What This Calculator Does
The Microapp Home Affordability Calculator answers the question that should come before any house hunting: how much home can you actually afford? You enter your income, monthly debts, down payment, and a real mortgage rate quote. The calculator applies the 28/36 debt-to-income rule that mortgage lenders use, plus the recurring housing costs (property tax, insurance, PMI if applicable, HOA), and returns the maximum home price you can sustain.
• Gross monthly income: $8,333
• Max PITI under 28% front-end: $2,333/month
• Of which property tax: ~$302, insurance: $125, PMI: ~$164
• Leaves for principal & interest: ~$1,743/month
• Implied loan amount: ~$262,000
• Max home price: ~$302,000
The front-end ratio is binding here. With $500/month in other debts, the back-end (36%) allows $3,000 − $500 = $2,500 for PITI, which is more than the front-end's $2,333 cap. So the housing cap wins.
The 28/36 Rule
The DTI (debt-to-income) rule is the standard mortgage underwriting test. Two ratios:
Front-end (28%): Monthly housing cost (PITI = Principal + Interest + Tax + Insurance, plus PMI if under 20% down, plus HOA) divided by gross monthly income.
Back-end (36%): All monthly debt payments (PITI + HOA + car loans + student loans + minimum credit card payments + child support + alimony) divided by gross monthly income.
Both rules apply at once. The lender approves the lower allowable home price. If you have no other debts, the front-end is usually binding. If you have meaningful other debt service, the back-end often binds because the housing budget shrinks dollar-for-dollar with non-mortgage payments.
What You Qualify For Is Not Your Budget
The 28/36 rule was designed by underwriters to identify the line at which borrowers stop defaulting on their mortgages. It is not the line at which they can also save for retirement, build an emergency fund, fund a kid's education, replace a furnace when it breaks, or take a vacation.
Consider what's left after housing and other debts at the lender's max:
| Use of income | At 28/36 (lender) | At 25/33 (comfortable) |
|---|---|---|
| Housing (PITI + HOA) | 28% | 25% |
| Other debt service | 8% | 8% |
| Federal income tax (avg) | ~13% | ~13% |
| FICA (SS + Medicare) | ~7.65% | ~7.65% |
| State + local tax (avg) | ~5% | ~5% |
| Subtotal: fixed obligations | ~62% | ~59% |
| Available for everything else | ~38% | ~41% |
That ~38% (or ~41%) covers retirement contributions (recommended 15% of gross), groceries, utilities (not in PITI), transportation costs beyond the car payment, childcare, healthcare beyond premiums, kids' education savings, emergency fund building, gifts, and discretionary spending. The math is tight at the comfortable budget and impossible for most households at the lender's max.
Pick the 25% front-end DTI option in the calculator if you want a budget that leaves room for the rest of your life.
The Down Payment Lever
A larger down payment increases your max affordable home price two ways: directly (every dollar of down payment is a dollar of home price), and indirectly (crossing 20% removes PMI, freeing budget for more loan).
Don't drain your emergency fund or skip retirement contributions to maximize down payment. Cash in the home is locked up — you can't get it back without selling. The "should I put 20% down" question depends on:
- Do you have 6+ months of emergency fund AFTER the down payment? If not, put less down.
- Are you capturing your full 401(k) employer match? If not, those contributions come first.
- Will the cash earn more in low-cost index funds (~7%/year real) than the PMI cost (~1% of loan for 5-7 years)? For younger buyers, usually yes.
For most first-time buyers, 5-10% down with PMI for 5-7 years is mathematically better than draining savings to put 20% down — provided you actually invest the difference.
The Other-Debt Lever
If the back-end ratio is binding, paying off non-mortgage debt before house hunting can move your max home price more than additional down payment. Example: $100k income, no other debts → max ~$340k home. Same household with $500/month in car payments → max ~$302k. The $38k gap comes from $6,000/year of debt service freed up.
Strategy: if you have credit card debt, car loans nearing payoff, or accelerable student loans, clearing those before house hunting often outperforms saving the same dollars for down payment — and improves your credit score in the process.
Property Tax Is Not Optional
The US effective property tax average is around 1.2% of home value, but the range is enormous:
| State range | Effective rate | $400k home, annual tax |
|---|---|---|
| Lowest (HI, AL, LA) | 0.3–0.5% | $1,200–$2,000 |
| Average (national) | ~1.2% | ~$4,800 |
| Highest (NJ, IL, TX) | 1.8–2.2% | $7,200–$8,800 |
Same home, $7,000/year of difference in monthly affordability impact. If you're moving across state lines, recompute the calculator with the new property tax rate before deciding what's in budget.
What Lenders Care About Beyond the 28/36 Rule
The 28/36 test is necessary but not sufficient. Other factors that move actual loan approvals:
- Credit score. 740+ gets the best rates. Below 620 typically pushes you to FHA. Score affects both approval probability and the rate offered.
- Employment history. Most lenders want 2+ years in the same field with steady income. Self-employed buyers need 2 years of tax returns showing income.
- Asset reserves. Most lenders require 2–6 months of mortgage payments in reserve after closing — proof you can weather a temporary income disruption.
- Loan type. Conventional, FHA, VA, USDA, and jumbo loans each have different DTI ceilings, down payment minimums, and insurance rules. FHA goes up to 43% back-end DTI; VA can go higher.
- Property type. Single-family homes get the best rates. Condos sometimes face restrictions (HOA financial health). Investment properties have higher down payment requirements (typically 20–25%).
What This Calculator Doesn't Capture
Closing costs. 2–5% of loan amount, paid at closing on top of down payment. On a $300k home with 10% down, plan for $6,000–$15,000 in closing costs as separate cash.
Long-term maintenance. Standard rule of thumb is 1–2% of home value per year for ongoing maintenance — $4,000–$8,000/year on a $400k home. Not in PITI but a real cost.
Utilities. Bigger home, bigger utility bill. Older homes can surprise you. Budget separately.
HOA increases. HOA fees almost always go up faster than inflation. If you're buying in a condo or planned community, ask for the HOA's reserve study before signing.
Property tax increases. Property taxes rise with assessments. Some states cap annual increases (California's Prop 13 at 2%); most don't. Budget room for tax to grow over the life of the loan.
Common Mistakes
Buying at the lender's max. Leaves no margin for retirement, family, or life events. Aim for 70–80% of max, or set the calculator to 25% front-end DTI.
Ignoring property tax differences between markets. Same home price in Texas (2% tax) vs Hawaii (0.4% tax) is a different affordability story. Recompute when shopping across state lines.
Forgetting that interest rate is the biggest knob. Going from 7% to 6% on a 30-year loan at this household's profile raises max home price by roughly $30k — about the same as a 10% larger down payment. Lock the rate when it's good, even if the home isn't.
Letting the pre-approval letter define your budget. Lenders profit from larger loans. Set your budget with this calculator, then ignore what the bank says you could borrow above it.
Educational Tool — Not a Loan Offer
This calculator implements the standard 28/36 DTI rule with iterative tax/insurance/PMI computation. Real lender approvals consider credit score, employment history, asset reserves, income stability, and dozens of other factors. For a real loan offer, get a Loan Estimate from at least three lenders and compare APRs (not just rates). For advice on whether buying makes sense at all versus renting in your market, a fee-only fiduciary financial planner is the right call.
Related Tools
For the forward direction — you have a target home price and want the monthly cost — use the Mortgage Calculator. The House Affordability Calculator answers the same affordability question with a simpler binary "lender max vs comfortable" toggle. For non-mortgage debt math, the Loan Calculator handles car loans and personal loans.