BMI Calculator
Calculate your Body Mass Index (BMI) and determine your weight category (Metric & Imperial).
Body Mass Index (BMI) is a single number that relates weight to height. It was devised in the 1830s by Belgian mathematician Adolphe Quetelet and adopted by public-health researchers in the 20th century because it is easy to compute from two commonly recorded measurements, and because at the population level it correlates reasonably well with body fat and health risk. This calculator computes BMI from your inputs, places it in the standard WHO category, and — equally important — lays out where BMI is a genuinely useful screening tool and where it is not.
The formula
- Metric:
BMI = weight (kg) / height (m)² - Imperial:
BMI = 703 × weight (lb) / height (in)²
The 703 in the imperial formula is a conversion factor so the result lands on the same scale as the metric version. The calculator handles both — enter your values in either system and switch units at any time.
WHO categories (adults, 20+)
| BMI | Category |
|---|---|
| < 16.0 | Severely underweight |
| 16.0–18.4 | Underweight |
| 18.5–24.9 | Normal weight |
| 25.0–29.9 | Overweight |
| 30.0–34.9 | Obesity class I |
| 35.0–39.9 | Obesity class II |
| ≥ 40.0 | Obesity class III |
Categories shift for children and teenagers (use age- and sex-specific percentile charts) and are adjusted in some regions: Asian-Pacific guidelines (WHO Asia consultation) typically lower the “overweight” threshold to 23.0 and “obesity” to 27.5 because body-fat percentage at a given BMI is higher in many East and South Asian populations.
Worked example
A 175 cm (1.75 m) person weighing 78 kg:
BMI = 78 / (1.75)² = 78 / 3.0625 ≈ 25.5
That lands in the “Overweight” band by the WHO scale, but just barely. Without more context (muscle mass, waist circumference, activity level), the number alone is not a medical verdict.
What BMI actually measures
BMI correlates with total body mass normalized for height. For most sedentary adults between 20 and 65, that correlation tracks body-fat percentage closely enough to be a useful screening signal: populations with average BMI ≥ 30 have measurably higher rates of type 2 diabetes, cardiovascular disease, and some cancers. Insurers, public-health agencies, and primary-care physicians use it because it is cheap to collect and broadly informative.
What BMI does not measure:
- Body fat directly. It cannot distinguish muscle from fat.
- Fat distribution. Visceral fat (around the organs) is much more dangerous than subcutaneous fat (under the skin). BMI treats both the same.
- Bone density or frame size. A person with a naturally larger skeleton can carry more weight healthily.
- Fitness. A sedentary person and an elite cyclist at the same weight and height get the same BMI.
- Age-related changes. Older adults lose muscle and often gain fat without changing weight much; BMI misses this shift.
When BMI misleads
- Muscular athletes. Rugby forwards, bodybuilders, powerlifters, and even serious recreational lifters often land in the “Overweight” or “Obesity I” bands despite low body fat. For this group BMI is essentially useless.
- Older adults. Sarcopenia (age-related muscle loss) can keep BMI in the “Normal” range while body-fat percentage quietly rises.
- Children. BMI for children is interpreted against percentile charts, not the adult cutoffs.
- Pregnant and postpartum women. BMI is not meaningful; clinicians use different measures.
- People with amputations or unusual body proportions. The formula breaks down when the proportions diverge from what Quetelet measured in the 1830s.
Better companion measurements
BMI is cheap and quick, but if you want a fuller health picture, add one or more of:
- Waist circumference. Men: < 94 cm low risk, 94–102 cm elevated, ≥ 102 cm high. Women: < 80 cm, 80–88 cm, ≥ 88 cm. Waist alone predicts cardiometabolic risk better than BMI for most adults.
- Waist-to-height ratio. Should ideally stay below 0.5 (“keep your waist to less than half your height”).
- Body-fat percentage. Measured via skinfold calipers, bioimpedance scales, or DEXA scans. More accurate than BMI but more expensive and trickier to do consistently.
- Resting heart rate and VO₂ max. Cardiovascular fitness often matters more than body composition for long-term health outcomes.
How to use this tool
- Choose your unit system (metric or imperial).
- Enter height and weight.
- Read the BMI value and category band.
- Remember that the number is a starting point, not a diagnosis.
Frequently asked questions
Is a “Normal” BMI always healthy?
Not necessarily. Someone with very low muscle mass and high body fat (“skinny fat”) can land in the normal band while carrying unhealthy metabolic indicators. Conversely, someone in the “Overweight” band with lots of muscle and active cardiovascular fitness may be perfectly healthy. BMI is one signal among several.
Why does BMI use height squared instead of cubed?
Because Quetelet observed in 1830s Belgian data that weight scaled roughly with height² rather than height³ across adults, presumably because taller people are not uniformly “scaled up” in every dimension. Modern analyses sometimes prefer the Ponderal Index (weight / height³) for very tall or very short individuals where BMI’s scaling breaks down.
What should I do if my BMI is outside the “Normal” range?
Talk to a doctor or registered dietitian. BMI is a screening flag, not a treatment plan — the underlying question is whether your body composition, blood markers, and activity level indicate risk, and those are medical conversations.
Are there BMI alternatives specific to athletes?
Body-fat percentage via skinfold calipers or DEXA is standard in athletic training contexts. Some sports use sport-specific norms rather than general-population data.
Does BMI work the same for men and women?
The formula is identical, but average body-fat percentage at a given BMI differs by sex — women typically have higher body-fat percentage than men at the same BMI. The WHO thresholds do not adjust for this; more refined assessments do.
Privacy note
Your height and weight are entered and calculated entirely in your browser. No values leave your device.
Disclaimer: This calculator is for informational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider for health decisions.