Source: https://guthlabs.ai/docs/instruments/bmi-calculator/mechanism/

instrument mechanism / verified-clean

# BMI Calculator mechanism

The inspectable method record for BMI Calculator, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The arithmetic formula is standard, but the instrument needs an explicit population scope and exact category mapping before release. The existing honest label is appropriate: BMI is an educational screening estimate, does not distinguish fat from muscle, and is not medical advice.

- BMI = mass in kilograms divided by height in meters squared is the standard adult BMI formula and is correct after unit conversion.

- The specification does not define the exact WHO category thresholds or whether categories apply to adults only; pediatric BMI requires age- and sex-specific percentiles rather than fixed adult bands.

- The formula field does not explicitly state the required conversion from pounds to kilograms and centimeters/inches to meters.

## Units and conventions

- Weight in kilograms is used directly; weight in pounds is converted using 1 lb = 0.45359237 kg.

- Height in centimeters is converted using 1 cm = 0.01 m; height in inches is converted using 1 in = 0.0254 m.

- Test categories assume adult WHO bands: underweight <18.5, normal 18.5-<25.0, overweight 25.0-<30.0, obesity >=30.0.

- BMI is treated as a screening index, not a direct measure of body-fat percentage or health status.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "weight": 70,
    "weight_unit": "kg",
    "height": 175,
    "height_unit": "cm"
  },
  "expected_outputs": {
    "BMI": 22.8571428571,
    "WHO_category": "Normal"
  },
  "working": "Height: 175 cm x 0.01 m/cm = 1.75 m. Height squared: 1.75 x 1.75 = 3.0625 m². BMI: 70 kg / 3.0625 m² = 22.8571428571. Since 18.5 <= 22.8571428571 < 25.0, category = Normal."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The hazard tests expose undefined behavior for zero height, a physically invalid but arithmetically defined zero-weight boundary, and a severe unit-convention ambiguity. The calculator must reject zero height and require unambiguous units; it should also validate nonpositive body measurements.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/bmi-calculator/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/bmi-calculator/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/bmi-calculator/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche2/batch10-output.jsonl`

- `BOUNDARY_HAZARD_CATALOG.json`