Body Mass Index (BMI) Calculator

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BMI range	Category	Copy
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Body Mass Index estimates relative weight for height and is commonly used to screen broad weight categories. It relies on mass and stature alone, so treat it as a screening cue instead of a diagnosis. These indices are simple snapshots that help track trends over time rather than precise measures of health.

Enter weight and height in either metric or imperial units to see BMI, its category band, BMI Prime, and a healthy weight range for your height. Select the classification preset that fits your population and use the target BMI slider to map a personalised goal weight. Results update instantly as you adjust units or values.

Treat boundaries with caution. A small change near a threshold can flip the label without reflecting a meaningful difference. Age, sex, ethnicity, pregnancy, and athletic body composition can shift interpretation.

This tool provides informational estimates and does not substitute professional advice.

Technical Details

Body Mass Index (BMI) is defined as mass divided by height squared. BMI Prime expresses BMI relative to the upper bound of the healthy band (25 kg/m²), and a healthy weight range derives from 18.5–24.9 kg/m² at your height. The tool includes WHO, Asia-Pacific, and NIH/CDC adult reference tables so you can compare interpretations across regions.

\begin{matrix} BMI & = & \frac{m_{kg}}{{h_{m}}^{2}} \\ BMI Prime & = & \frac{BMI}{25} \end{matrix}

Symbols and units
Symbol	Meaning	Unit/Datatype	Source
m	Body mass	kg	Input (kg or lb → kg)
h	Stature	m	Input (cm/m/in → m)
BMI	Body Mass Index	kg/m²	Derived
BMI′	BMI Prime (BMI/25)	unitless	Derived

Worked example

Weight 70 kg, height 1.70 m.

\begin{matrix} BMI & = & \frac{70}{{(1.70)}^{2}} \approx 24.2 \\ Prime & = & \frac{24.2}{25} \approx 0.968 \end{matrix}

Interpretation: BMI in the “Normal” band, BMI Prime near 0.97 indicates you are slightly below the international threshold of 25.

BMI bands (adults)

BMI categories
Category	Lower	Upper	Implication
Severely Underweight	0	16.49	Clinical concern; seek guidance.
Underweight	16.5	18.49	Below recommended range.
Normal	18.5	24.9	Typical healthy band.
Overweight	25	29.9	Elevated risk indicator.
Obese I	30	34.9	Increased clinical risk.
Obese II	35	39.9	High clinical risk.
Obese III	40	100	Very high clinical risk.

Units, precision, and rounding

Weight: kg or lb; height: cm, m, or in.
Internal math converts to kg and m; displayed BMI uses one decimal and BMI Prime three decimals.
Healthy weight range uses 18.5–24.9 kg/m² with one‑decimal rounding.

Input & validation

Validation rules
Field	Type	Accepted values	Notes	Placeholder
weight	number	> 0	kg or lb; converts to kg	70
height	number	> 0	cm/m/in; converts to m	170
age	number	0–120	Optional; not used in math	—
sex	enum	male/female	Optional; not used in math	—

I/O formats

Inputs and outputs
Input	Accepted Families	Output	Encoding/Precision
Weight, height	Numbers with units	BMI, BMI Prime	1–3 decimals as noted
—	—	Healthy weight range	One decimal; kg and lb
—	—	CSV/JSON exports	Plain CSV; UTF‑8 JSON

Privacy & compliance

Calculations run in your browser; no data is sent to a server. These outputs are estimates and not medical advice.

How‑to Guide

Enter Weight and pick kg or lb.
Enter Height and pick cm, m, or in.
Select a BMI classification preset or leave the WHO default.
Adjust the Target BMI slider to preview an exact goal weight.
Review BMI, category, BMI Prime, and healthy range; copy or download if needed.

Example: 70 kg, 170 cm → BMI ≈ 24.2 (Normal), Prime ≈ 0.968.

Assumptions & limitations

Indices screen broad categories; they do not diagnose health.
Muscular, pregnant, growing, or older individuals may not fit BMI bands well.
Rounding at one decimal can shift labels at boundaries.

Edge cases & error sources

Zero or negative height/weight suppresses results.
Mixed units without switching selectors yield incorrect values.
Very small heights or extreme weights can exceed typical adult ranges.
Locale formatting affects displayed separators; exports keep plain ASCII digits.

FAQ

Is my data stored?

No. Inputs and outputs stay on your device; exports are created locally.

Can I use pounds and inches?

Yes. The app converts lb→kg and in→m before calculation; results display consistently.

Which number should I watch?

Track BMI trend over time and watch BMI Prime or the healthy range delta to see how far you sit from preferred bands. Use the healthy range as a planning guide.

Does BMI apply to athletes?

BMI can misclassify very muscular people; pair it with performance markers and body composition measures when available.