BMI Calculator
FAQs
- What is BMI?
- Body Mass Index (BMI) is a value derived from your weight and height — specifically weight in kilograms divided by the square of height in metres (kg/m²). It is a screening tool, not a diagnosis.
- How is BMI calculated?
- BMI = weight (kg) ÷ height (m)². For imperial inputs, the calculator first converts pounds and inches to kilograms and metres before applying the same formula.
- What are the WHO BMI categories for adults?
- Under 18.5: Underweight. 18.5–24.9: Normal weight. 25.0–29.9: Overweight. 30.0–34.9: Obese class I. 35.0–39.9: Obese class II. 40.0 and above: Obese class III.
- Is BMI accurate for everyone?
- No. BMI does not distinguish between muscle and fat, so it can misclassify athletes, pregnant or breastfeeding people, older adults, and growing children. Some population groups also have different healthy ranges. Use BMI as a starting point, not a verdict.
- What is a healthy BMI?
- For most adults aged 18–64, the WHO considers a BMI between 18.5 and 24.9 the healthy range. Your doctor may use a different range depending on age, ethnicity, and medical history.
- Should I use metric or imperial units?
- Either is fine — the calculator converts internally to kilograms and metres before computing BMI. Pick whichever you can measure most accurately.
What is BMI?
Body Mass Index (BMI) is a value derived from your weight and height. It is calculated as weight in kilograms divided by the square of height in metres (kg/m²). The World Health Organization uses BMI as a population-level screening tool to flag whether an adult may be underweight, in a healthy weight range, overweight, or obese for their height.
BMI is a starting point, not a diagnosis. Muscle mass, age, ethnicity, and pregnancy can all shift what a healthy number looks like for you.
BMI Formula
BMI = weight (kg) ÷ height (m)²
For imperial inputs, this calculator first converts pounds and inches to kilograms and metres, then applies the same formula. The result is unitless (kg/m²) and is identical regardless of which input units you used.
Worked example
An adult who weighs 70 kg and is 1.75 m (175 cm) tall has a BMI of:
70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.86 kg/m²
That falls in the Normal weight range (18.5 – 24.9).
WHO BMI Categories for Adults
| BMI (kg/m²) | Category |
|---|---|
| Below 16.0 | Underweight (severe) |
| 16.0 – 16.9 | Underweight (moderate) |
| 17.0 – 18.4 | Underweight (mild) |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight (pre-obese) |
| 30.0 – 34.9 | Obese (class I) |
| 35.0 – 39.9 | Obese (class II) |
| 40.0 or above | Obese (class III) |
How to Use This Calculator
- Enter your weight and pick the matching unit (kg, g, lb, oz, or stone).
- Enter your height and pick its unit (m, cm, mm, ft, or in).
- Your BMI value and matching WHO category appear instantly below the inputs.
Limitations of BMI
BMI is widely used because it is cheap to measure and reproducible. It has known limitations:
- Muscle vs. fat: athletes and people with high muscle mass can score as overweight or obese despite being lean.
- Age: body composition shifts with age; older adults may carry more body fat at the same BMI.
- Ethnicity: some populations have a higher health risk at lower BMI cutoffs.
- Pregnancy and lactation: BMI is not validated during pregnancy or while breastfeeding.
- Children and teens: use age- and sex-specific BMI percentile charts, not these adult cutoffs.
For an individual health assessment, consult a healthcare professional who can also weigh waist circumference, body composition, fitness level, and family history.
Sources & methodology
This calculator applies the standard Body Mass Index formula — weight in kilograms divided by height in metres squared (kg/m²), historically known as the Quetelet index — and classifies the result against the World Health Organization adult BMI cut-offs.
- World Health Organization, Obesity and overweight (fact sheet) — defines BMI and the ≥25 (overweight) and ≥30 (obese) thresholds.
- World Health Organization, Body mass index (BMI) classification — the full underweight-to-obese category bands used above.
BMI is a screening tool, not a diagnosis. For an individual assessment, consult a qualified healthcare professional.