BMI Calculator

BMI: what the number means

Body Mass Index (BMI) is a single number — weight in kilograms divided by the square of height in metres — used by the World Health Organization and many health systems as a quick screen for body-weight categories. It's not diagnostic; it's a flag. The classic adult thresholds are: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and over obese (split into Class I/II/III at 35 and 40). This tool computes the value and the category from height and weight in metric or imperial.

The formula behind the score

• Metric: BMI = kg ÷ (m × m). 70 kg at 1.75 m → 70 / 3.0625 = 22.9.
• Imperial: BMI = (lb × 703) ÷ (in × in). The tool converts to metric internally for accuracy.
• The WHO categories are the same regardless of unit system — BMI itself is unitless.

Self-checks, forms, and trend tracking

• Quick self-check or to fill in a form that asks for it (insurance, fitness apps, gym intake).
• Comparing a value across populations or studies.
• Tracking direction of change over time (rising, stable, falling) — the trend is more useful than any single reading.

Where BMI falls short

• BMI does not measure body composition. Muscle weighs more than fat, so a fit, muscular person can score "overweight" while having low body-fat. Conversely, a low-muscle person can score "normal" while being unhealthy ("skinny fat").
• It's an adult metric. For children and adolescents (under 18) use the age- and sex-specific BMI percentile charts instead.
• Pregnancy isn't supported. BMI doesn't apply during pregnancy; talk to your healthcare provider.
• Ethnicity matters. Several health bodies (NHS, WHO Asia-Pacific guidance) use lower thresholds (overweight ≥23, obese ≥27.5) for South Asian, Chinese, and other groups, because cardiovascular risk rises at lower BMIs.
• Tall vs short people. The squared-height formula systematically over-classifies tall people as "underweight" and short people as "overweight" — alternative formulas (Trefethen's BMI uses height^2.5) try to correct for this.
• Not medical advice. If you're concerned about your weight, talk to a clinician. They have the rest of the picture (waist circumference, blood pressure, blood-work, lifestyle) that a single number doesn't.

Population thresholds vs individual reality

• BMI was invented in the 1830s for population statistics, not individual diagnosis. Adolphe Quetelet's original Quetelet Index aimed to characterise the "average man" across a population. Its use as an individual diagnostic tool came a century later and was always controversial. For a single person, waist circumference (men >102 cm / women >88 cm indicates elevated risk) is generally a better cardiovascular signal than BMI alone.
• The "overweight paradox" is real but contested. Multiple large meta-analyses find that adults in the "overweight" BMI range (25-29.9) have slightly lower all-cause mortality than the "normal" range. Possible explanations: residual confounding (smokers and the chronically ill tend to weigh less), the population-statistics origin of the threshold, and that small reserves of fat may be protective in illness or injury. Don't interpret the BMI categories as a fitness target.
• The thresholds are not the same everywhere. WHO Asia-Pacific guidance shifts the overweight threshold to 23 and the obese threshold to 27.5 for South Asian, Chinese, and Southeast Asian populations, because cardiovascular and diabetes risk rises at lower BMIs in those groups. The US/Europe thresholds (25/30) were derived from European-descent populations and don't transfer cleanly.
• The squared-height denominator is mathematically arbitrary. Quetelet picked it because it produced numbers that varied less with height in his sample. For very tall and very short people, the formula systematically misfires — Trefethen's revision (height^2.5) is more accurate but never displaced the original because the institutional cost of switching is enormous. If you're under 165 cm or over 195 cm, treat your BMI as approximate.
• Trends matter more than absolute values. Whatever your starting BMI, a stable trend over years signals different things than a rapid change. Rapid unintended loss in adults often signals underlying disease; rapid gain often signals lifestyle or medication change. A clinician interpreting the same number alongside the trend, alongside lab work and your individual history, will reach a much better conclusion than any web calculator.

Step-through calculation

In metric mode, enter height 178 cm and weight 75 kg. The height converts to 1.78 m and BMI = 75 ÷ (1.78 × 1.78) = 23.7 — landing in the Normal weight band, with the marker sitting on the 0–40 scale and a healthy-weight range of about 58.6 – 78.9 kg shown for that height. Switch to imperial and enter 5 ft 10 in / 165 lb and you get the same figure — internally it converts inches to metres (× 0.0254) and pounds to kilograms (× 0.453592) before applying the identical formula.

The WHO categories and healthy-weight range

What are the category cut-offs? Under 18.5 is underweight, 18.5–24.9 normal, 25–29.9 overweight, and 30+ obese (split into Class I 30–34.9, II 35–39.9, III 40+). These are the standard WHO adult thresholds the tool uses verbatim.

How is the "healthy weight range" worked out? It inverts the formula for your height: the lower bound is 18.5 × height² and the upper is 24.9 × height², then converts back to your chosen units. It's the weight span that would put your BMI in the normal band.

Is BMI accurate for me personally? Treat it as a population-level screening figure, not a diagnosis. It doesn't distinguish muscle from fat, and it doesn't account for age, sex, ethnicity, body composition or pregnancy — a lean, muscular person can read "overweight". Use it as a rough flag and speak to a clinician for a real assessment.

Does it work for children? No — childhood BMI is read against age-and-sex percentile charts, not these fixed adult cut-offs. This calculator is for adults.