BMI Calculator — Free Body Mass Index, Metric & Imperial
Enter your weight and height to get your Body Mass Index using the formula BMI = weight (kg) ÷ height (m)². See your CDC/WHO category and the healthy weight range for your height. Free, no signup, and 100% client-side — your numbers never leave your browser.
BMI is a population-level screening tool, not a medical diagnosis. It does not distinguish muscle from fat and can be misleading for athletes, pregnant individuals, the elderly, very short or very tall people, and some non-European populations. Consult a qualified clinician for personalised health advice.
Metric & Imperial Toggle
Switch between kilograms / centimetres and pounds / feet+inches with one click. We convert internally so the formula is always the canonical metric form.
Full WHO Category Bands
Underweight, Healthy, Overweight, Obese I/II/III — with WHO thresholds and a clear health-risk note for each category, not just the high-level label.
Healthy Weight Range
For your entered height, we compute the weight range that would land you in the WHO healthy band (BMI 18.5–24.9). Shown in your selected unit system.
100% Client-Side
Height, weight, and BMI calculations stay entirely on your device. No upload, no logging, no analytics tied to the numbers you enter.
BMI Calculator: Body Mass Index from Weight and Height
Body Mass Index (BMI) is your weight in kilograms divided by your height in metres squared: BMI = weight (kg) ÷ height (m)². This free calculator runs that formula on your input, places the result in a CDC/WHO category, and shows the healthy weight range for your height. It accepts metric or imperial units and runs 100% in your browser — no signup, no upload.
How to calculate your BMI
- Choose your unit system: Metric (kg, cm) or Imperial (lb, ft+in). Switching units clears the fields.
- Enter your weight — kilograms in metric mode, pounds in imperial mode.
- Enter your height — centimetres in metric, or feet and inches in imperial.
- Read your BMI and category card. It updates instantly as you type; no submit button.
- Check the healthy weight range shown for your height, then read the limitations below before acting on the number.
The exact BMI formula, with every variable defined
BMI was devised by Belgian statistician Adolphe Quetelet in the 1830s as a population-level measure. The modern definition adopted by the CDC and WHO is a single ratio. This tool computes it in metric form and converts imperial input internally, so both unit modes return the same number.
Metric formula (the canonical form this tool uses)
BMI = weight (kg) ÷ height (m)²Imperial shortcut (same one-decimal result)
BMI = 703 × weight (lb) ÷ height (in)²- weight (kg) — body mass in kilograms.
- height (m) — standing height in metres (centimetres ÷ 100).
- weight (lb) — body mass in pounds.
- height (in) — standing height in inches (feet × 12 + inches).
- 703 — the unit-conversion constant that maps lb/in² onto kg/m².
Internally this calculator never uses the 703 shortcut: in imperial mode it multiplies pounds by 0.45359237 to get kilograms and inches by 2.54 to get centimetres, then applies the metric formula. The result is rounded to one decimal place for display. The category cut-points come straight from the CDC Adult BMI Categories and the WHO obesity fact sheet.
Worked examples: input → step-by-step → result
Metric · 70 kg, 175 cm
height = 175 cm = 1.75 m
BMI = 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.857…
BMI = 22.9 → Healthy (18.5–24.9)
Healthy weight range for 175 cm: 18.5 × 1.75² = 56.7 kg to 25 × 1.75² = 76.6 kg.
Imperial · 154 lb, 5 ft 9 in
height = 5×12 + 9 = 69 in = 175.26 cm = 1.7526 m
weight = 154 × 0.45359237 = 69.85 kg
BMI = 69.85 ÷ (1.7526)² = 22.74… (703 shortcut: 703 × 154 ÷ 69² = 22.74)
BMI = 22.7 → Healthy (18.5–24.9)
Edge case · muscular build
A 100 kg, 180 cm athlete computes BMI = 100 ÷ 1.8² = 30.9 → Obese Class I, even at 12% body fat. The formula has no input for muscle mass, so it misclassifies dense, lean physiques. Read the result alongside body fat percentage, not on its own.
CDC & WHO BMI Categories & Risk Bands
| BMI Range | Category | Population Risk Level |
|---|---|---|
| Below 18.5 | Underweight | Low (but malnutrition/illness risk) |
| 18.5 – 24.9 | Healthy | Lowest — baseline reference |
| 25.0 – 29.9 | Overweight | Increased |
| 30.0 – 34.9 | Obese I | High |
| 35.0 – 39.9 | Obese II | Very high |
| 40.0 and above | Obese III | Extremely high |
Risk is a population-level statistical association, not a personal prediction.
Six Situations Where BMI Misclassifies You
1. Athletes & Muscular Builds
Muscle is denser than fat. An NFL linebacker can register as obese while having sub-10% body fat. BMI cannot see body composition.
2. The Elderly
Sarcopenia (age-related muscle loss) lowers BMI even as visceral fat rises. A “healthy” BMI in an 80-year-old may mask metabolic risk.
3. South Asian & East Asian Populations
Some bodies show metabolic disease risk earlier than the European-derived thresholds suggest. The WHO supports 23 as the overweight cut-point for South Asians.
4. Pregnant or Post-Partum
BMI categories don't apply during pregnancy or in the first months post-partum. Use specialised obstetric guidelines instead.
5. Children & Adolescents
Paediatric BMI uses age-and-sex percentile charts (CDC / WHO growth standards), not the adult cut-points. Different tool.
6. Very Short or Very Tall
BMI scales with height squared, but humans don't. Tall people are systematically over-classified, short people under-classified.
Metrics That Improve on BMI for Individual Assessment
| Metric | What It Measures | How To Get It |
|---|---|---|
| Waist circumference | Abdominal fat (the dangerous kind) | Tape measure at navel level |
| Waist-to-height ratio | Abdominal fat relative to body size | Waist / height; target ≤ 0.5 |
| Body fat percentage | Actual lean vs adipose mass | DEXA scan, bioimpedance scale, calipers |
| Resting heart rate | Cardiovascular fitness | Fitness tracker, manual pulse-count |
| HbA1c / lipid panel | Metabolic disease markers | Annual physician blood test |
A modern, evidence-based annual check-up uses BMI as one of five or six inputs — not as the headline. If your BMI surprises you, ask your clinician to interpret it alongside at least waist circumference and a basic blood panel.
The nuance most BMI calculators skip: ethnicity-adjusted cut-points
The standard 25/30 thresholds were derived largely from white European adults and under-flag risk in some Asian populations. A WHO expert consultation (Lancet, 2004) found that type-2-diabetes and cardiovascular risk in many Asian groups rises at BMIs below 25, and recommended public-health trigger points at 23 (overweight) and 27.5 (obese). So a BMI of 24 reads “healthy” on the international scale but may already signal elevated risk for a South Asian adult.
BMI also has a built-in height bias: because it divides by height squared while body mass scales closer to height cubed, the formula systematically over-classifies tall people and under-classifies short people. Treat any single BMI number as a screening flag, not a measurement of body fat.
Medical disclaimer
For general informational purposes only — not medical advice. BMI is a screening estimate, not a diagnosis, and it cannot account for muscle mass, fat distribution, age, or ethnicity. Consult a qualified healthcare provider before making decisions about your health.
Last reviewed: June 2, 2026 · Cut-points verified against the CDC and WHO. Runs 100% in your browser — no uploads, nothing leaves your device.
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