BMI Calculator

BMI is calculated by dividing your weight in kilograms by your height in meters squared (kg/m²). For imperial units, the formula is (weight in pounds ÷ height in inches²) × 703. Simply enter your height and weight, and our calculator does the math instantly.

The WHO BMI classification: Underweight (below 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 and above). Your BMI category helps assess potential health risks.

Based on your height, our calculator shows your healthy weight range where BMI falls between 18.5 and 24.9. This gives you a target range for weight management, whether you need to lose or gain weight.

While the BMI formula is identical for both genders, body composition differs. Women naturally have higher body fat percentages. Our calculator provides the same healthy BMI range (18.5-24.9) for both, with optional gender input for context.

BMI is a useful screening tool but doesn't measure body fat directly. It may overestimate fat in athletes and underestimate it in older adults. For a complete health picture, consider waist circumference, body fat percentage, and other health markers.

Unlike adults, children and teens ages 2–19 are assessed using age- and sex-specific BMI percentiles, not fixed number categories. A child's BMI is plotted against CDC growth charts: below the 5th percentile is underweight, 5th–84th is healthy weight, 85th–94th is overweight, and 95th or above is obese. This is because growing bodies have different body-fat profiles at each age. A BMI of 19 might be healthy for a 15-year-old but overweight for a 6-year-old. The CDC BMI Percentile Calculator for Children and Teens uses the same height and weight inputs but interprets them against percentile tables. If you're calculating BMI for a child, use the number from this calculator as input to percentile tools — and always consult a pediatrician for interpretation.

BMI and body-fat percentage answer different questions. BMI is a quick screening tool based only on height and weight — it takes 5 seconds and requires no equipment. Body-fat percentage is a direct measurement of fat mass relative to total body mass, requiring calipers, DEXA scan or bio-impedance scale. Use BMI for population-level screening, tracking trends over time, or when you don't have access to body-composition tools. Use body-fat percentage if you are a competitive athlete, heavily muscled, over 65, pregnant, or specifically working on body recomposition. BMI classifies many muscular athletes as 'overweight' when their body-fat percentage is actually low. For most adults without extreme body composition, BMI tracks body-fat percentage closely enough to be useful as a screening tool.

BMI is a screening tool designed for the general adult population and is less accurate for specific groups. Competitive athletes and bodybuilders often have BMI above 25 from muscle mass, not fat — use body-fat measurement instead. Adults over 65 lose muscle mass while retaining fat, so their BMI may read normal even when body-fat is elevated; waist circumference is a better marker. Pregnant and breastfeeding women should use pre-pregnancy BMI as reference, not current weight. People with amputations and those at very tall or very short stature (below 5'0 or above 6'4) may need adjusted interpretation. If any of these apply to you, treat BMI as one data point alongside waist circumference, body-fat percentage, and clinical guidance from your healthcare provider.

The BMI formula is identical for women and men, but women's bodies go through phases where interpretation needs nuance. During pregnancy, do not use current BMI — your healthcare provider tracks weight gain against pre-pregnancy BMI using Institute of Medicine (US) or NHS (UK) guidelines. After childbirth, allow 6–12 months for weight normalization before using BMI as a benchmark. At menopause and beyond, many women gain 5–10 lbs (2–5 kg) due to hormonal shifts and muscle loss, pushing BMI up even without lifestyle changes; waist circumference often captures this risk more accurately than BMI alone. Women with polycystic ovary syndrome (PCOS) frequently have BMI in the overweight range with insulin resistance — a healthy BMI for a woman with PCOS is best set with her doctor. In short: BMI 18.5–24.9 is the same target for women, but context (life stage, waist measurement, hormonal status) should shape how you act on the number.

Waist-to-Height Ratio (WHtR) is increasingly recommended alongside BMI by the UK NHS (2023 guideline update), Lancet Diabetes & Endocrinology, and several US cardiology bodies. The formula is simple: waist circumference ÷ height (same units). A WHtR under 0.5 is considered healthy for most adults regardless of sex — the 'keep your waist to less than half your height' rule. WHtR captures abdominal fat — the visceral fat that drives cardiovascular and metabolic disease — far better than BMI, which can't distinguish muscle from fat or tell fat distribution. Use WHtR with BMI: a 'normal' BMI with WHtR above 0.5 still carries elevated metabolic risk (sometimes called 'skinny fat'), and a high BMI with WHtR below 0.5 is usually muscle, not health risk. To measure waist correctly: stand, exhale normally, measure at the midpoint between the lowest rib and the top of the hip bone, keeping the tape horizontal and snug but not compressing skin.

Our calculator outputs two supplementary metrics alongside your BMI. BMI Prime is your BMI divided by 25 (the upper threshold of the healthy range). A BMI Prime of 1.0 means you're exactly at the overweight borderline; 0.8 means you're at the bottom of the healthy range; 1.2 means you're 20% above the overweight cutoff. It makes it easy to see how far above or below the healthy-weight ceiling you are at a glance. Ponderal Index (weight ÷ height³) is an alternative to BMI that accounts better for height — particularly useful for very tall or very short adults whom BMI can misclassify. A typical healthy Ponderal Index is 11–15 kg/m³. Both metrics are informational only; BMI remains the primary screening tool used by the CDC, NHS, Health Canada, and Heart Foundation Australia.

Understanding obesity prevalence helps contextualize your BMI. In the United States, about 42% of adults have obesity (BMI 30+) per CDC NHANES data. In the United Kingdom, roughly 26% of adults have obesity and another 38% are overweight per NHS Digital. Canada reports around 27% adult obesity per Statistics Canada 2022. Australia has about 32% adult obesity per Australian Institute of Health and Welfare. Beyond national averages, your individual risk depends on BMI category, waist circumference, family history, and lifestyle. A BMI in the overweight range (25–29.9) isn't uncommon — more than a third of adults in each of these countries fall in this band — but it is associated with gradually rising risk of type 2 diabetes, cardiovascular disease, and certain cancers. Small, sustained changes (0.5–1 lb / 0.25–0.5 kg weight loss per week) moving toward a healthy BMI produce meaningful health benefits.