older individuals may have normal BMI but high body fat. It also doesn't account for fat distribution (visceral vs. subcutaneous fat), which is a stronger predictor of metabolic health. Metrics like WHtR, RFM, and ABSI improve on BMI's limitations.
What is the Waist-to-Height Ratio (WHtR) and why does it matter?
WHtR = waist circumference / height. A WHtR below 0.5 is considered healthy for most adults ("keep your waist to less than half your height"). WHtR is a stronger predictor of cardiovascular and metabolic risk than BMI because it specifically measures central/visceral obesity, which surrounds the organs and is metabolically active.
How does the Navy Method body fat % work?
The US Navy Method (Hodgdon & Beckett 1984) estimates body fat using circumference measurements. For men: it uses height, waist, and neck. For women: height, waist, hip, and neck. The formula is: 495 / (1.0324 − 0.19077×log₁₀(waist−neck) + 0.15456×log₁₀(height)) − 450. This method is accurate to within 3–4% of DEXA scan results and is used by the US military.
What is Relative Fat Mass (RFM) and why is it better than BMI?
RFM (Woolcott & Bergman 2018) uses only height and waist: 64 − 20 × (height/waist) for men; 76 − 20 × (height/waist) for women. Research shows RFM has a higher correlation with DEXA-measured body fat than BMI (r=0.80 vs. r=0.68). It's free from BMI's muscle-mass bias because waist measurement reflects fat better than total body weight.
What does my ABSI score mean?
ABSI (A Body Shape Index) measures how much your waist is larger than predicted for your BMI and height: waist / (BMI^(2/3) × height^(1/2)). A higher ABSI indicates more central/abdominal fat relative to overall body size, which is associated with higher all-cause mortality independent of BMI. A Z-score above 0.798 indicates elevated risk; above 1.645 is very high risk.
How do I calculate my ideal body weight?
This calculator uses 4 clinical formulas: Devine (1974) is most used in pharmacology; Robinson (1983) adjusts Devine slightly; Miller (1983) produces lower estimates; Hamwi (1964) is used in dietetics. All are based on height and sex. None of these formulas account for frame size or muscularity — they're reference points, not rigid targets.
What is a healthy BMI for Asian populations?
The WHO Asia-Pacific guidelines and many national health authorities in Asia use lower cutoffs: Overweight ≥23 kg/m² and Obese ≥27.5 kg/m². This is because studies show East Asian and South Asian populations have higher body fat percentages and higher cardiometabolic risk at the same BMI compared to European populations. Enable the ethnicity adjustment to apply these cutoffs.
How does BMI differ for children vs. adults?
For children and adolescents aged 2–19, raw BMI values are not used. Instead, BMI-for-age percentiles based on the CDC 2000 Growth Charts are used. A child at the 85th percentile or higher is considered overweight; 95th percentile or higher is obese. This calculator automatically switches to children's mode when age is under 20 and computes the CDC BMI percentile using the LMS method (Box-Cox transformation).
What body shape is healthiest?
Research consistently shows that "pear-shaped" fat distribution (hips/thighs) is metabolically safer than "apple-shaped" (abdomen/visceral). Visceral fat releases inflammatory cytokines and free fatty acids directly into the portal circulation. Waist-to-hip ratio (WHR) ≥0.90 in men and ≥0.85 in women indicates high risk per WHO guidelines. However, reducing total body fat and maintaining muscle mass is more actionable than targeting shape.
How does muscle mass affect BMI?
BMI cannot distinguish muscle from fat. A person with 200 lbs of weight — 180 lbs muscle vs. 120 lbs muscle — has the same BMI but very different health profiles. This is called the "athlete paradox." For muscular individuals, body fat % metrics (Navy Method, RFM, DEXA) and waist-based metrics (WHtR, ABSI) are far more useful than BMI. Enable the Athlete toggle to see adjusted risk interpretations.
What is BRI (Body Roundness Index)?
BRI (Thomas et al. 2013) models the human body as an ellipse and calculates a "roundness" score from 1–15. BRI = 364.2 − 365.5 × √(1 − ((waist/2π)/(0.5 × height))²). A BRI under 2 indicates a very lean body; 2–4 is healthy; 4–6 suggests excess body fat; above 6 indicates high adiposity. BRI correlates strongly with visceral fat and cardiovascular risk.
How accurate are BMI-based body fat % estimates?
The Deurenberg equation (used in this calculator's primary BF% estimate) has a standard error of ~3–4%. The Navy Method has ~3.5% error vs. hydrostatic weighing. RFM has ~4% error vs. DEXA. All circumference-based methods outperform BMI alone but are less accurate than DEXA, hydrostatic weighing, or BOD POD. For most practical purposes, these estimates are sufficient for tracking trends over time.
Is BMI valid for older adults?
BMI is less accurate for older adults (65+) because body composition changes with age — muscle mass decreases (sarcopenia) while fat mass often increases, even at the same weight. A "normal" BMI in an older adult may mask sarcopenic obesity (low muscle + high fat). Waist circumference and body fat % are more informative for this group. Some evidence suggests a BMI of 25–27 may be optimal for longevity in adults over 65.
BMI — Body Mass Index: weight(kg)/height(m)². A screening tool, not a diagnostic.
BMI Prime — Your BMI divided by 25 (the upper normal limit). Values <1 = normal range.
RFM — Relative Fat Mass: a waist+height-based body fat estimate with less muscle-mass bias than BMI.
ABSI — A Body Shape Index: waist relative to height and BMI. Predicts mortality risk beyond BMI.
BRI — Body Roundness Index: elliptic model of body shape on a 1–15 scale. <4 is healthy.
WHtR — Waist-to-Height Ratio: waist/height. Under 0.5 is healthy for most adults.
WHR — Waist-to-Hip Ratio: waist/hip. Men ≥0.90, Women ≥0.85 = high cardiovascular risk (WHO).
IBW — Ideal Body Weight: clinically-derived formulas (Devine, Robinson, Miller, Hamwi) for target weight by height.
Visceral Fat — Fat stored around internal organs. More metabolically dangerous than subcutaneous fat.
Sarcopenia — Age-related loss of muscle mass; can coexist with excess fat (sarcopenic obesity).
LMS Method — Box-Cox transformation used in CDC growth charts: Z = ((BMI/M)^L − 1) / (L×S). L=power, M=median, S=CV.
Ponderal Index — weight(kg)/height(m)³. Preferred over BMI for newborns; less sensitive to height in adults.