BMI calculator
Body Mass Index and category.
How to use the BMI calculator
- Pick your unit system: Metric (centimetres and kilograms) or Imperial (feet + inches and pounds). The form swaps instantly and the weight label switches between kg and lb.
- Enter your height — one box for centimetres, or two boxes in imperial: feet, then the remaining inches (5′7″ is
5and7). - Enter your weight in the unit shown.
- Read the BMI, rounded to one decimal, and its category: Underweight (below 18.5), Normal (18.5–24.9), Overweight (25–29.9) or Obese (30+).
- There's no button — results recalculate on every keystroke, so nudge the weight up or down to see where the category boundaries sit for your height.
Common uses
- Get a quick screening check of where you sit in the standard adult BMI bands before a doctor's visit or a training program.
- Fill in the BMI field on insurance, gym and health questionnaires without hunting for the formula.
- Track direction over time — recompute monthly with the same scale and time of day and watch the trend, not single readings.
- Find your personal boundary weights: hold height steady and adjust weight until the category flips, to see in kg or lb what the 18.5 and 25 cut-offs mean for you.
- Switch someone else's figures between metric and imperial on the spot — no separate conversion step.
Tips & limitations
- BMI is a screening number, not a diagnosis. It compares weight to height only and can't tell muscle from fat — muscular people often read Overweight, and a "Normal" BMI doesn't guarantee healthy body composition.
- The adult cut-offs here don't apply to children and teens, who are assessed with age- and sex-specific percentile charts this tool doesn't include; BMI is also less meaningful in pregnancy and old age.
- Metric and imperial can disagree in the last decimal: the imperial formula uses the conventional rounded factor
703, so equivalent measurements may differ by about 0.1. - Some health authorities use lower thresholds for certain populations (parts of Asia treat 23 as overweight); this tool shows only the standard international bands.
- Height is squared in the formula, so it's the biggest error source — measure without shoes, and enter inches as the remainder past whole feet.
How it's built & why it's safe
This is the plain BMI formula computed in your browser as you type: weight ÷ height² in metric (kilograms and metres, your centimetres divided by 100), or 703 × pounds ÷ inches² in imperial — 703 being the standard factor that maps pounds and inches onto the metric definition. The category is a four-band lookup at 18.5, 25 and 30. There's no server, no account and no storage: your height and weight are never saved or sent anywhere, which matters for health data. It's a rough screening number, not medical advice.
Related tools: Unit Converter · Age Calculator · Percentage Calculator
Further reading: Why Toolkit runs entirely in your browser (and why that matters)
Frequently asked questions
What formula does this calculator use?
The standard Body Mass Index definition: weight in kilograms divided by height in metres squared. In imperial mode it uses the equivalent 703 x pounds / inches squared, the same factor the CDC uses. The result is shown to one decimal place.
What do the categories mean?
They're the standard adult bands: below 18.5 is Underweight, 18.5 to 24.9 Normal, 25 to 29.9 Overweight, and 30 or above Obese. The boundaries are population-level screening thresholds, not personal targets.
I lift weights — why does it say I'm overweight?
BMI only compares weight to height; it can't see how much of that weight is muscle. Muscle is denser than fat, so athletic people often land in the Overweight band while perfectly healthy — a known limitation of BMI itself, not a bug.
Does it work for children?
No. The adult cut-offs don't apply to anyone still growing — children and teenagers are assessed with BMI-for-age percentile charts that account for age and sex. Ask a pediatrician or use a dedicated child percentile tool.
Why do metric and imperial give slightly different results?
The imperial 703 factor is a rounded convention (the exact conversion is about 703.07), so the two modes can differ by roughly 0.1 in the final digit. Both are correct to the precision BMI deserves.
Is my height and weight stored anywhere?
No. The calculation happens in your browser on each keystroke, nothing is uploaded or written to storage, and reloading the page resets the fields to their defaults.
What's a healthy BMI to aim for?
The Normal band runs 18.5 to 24.9, but a healthy weight is individual — body composition, age, ethnicity and medical history all matter. Use the number as a rough gauge and talk to a clinician before acting on it.