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BMI Calculator — Body Mass Index for Adults

Calculate your Body Mass Index (BMI) in metric or imperial units, see your category (Underweight / Normal / Overweight / Obese), and find your healthy weight range. WHO thresholds, in-browser, no signup.

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How to use

  1. Step 1: Choose metric (cm/kg) or imperial (ft/in/lbs)
  2. Step 2: Enter your height
  3. Step 3: Enter your weight
  4. Step 4: Optionally enter age and sex for context
  5. Step 5: Read your BMI, category, and healthy weight range

How BMI works

BMI is calculated using the formula BMI = weight (kg) / [height (m)]². For imperial units, the equivalent is BMI = (weight (lbs) / [height (in)]²) × 703. Either way, the result is a single number that places adults aged 20+ into one of four categories defined by the World Health Organization (WHO).

  • Below 18.5 — Underweight
  • 18.5 – 24.9 — Normal weight
  • 25.0 – 29.9 — Overweight
  • 30.0 and above — Obese (Class I / II / III)

These ranges were defined for population-level public-health analysis, not individual diagnosis. They were developed in the 19th century by Belgian statistician Adolphe Quetelet (originally called the Quetelet Index), and adopted by WHO in 1995 as a rough screening proxy for obesity risk.

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Examples

Example 1 — 175 cm, 70 kg
Height1.75 m
Weight70 kg
BMI70 / 1.75² = 22.86
CategoryNormal
Example 2 — 5'10" (178 cm), 200 lbs (90.7 kg)
Height1.78 m
Weight90.7 kg
BMI90.7 / 1.78² = 28.62
CategoryOverweight
Example 3 — 160 cm, 50 kg
Height1.60 m
Weight50 kg
BMI50 / 1.60² = 19.53
CategoryNormal

Frequently asked questions

A BMI (Body Mass Index) calculator is a tool that uses your height and weight to estimate whether your body weight falls in a healthy range. The result is a single number plus a category (underweight, normal, overweight, or obese) based on World Health Organization (WHO) thresholds.

Why use a BMI calculator?

BMI is the single most-used screening number in clinical and public-health practice. People use it to:

  • Get a quick first-pass read on body weight relative to height
  • Track trends over time as part of a weight-loss or weight-gain plan
  • Determine eligibility for certain medications and surgeries (bariatric surgery typically requires BMI ≥ 40, or ≥ 35 with comorbidities)
  • Compare against population norms in epidemiology and insurance underwriting

BMI limitations & alternatives

BMI does not measure body composition — only the relationship between height and weight. Its blind spots:

  • Athletes and bodybuilders often score in the "overweight" or "obese" range despite low body fat, because muscle is denser than fat.
  • Older adults can lose muscle mass while gaining fat ("sarcopenic obesity") without BMI changing much.
  • Ethnic differences exist — South Asian populations face higher metabolic risk at lower BMI thresholds; some health authorities use BMI ≥ 23 as overweight for South Asians.
  • Pregnant women should not use standard BMI categories.
  • Children and teens need BMI-for-age percentile charts, not adult thresholds.

Better complements: waist circumference, waist-to-hip ratio, body-fat percentage (skinfolds, BIA, DEXA), and metabolic markers (fasting glucose, lipids, blood pressure).

Common mistakes & tips

  • Mixing units. Don't enter height in cm with weight in lbs — use the toggle.
  • Treating BMI as a diagnosis. It's a screening tool. Always pair with a physician's assessment.
  • Comparing to "ideal" celebrity BMIs. Healthy ranges are wide for a reason — anywhere from 18.5 to 24.9 is normal.
  • Daily weighing. Weight fluctuates 1-2 kg day-to-day. Track weekly averages instead.

⚠️ Disclaimer: BMI is a general indicator and not a substitute for medical advice. Consult a healthcare professional for diagnosis and treatment.