Source: https://guthlabs.ai/docs/instruments/debt-to-income-ratio-calculator/mechanism/

instrument mechanism / verified-clean

# Debt-to-Income Ratio Calculator mechanism

The inspectable method record for Debt-to-Income Ratio Calculator, kept separate from implementation and qualification.

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

## Model scope

The core ratio is standard when all inputs are monthly and gross income is pre-tax. The exact ambiguity requiring review is the undefined composition of total monthly debt payments and the unspecified behavior for zero income. Retain the educational-only label and explicitly state that lender definitions, thresholds, and underwriting decisions vary.

- The ratio formula is correct for monthly debt payments divided by gross monthly income, but the stated formula omits the multiplication by 100 required when the output is labeled as a percentage: DTI%=total_monthly_debt_payments/gross_monthly_income×100.

- The specification does not define which obligations must be included in total monthly debt payments, such as housing, installment loans, credit-card minimums, leases, alimony, or other recurring obligations.

- The 36%/43% lender-threshold note is not a computed formula and must be presented as illustrative only, because applicable underwriting thresholds and definitions vary by lender and loan program.

## Units and conventions

- Every debt-payment list element is a positive monetary amount in the same currency and monthly period.

- Gross monthly income is pre-tax income, expressed in the same currency and monthly period as the debt payments.

- If income is supplied from an annual figure, it must first be divided by 12; annual income must not be divided directly into monthly debt payments.

- DTI percentage is calculated as (sum of monthly debt payments ÷ gross monthly income) × 100.

- The illustrative 36% and 43% figures are percentages, not decimal ratios, and are not universal lending rules.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "monthly_debt_payments": [
      1500,
      300,
      200
    ],
    "gross_monthly_income": 5000
  },
  "expected_outputs": {
    "total_monthly_debt_payments": 2000,
    "dti_percent": 40,
    "threshold_note": "Illustrative reference only: 36% and 43%; actual lender criteria vary."
  },
  "working": "1. Sum debt payments: 1,500 + 300 + 200 = 2,000. 2. Divide by gross monthly income: 2,000 ÷ 5,000 = 0.40. 3. Convert to a percentage: 0.40 × 100 = 40%. 4. Result: DTI = 40%."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

A zero gross monthly income produces an undefined division; an implementation that does not reject it may expose Infinity or an invalid DTI percentage.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/debt-to-income-ratio-calculator/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/debt-to-income-ratio-calculator/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/debt-to-income-ratio-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-tranche5/tranche5-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`