Buying a home is almost certainly the largest financial decision most people will ever make. Yet the majority of first-time buyers walk into the process without a clear sense of what they can actually afford — and end up either stretching dangerously thin on a budget that doesn't leave room for emergencies, or undershooting and renting longer than necessary.
The good news: the math is not complicated. With a few numbers and a basic understanding of how mortgages work, you can arrive at a confident, realistic affordability figure before you ever speak to a lender.
The 28% Rule — Your First Affordability Filter
The most widely used mortgage affordability guideline in the US is the 28% rule: your monthly mortgage payment (principal + interest + taxes + insurance, sometimes called PITI) should not exceed 28% of your gross monthly income.
If your household earns $7,000/month before taxes, your maximum monthly mortgage payment would be $7,000 × 0.28 = $1,960. This isn't a magic number — it's a ceiling designed to keep housing costs from consuming your financial life. Many households successfully carry slightly higher ratios, but 28% is a well-established starting point.
The 36% Debt-to-Income Rule
Mortgage lenders also examine your total debt-to-income ratio (DTI) — the percentage of your gross monthly income going toward all debt payments combined: mortgage + car loans + student loans + credit card minimums + any other recurring debt.
Most conventional lenders want your total DTI at or below 36%. FHA loans sometimes allow up to 43–45%. Staying well below 36% gives you budget breathing room, improves your chance of approval, and often qualifies you for a lower interest rate.
How Much House Can You Actually Afford?
Here's a practical step-by-step approach:
- Determine your gross monthly household income
- Apply the 28% rule to find your maximum monthly payment
- Back-calculate from that monthly payment to a loan amount using current interest rates
- Add your expected down payment to get your maximum purchase price
A Real Example
Household income: $90,000/year = $7,500/month gross
28% of monthly income: $7,500 × 0.28 = $2,100/month
Estimated taxes + insurance: $450/month
Available for principal + interest: $2,100 − $450 = $1,650/month
At 7.0% over 30 years, $1,650/month supports a loan of approximately: $247,500
With a 20% down payment: purchase price = $247,500 ÷ 0.80 = $309,375
Use our Mortgage Calculator to run these numbers with your actual figures — it handles the amortization math and lets you adjust rate, term, and down payment instantly.
Down Payment — How Much Do You Really Need?
The classic advice is 20% down to avoid Private Mortgage Insurance (PMI). PMI typically adds 0.5–1.5% of the loan amount annually — that's an extra $100–$300/month on a $250,000 loan. Over 7 years (a common home ownership period), that's $8,400–$25,200 extra cost.
However, many buyers successfully put down 3–10% and accept PMI for a few years to enter the market sooner. If home values in your area are appreciating faster than you can save, buying earlier with PMI can actually be the mathematically better choice. Once your equity reaches 20%, PMI can be cancelled.
The Hidden Costs Most First-Time Buyers Forget
Your down payment is only the beginning. A $300,000 home typically requires much more cash upfront:
- Closing costs: 2–5% of purchase price ($6,000–$15,000)
- Home inspection: $300–$600
- Moving costs: $1,000–$5,000+ depending on distance
- Immediate repairs and updates: variable, but budget at least $2,000–$5,000
- Furniture and appliances: if the home is unfurnished
- Ongoing maintenance reserve: budget 1% of home value annually ($3,000/year on a $300k home)
Many financial advisors suggest having a 6-month emergency fund in addition to your down payment and closing costs before buying. This prevents a broken furnace or job disruption from becoming a mortgage crisis.
Why the Interest Rate Changes Everything
A seemingly small rate difference has enormous long-term impact. On a $250,000 loan:
| Interest Rate | Monthly Payment (P+I) | Total Interest Paid (30yr) |
|---|---|---|
| 5.0% | $1,342 | $233,139 |
| 6.5% | $1,580 | $318,868 |
| 7.5% | $1,748 | $379,175 |
The difference between a 5% and 7.5% rate on the same loan is over $145,000 in total interest. This is why improving your credit score before applying — even by 30–40 points — can be worth months of effort.
Frequently Asked Questions
How much income do I need for a $300,000 mortgage?
With a 20% down payment, your loan would be $240,000. At 7% over 30 years, the monthly principal + interest payment is approximately $1,597. Adding typical taxes and insurance ($350–$500/month), total PITI is roughly $2,000–$2,100/month. To keep housing below 28% of income, you'd need gross monthly income of at least $7,100–$7,500, or about $85,000–$90,000 annually.
What credit score do I need to get a mortgage?
Conventional loans typically require a minimum score of 620, though you'll access the best rates with 740 or above. FHA loans accept scores as low as 580 (with 3.5% down) or 500 (with 10% down). Each 20-point improvement in your credit score can potentially lower your rate by 0.125–0.25%, saving tens of thousands over the loan's life.
Is it always better to put 20% down?
Not always. If putting 20% down depletes your emergency fund, you're one unexpected expense away from financial stress. And if home prices are rising quickly in your target market, buying sooner with PMI might result in more equity gain than you'd have saved by waiting. Run the numbers both ways with current appreciation rates before deciding — it's a genuinely case-by-case decision.
How does the mortgage calculator formula work?
The standard mortgage amortization formula is: M = P[r(1+r)ⁿ]/[(1+r)ⁿ−1], where P is the loan principal, r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments (months). Each payment covers that month's interest first, with the remainder reducing the principal. In the early years of a mortgage, the majority of each payment goes toward interest — which is why extra principal payments in year 1–5 have an outsized impact on total interest paid.