Most affordability calculators start with income and spit out a home price. That's the easy part. The part that actually moves the number for a large share of US buyers — anyone under 20% down — is PMI, an extra monthly cost that doesn't show up until you dig into the loan terms.
The Basic Affordability Math
Lenders generally work backward from your gross income and target DTI ratios (see our DTI Calculator for the full breakdown) to find a maximum PITI payment, then work forward from that payment, your rate, and your down payment to a maximum home price.
Roughly: Max monthly housing payment (28% of gross income) → Max loan amount at your rate and term → Add back your down payment → Max home price.
Where PMI Enters the Picture
If your down payment is under 20% on a conventional loan, PMI gets added on top of principal, interest, taxes, and insurance — and it counts against your DTI ratio, which means it directly shrinks the loan amount you can qualify for, not just adds a cost after the fact.
| Down Payment | PMI (approx, avg credit) | Effect on Affordability |
|---|---|---|
| 20%+ | $0 | Full affordability ceiling from income/DTI |
| 10% | ~0.55%/year | Reduces max loan by roughly 3-5% vs. no-PMI scenario |
| 5% | ~0.85%/year | Reduces max loan more, plus a larger loan balance to insure |
| 3% | ~1.1%+/year | Largest PMI drag, though the lowest cash-to-close |
PMI rates aren't flat — they scale with credit score and loan-to-value, so a 620 credit score borrower at 5% down can pay meaningfully more in PMI than a 760 credit score borrower at the identical down payment.
Worked Example
A household with $8,400 gross monthly income and minimal other debt, targeting a 28% front-end ratio, has roughly $2,350/month available for PITI (plus PMI if applicable). At 6.47% on a 30-year fixed with 10% down and PMI factored in, that translates to a maximum home price in the high $300,000s to low $400,000s, depending on local tax rates. The same income with 20%+ down, no PMI drag, can typically support a price 5-8% higher.
Common Mistakes
Buyers frequently run affordability numbers assuming 20% down "because that's what everyone says," then get pre-approved at a smaller amount than the calculator showed once PMI enters actual underwriting.
Buyers also forget that affordability calculators use gross income, and treat the output as a hard target rather than a ceiling — comfortable spending and maximum qualifying capacity are usually two different numbers, and it's worth deciding which one you're actually solving for.
Where This Calculator Has Limits
It can't fully model credit-score-tiered PMI pricing without your actual score. It also doesn't factor in local cost-of-living pressures outside the mortgage itself — two buyers who "afford" the identical house on paper can have very different comfort levels depending on childcare, commute costs, or other fixed expenses the DTI math never sees.
Frequently Asked Questions
Does a higher down payment always mean I can afford more house?
Usually yes, for two reasons — it shrinks the loan you need, and it can eliminate PMI entirely above 20% down, freeing up more of your DTI room for the loan itself.
Can PMI be removed later even if I start under 20%?
Yes — once you reach 20% equity through paydown or appreciation, you can typically request PMI cancellation on a conventional loan.
Is affordability the same as what a lender will pre-approve me for?
It's an estimate. Actual pre-approval also weighs credit score, employment history, and asset documentation that a simple calculator doesn't capture.
Does this account for closing costs?
No — closing costs (typically 2-5% of loan amount) are a separate cash requirement on top of the down payment, not part of the ongoing affordability math.
Should I max out what I qualify for?
Not necessarily — qualifying capacity is a ceiling based on debt ratios, not a recommendation for comfortable monthly spending.
Related Tools
DTI Calculator · Down Payment Calculator · PITI Calculator
Educational content, not financial advice. PMI rates vary by lender, credit score, and loan-to-value ratio — confirm your exact affordability figures with a licensed mortgage lender. Written by the MortgagePro Global team.