BMI Calculator
A body mass index calculator that accepts weight and height in either metric or imperial units and returns BMI plus the WHO weight category. The tool also computes the healthy weight band for the entered height, so the number you get is paired with the range it came from. Everything runs in this browser tab — no weight, no height, and no result is uploaded anywhere.
Use this without the search next time. Prathom Workbench puts Prathom's tools in your toolbar.
Add to Chrome — freeWhat it does
- Metric and imperial unit input
- WHO BMI category lookup
- Healthy weight range for the entered height
- One-decimal precision without rounding surprises
- Browser-local calculation, nothing uploaded
How to use BMI Calculator
- 1
Choose your unit system
Pick metric or imperial. Metric takes weight in kilograms and height in centimeters. Imperial takes weight in pounds and height in feet and inches, which means you can enter five feet nine inches without converting either value first. The result is the same in either mode.
- 2
Enter weight and height
Type your weight and your height. Heights beyond three meters or weights beyond seven hundred kilograms are rejected rather than producing an absurd BMI. Negative or zero entries are ignored because the formula is undefined on them.
- 3
Read the BMI and category
The main stat is your BMI to one decimal place. The category line is the WHO band that contains the value: underweight, healthy weight, overweight, or one of three obesity classes. The healthy weight range given below is the band for the height you entered, not a generic chart.
- 4
Copy the result
Press Copy result to put the BMI value, category, and healthy weight band on the clipboard. The Clear button removes the entries so you can run the calculation for someone else without first deleting fields one at a time.
How it works
The BMI is weight / height², with weight in kilograms and height in meters.
Two unit conversions are done before the formula runs: pounds to kilograms via
the exact factor 0.45359237, and feet-plus-inches to meters via 2.54
centimeters per inch. Once the inputs are in SI units, the formula is one
division away and the result is formatted to one decimal place.
The category lookup walks an ordered list of thresholds. Each entry is a
(threshold, label) pair; the first threshold larger than the BMI is the band
it belongs to. The five bands are the WHO cut-offs — underweight below 18.5,
healthy from 18.5 to 24.9, overweight from 25 to 29.9, and three obesity
classes at 30, 35, and 40 — so the categorization matches the definitions used
by clinical references rather than an arbitrary house scale.
The healthy weight range is computed in reverse from the same formula: 18.5 × height² and 24.9 × height² give the weights that would land on each end of the healthy band for the entered height. This is why the range is specific to the height you typed — a person who is 160 cm tall carries a different band than one who is 190 cm, and a generic fifty-to-eighty kilogram chart figure is too coarse to be useful.
What BMI does and does not measure
BMI measures mass relative to height. It does not measure body composition, distribution of fat, bone density, lean mass, frame size, age, sex, or cardiovascular fitness. The formula's strength is its simplicity: every adult with a scale and a tape measure can compute it, which is why it is the standard population-level metric for weight status reporting.
Its weakness is exactly the same simplicity. Two adults with identical BMI can have very different body compositions, and clinical decisions based on BMI alone — particularly in borderline cases or across populations where the formula is known to mis-estimate — require a clinician's interpretation. This tool computes the number accurately, but it cannot measure the context that gives the number its meaning.
Why the conversion factors are exact
The international yard-and-pound agreement of 1959 defined one pound as exactly 0.45359237 kilograms and one inch as exactly 2.54 centimeters. These are not approximations; they are the definitions by which the units exist in common use. The conversions this tool performs are therefore mathematically exact, and any small disagreement between metric and imperial entries comes from the rounding in the input, not from the conversion.
Privacy from the formula up
Weight and height are personal data, and a calculator that sends them anywhere is asking for information it does not need. This tool does not make a network request, does not retain the value after the page closes, and does not write anything to disk. The privacy claim is verifiable: open your browser's Network tab, change an input, and observe that no request is made. The calculation is honest about being nine lines of arithmetic you can audit without sending the inputs anywhere.
Examples
A height inside the healthy band
A common adult entry producing a BMI comfortably inside the 18.5–24.9 band. The healthy weight range is calculated for 175 cm specifically, which is why it differs from a generic 50–80 kg chart figure.
The same person, entered in imperial
The small difference from the metric example comes from the impounded precision of 154 lb rather than 70 kg. Both entries are physically the same person and round to the same category.
Frequently asked questions
What formula does this BMI calculator use?
The standard one defined by the World Health Organization: weight in kilograms divided by height in meters squared. Imperial inputs are converted using exact factors — one pound is exactly 0.45359237 kilograms and one inch is exactly 2.54 centimeters — before the formula is applied. The category bands are the WHO cut-offs: underweight below 18.5, healthy 18.5 to 24.9, overweight 25 to 29.9, and obesity at 30 and above, divided into class one through three at 30, 35, and 40.
Why does BMI categorize people inaccurately?
BMI cannot tell the difference between muscle and fat, and it does not account for body shape, age, sex, or bone density. An athlete with high lean mass can register as overweight; an older adult can register as healthy while having low muscle. The number is intentionally simple so it can be measured at population scale, which is also the reason it is a weak individual health indicator. Treat the category as a screening signal, not a diagnosis, and discuss any clinical question with a qualified practitioner.
Are the metric and imperial results guaranteed identical?
Mathematically yes, when the inputs describe exactly the same body. In practice small differences appear because imperial inputs are typically rounded before entry — 154 lb is not exactly 70 kg, and 5 foot 9 inches is not exactly 175 cm. The conversion factors themselves are exact, defined by international agreement in 1959, so once the inputs are expressed the underlying meters and kilograms produce the same BMI to irrelevant floating-point rounding.
Is BMI suitable for children, athletes, or pregnant people?
Not as an isolated number. For children under eighteen, BMI is plotted on age-and-sex percentile charts rather than the adult bands, so the category line on this page is not meaningful for them. For very lean adults the formula overestimates body fat proportion, and during pregnancy the tool is simply not the right measurement. The healthy weight range output is therefore most useful for average-build adults, and least useful exactly where the formula's simplifications break down.