# Debt-to-Income Ratio Calculator

> Computes the back-end debt-to-income ratio (all monthly debt payments including housing divided by gross monthly income) and the front-end ratio (housing payment alone), and compares them with the 28/36 guideline and the 43 % qualified-mortgage benchmark.

- Calculator id: `debt-to-income` · Category: Finance (`finance`) · Tool name: `calculate_debt_to_income_ratio`
- Canonical page: https://tttkmbb.com/finance/debt-to-income · This document: https://tttkmbb.com/finance/debt-to-income.md · JSON definition: https://tttkmbb.com/finance/debt-to-income.json

## Purpose

Computes the back-end debt-to-income ratio (all monthly debt payments including housing divided by gross monthly income) and the front-end ratio (housing payment alone), and compares them with the 28/36 guideline and the 43 % qualified-mortgage benchmark.

**Use when:** You want to check whether a proposed mortgage or other loan fits typical lender DTI limits, or how much monthly debt capacity remains.

**Do not use when:** You need to know how long it takes to pay a balance off (use debt-payoff), or the amount of a down payment (use down-payment).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `gross_monthly_income` | number |  | required | Total pre-tax income per month (salary, bonuses, other verifiable income). (> 0, max 100000000) |
| `monthly_debts` | number_list |  | optional | Minimum monthly payments on non-housing debts: car loans, student loans, credit-card minimums, alimony. Omit if you use other_monthly_debt. |
| `other_monthly_debt` | number |  | optional, default 0 | Additional non-housing debt payments not itemised in monthly_debts (or the whole total if you have it). (min 0, max 100000000) |
| `housing_payment` | number |  | optional | Proposed or current monthly housing cost: rent, or mortgage principal, interest, taxes, insurance and HOA (PITI). Enables the front-end ratio. (min 0, max 100000000) |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `total_monthly_debt_payments` | number |  | Sum of monthly_debts + other_monthly_debt + housing_payment. |
| `back_end_dti_percent` | number | % | total_monthly_debt_payments / gross_monthly_income × 100. |
| `front_end_dti_percent` | number | % | housing_payment / gross_monthly_income × 100 (only when housing_payment is given). |
| `assessment` | string |  | Band of the back-end ratio: Good (≤ 36 %), Acceptable (36–43 %), High (43–50 %) or Very high (> 50 %). |
| `max_housing_payment_at_28_percent` | number |  | 0.28 × gross_monthly_income (front-end guideline). |
| `max_total_debt_at_36_percent` | number |  | 0.36 × gross_monthly_income (conservative back-end guideline). |
| `max_total_debt_at_43_percent` | number |  | 0.43 × gross_monthly_income (qualified-mortgage benchmark). |
| `remaining_capacity_at_43_percent` | number |  | max_total_debt_at_43_percent − total_monthly_debt_payments (negative when already above 43 %). |

## Formula

`back_end_dti = (Σ monthly_debts + other_monthly_debt + housing_payment) / gross_monthly_income × 100; front_end_dti = housing_payment / gross_monthly_income × 100`

Bands follow common US underwriting practice: the 28/36 rule (housing ≤ 28 %, all debt ≤ 36 %) and the 43 % back-end cap of the original 2014 Qualified Mortgage rule; since 2021 the General QM definition uses a price-based test, but 43 % remains a widely used benchmark and Fannie Mae allows up to 50 % with compensating factors. Utilities, groceries and insurance are not debts.

## Data Sources

- CFPB – What is a debt-to-income ratio? — https://www.consumerfinance.gov/ask-cfpb/what-is-a-debt-to-income-ratio-en-1791/ (government, retrieved 2026-09-24)
- CFPB – Ability-to-Repay and Qualified Mortgage Standards (Regulation Z) — https://www.consumerfinance.gov/rules-policy/final-rules/ability-to-repay-and-qualified-mortgage-standards-under-the-truth-in-lending-act-regulation-z/ (government, retrieved 2026-09-24)
- Investopedia – Debt-to-Income (DTI) Ratio: What's Good and How to Calculate It — https://www.investopedia.com/terms/d/dti.asp (reference, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/debt-to-income?gross_monthly_income=…`
- `POST https://tttkmbb.com/api/v1/calculate/debt-to-income` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/debt-to-income · OpenAPI operationId `calculate_debt_to_income_ratio` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "debt-to-income", "inputs": {…}}`

## Example

- 6,000 income, debts 400/250/150, housing 1,500: inputs `{"gross_monthly_income":6000,"monthly_debts":[400,250,150],"housing_payment":1500}` → `{"total_monthly_debt_payments":2300,"back_end_dti_percent":38.33,"front_end_dti_percent":25,"assessment":"Acceptable (36–43 %)","max_housing_payment_at_28_percent":1680,"max_total_debt_at_36_percent":2160,"max_total_debt_at_43_percent":2580,"remaining_capacity_at_43_percent":280}`
- 8,000 income, 1,600 total debt, no housing given: inputs `{"gross_monthly_income":8000,"other_monthly_debt":1600}` → `{"total_monthly_debt_payments":1600,"back_end_dti_percent":20,"assessment":"Good (≤ 36 %)","max_total_debt_at_43_percent":3440}`

```
GET https://tttkmbb.com/api/v1/calculate/debt-to-income?gross_monthly_income=6000&monthly_debts=400%2C250%2C150&housing_payment=1500
```

## Limitations

You need to know how long it takes to pay a balance off (use debt-payoff), or the amount of a down payment (use down-payment). Bands follow common US underwriting practice: the 28/36 rule (housing ≤ 28 %, all debt ≤ 36 %) and the 43 % back-end cap of the original 2014 Qualified Mortgage rule; since 2021 the General QM definition uses a price-based test, but 43 % remains a widely used benchmark and Fannie Mae allows up to 50 % with compensating factors. Utilities, groceries and insurance are not debts. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Gross or net income?**

Lenders use gross (pre-tax) income, so enter pay before taxes and deductions; using take-home pay overstates the ratio.

**Which payments count as debt?**

Recurring obligations that appear on a credit report or court order: loans, credit-card minimum payments, leases, child support. Utilities, phone plans, insurance and groceries are excluded.

## Related

- [Mortgage Payment Calculator](https://tttkmbb.com/finance/mortgage-payment.md) — Compute the PITI housing payment to enter here.
- [Down Payment Calculator](https://tttkmbb.com/finance/down-payment.md) — Loan amount and PMI for the purchase.
- [Debt Payoff Calculator](https://tttkmbb.com/finance/debt-payoff.md) — Clear a balance to lower the ratio.