# How to Calculate Your Mortgage Affordability: Rules, Ratios, and 2026 Limits

Reading Time — 18 minutes

Last updated: April 20, 2026

## Introduction
Figuring out how much mortgage you can afford is one of the most important — and most stressful — steps in the homebuying journey. Borrow too much and you risk financial strain for decades. This mortgage affordability calculator guide gives you everything you need to calculate mortgage affordability with confidence: proven rules of thumb, formula breakdowns, worked examples at four different income levels, and a clear framework for deciding what truly fits your budget.

## How to Use a Mortgage Affordability Calculator
A mortgage affordability calculator takes your financial inputs and estimates the maximum loan — and home price — you can reasonably afford. Below is a step-by-step walkthrough you can follow manually or use alongside any online calculator tool.

### Inputs you'll need:
|     |     |
| --- | --- |
| **Input** | **Where to Find It** |
| Gross annual income (pre-tax) | Pay stubs or tax return |
| Monthly debt payments | Credit card minimums, auto loans, student loans |
| Down payment amount | Savings earmarked for purchase |
| Estimated interest rate | Current lender quotes or Freddie Mac's [Primary Mortgage Market Survey](https://www.freddiemac.com/pmms) |
| Loan term | Typically 30 or 15 years |
| Estimated property tax rate | County assessor website |
| Homeowners insurance estimate | Insurance quote or local average |

### Step 1 — Determine Your Gross Monthly Income
Start with your **gross monthly income** — that's your total pre-tax earnings divided by 12. If you earn a $100,000 annual salary, your gross monthly income is **$8,333**.

Include all stable, documentable income sources:
- **Base salary or hourly wages**
- **Bonuses and commissions**
- **Self-employment income**
- **Rental income, alimony, or investment income**

### Step 2 — Calculate Your Monthly Debts
List every recurring monthly debt obligation that shows up on your credit report:
- Minimum credit card payments
- Auto loan or lease payments
- Student loan payments
- Personal loans
- Child support or alimony

### Step 3 — Apply the 28/36 Rule
The **28/36 rule** is the most widely used mortgage affordability rule of thumb. It sets two spending ceilings:
- **28%** of gross monthly income for housing costs (your front-end ratio)
- **36%** of gross monthly income for all debt payments combined, including housing (your back-end ratio)

### Step 4 — Factor in Your Down Payment
Your down payment directly affects how much you need to borrow — and whether you'll pay private mortgage insurance (PMI).
- **20% down** eliminates PMI and gives you instant equity.
- **3%–5% down** is possible with conventional and FHA loans, though PMI will add to your monthly costs.

### Step 5 — Account for Interest Rate and Loan Term
Even a small difference in your interest rate dramatically changes what you can afford.
|     |     |     |     |
| --- | --- | --- | --- |
| **Loan Amount** | **Rate** | **Term** | **Monthly P&I Payment** |
| $350,000 | 6.5% | 30 years | $2,212 |

## The 28% Rule: Your Front-End Ratio Explained
The **28% rule mortgage** guideline states that your total monthly housing costs should not exceed **28% of your gross monthly income**.

### What Counts as "Housing Costs"
Your front-end ratio includes your full **PITI payment**:
- **P** rincipal
- **I** nterest
- **T** axes (property taxes)
- **I** nsurance (homeowners insurance)

### The Formula
> **Front-end ratio = Total monthly housing costs ÷ Gross monthly income**

## The 36% Rule: Your Back-End DTI Ratio Explained
The **36% rule** addresses your **back-end ratio** — your **total DTI (debt-to-income) ratio**.

### The Formula
> **Back-end DTI ratio = (Total monthly housing costs + All other monthly debts) ÷ Gross monthly income**

## Mortgage Affordability Rules of Thumb Compared
Here's how the most common rules of thumb stack up:
|     |     |       |     |
| --- | --- | ----- |---|
| **Rule** | **What It Says** | **Formula** | **Best For** |
| **28% Rule** | Housing costs ≤ 28% of gross income | Gross monthly income × 0.28 | Conservative housing budget ceiling |
| **36% Rule** | Total debt ≤ 36% of gross income | (All monthly debts) ÷ gross income ≤ 0.36 | Full debt-load check |
| **3× Annual Income** | Home price ≤ 3× gross annual income | Annual income × 3 | Quick estimate |

## How Much Mortgage Can I Afford? Examples by Income Level
Below are worked examples at four income levels using these shared assumptions:

**Assumptions for all examples:**
- 30-year fixed mortgage at **6.75% interest rate**
- **10% down payment**
- Property tax rate: **1.1% of home value annually**

### Mortgage Affordability on a $50,000 Salary
|     |     |
| --- | --- |
| **Metric** | **Value** |
| Gross monthly income | $4,167 |
| Max housing payment (28%) | $1,167 |
| Estimated max home price | **~$165,000** |

### Mortgage Affordability on a $100,000 Salary
|     |     |
| --- | --- |
| **Metric** | **Value** |
| Gross monthly income | $8,333 |
| Max housing payment (28%) | $2,333 |
| Estimated max home price | **~$345,000** |

## Key Factors That Affect How Much Mortgage You Can Afford
### What Lenders Evaluate
1. **DTI ratio**
2. **Credit score**
3. **Loan-to-value (LTV) ratio**

## Frequently Asked Questions
### How much mortgage can I afford on a $100K salary?
Using the 28/36 rule with a 30-year loan at approximately 6.75%, a $100,000 salary supports a maximum housing payment of about **$2,333 per month**.

### What is the 28/36 rule for mortgages?
The 28/36 rule is a mortgage affordability guideline saying your **monthly housing costs should not exceed 28%** of your gross monthly income and your **total monthly debt payments should not exceed 36%**.