Mortgage Affordability Calculator

Background.

A mortgage affordability calculator answers the question every prospective homebuyer asks first: how much house can I actually afford? Not how much house a real-estate agent thinks you can afford, and not how much house a lender's preapproval will quote you — the honest number that comes out when you stop treating the mortgage as an isolated line item and start treating it as one slice of your total monthly cash flow.

The framework underwriters use is a pair of debt-to-income ratios. The front-end ratio caps your monthly housing payment as a percentage of gross monthly income. The back-end ratio caps housing plus every other recurring debt obligation — car loans, student loans, minimum credit-card payments, child support — as a percentage of the same gross income.

The classic 28/36 rule, written into Fannie Mae's Selling Guide B3-6-02, sits at the conservative end: housing under 28%, total debt under 36%. The Federal Housing Administration loosens this to 29/41 for FHA-insured loans, recognising that FHA borrowers tend to have less cushion to absorb shocks but tighter underwriting on the loan itself. Conventional lenders working with the Consumer Financial Protection Bureau's Qualified Mortgage (QM) rule under 12 CFR §1026.43 will stretch as high as 31/43 when the borrower has heavy student-loan debt but documented income stability — the structure the FHFA explicitly permits when a higher back-end ratio is offset by compensating factors. Picking the right rule is the first decision this calculator forces you to make, because the same income produces very different affordability ceilings under each one.

The second thing this calculator captures, that simple monthly-payment calculators do not, is that your real housing cost is not just principal and interest. It is PITI — principal, interest, taxes, and insurance — plus any HOA dues. Property tax in the United States averages roughly 1% of home value annually but ranges from under 0.3% in Hawaii to over 2.2% in New Jersey, and it scales with the price of the house, which means a more expensive home pushes its own tax bill up and compresses the room left for principal and interest.

Homeowners insurance adds another $100–$200 per month for most single-family homes. Private mortgage insurance (PMI) kicks in on conventional loans whenever your down payment is below 20% of the purchase price — that is, whenever your loan-to-value ratio exceeds 80% — and typically runs 0.3% to 1.5% of the loan amount per year until you reach 78% LTV under the Homeowners Protection Act. HOA dues, which the lender will absolutely include in your DTI calculation, can add another $150–$600 per month for condominium and planned-community properties. All of this comes out of the same DTI cap that has to cover the mortgage itself, so the more your tax, insurance, and HOA take, the smaller the loan — and the smaller the house — that fits inside the rule.

This calculator solves that interaction directly. You enter your gross income, your monthly debts, the cash you have ready for a down payment, the rate the lender quoted you, the loan term, your county's effective property-tax rate, your insurance estimate, any HOA dues, and the DTI rule that fits your loan program. The engine computes your PITI cap from your income, subtracts insurance and HOA, then runs a fixed-point iteration on price and tax — because tax depends on the price you can afford and the price you can afford depends on the tax — until the two converge. The output is your maximum supportable purchase price, the maximum loan that supports, the full PITI breakdown at that price, and the front-end and back-end DTI ratios you will land on.

Treat the result as a ceiling, not a target. The 28/36 rule was written for the underwriter, not for you — it tells the bank when they can sell the loan to Fannie Mae, not whether you will be comfortable making the payment in year seventeen when the roof needs replacing and your kids start college.

What is mortgage affordability calculator?

Mortgage affordability is the largest home purchase price a borrower can support under a lender's debt-to-income (DTI) rules, given the borrower's income, recurring debts, available down payment, and the costs of the loan itself. It is distinct from mortgage preapproval — preapproval is the lender's written estimate of what they will lend you, often based on a less complete picture; affordability is the number that falls out of an explicit application of underwriting rules to your finances. The two standard DTI ratios are the front-end ratio (housing expense / gross income) and the back-end ratio (housing expense + other debts / gross income). Different loan programs apply different caps. Fannie Mae's conventional conforming loans use 28% front and 36% back as the comfort target, with overlay flexibility up to 45% back when compensating factors exist. FHA loans, per HUD Handbook 4000.1 Section II.A.4.b, target 29% front and 41% back. The CFPB's Qualified Mortgage rule caps total back-end DTI at 43% for the safe-harbor QM designation, which is where most conventional loans for borrowers with significant student debt land. Housing expense in every program means full PITI — principal, interest, taxes, and insurance — plus PMI if applicable and any HOA dues, not just the mortgage payment itself.

How to use this calculator.

1. 01Enter your gross annual household income. Use the pre-tax figure your lender will verify against W-2s or two years of 1099s. Bonus, commission, and overtime only count if you have a documented two-year history.
2. 02Enter your monthly debt payments. Add up the minimum monthly amounts on car loans, student loans (including income-driven repayment amounts), credit cards, personal loans, child support, and alimony. Do not include utilities, groceries, or insurance premiums.
3. 03Enter your planned down payment in cash. Anything under 20% of the eventual purchase price will trigger PMI on a conventional loan or upfront MIP plus annual MIP on an FHA loan.
4. 04Enter the annual interest rate the lender quoted, or use the Freddie Mac weekly average as a starting estimate. The default of 7% leaves a small cushion above current market rates.
5. 05Pick a loan term. Most buyers default to 30 years; 15-year terms cut total interest in half but raise the monthly payment by 40–50%, which shrinks the affordable price by a similar percentage.
6. 06Enter the property-tax rate for the county where you plan to buy. County assessor websites publish the effective rate; if you don't know it, leave the default 1.2% — close to the national median.
7. 07Enter your homeowners insurance estimate (annual). $1,500/year is a reasonable national average for a single-family home; condo HO-6 policies run lower, coastal and wildfire zones much higher.
8. 08Enter any monthly HOA dues for the type of property you are targeting. Lenders include HOA in the back-end ratio.
9. 09Pick a DTI rule. Use 28/36 for a conservative conventional benchmark, 29/41 for FHA, or 31/43 if you carry significant student-loan or other consumer debt and need the back-end stretch.
10. 10Read the max home price as a ceiling. The front-end and back-end ratios will tell you which side of the rule you are bumping up against — if the front-end is lower than the rule's limit, your debts are constraining you; if the front-end is hitting the limit, your income is.

The formula.

The calculator works in two stages. First it computes your PITI cap — the maximum monthly housing payment the DTI rule allows. The front-end constraint gives PITI_front = (monthly income) × front limit. The back-end constraint gives PITI_back = (monthly income) × back limit − monthly debts. Your PITI cap is the smaller of the two, because both rules must be satisfied. Second, it computes the home price that exactly consumes that cap. This is harder than it looks because property tax is a percent of home price, not a fixed dollar amount, so tax depends on price and price depends on tax. The calculator handles this with a fixed-point iteration. It starts with price = 0, computes the implied monthly tax (price × annual tax rate / 12), subtracts that tax plus monthly insurance plus HOA from the PITI cap to get the maximum principal-and-interest payment available, then uses the standard present-value annuity formula loan = PI × (1 − (1+r)^−n) / r to back out the supported loan amount. Add the down payment and you have a new price; feed that price back in and recompute. Ten iterations is always sufficient to converge to within a dollar. The final outputs report the converged max home price, the loan amount (price minus down payment), the PITI cap, the breakdown into principal-and-interest plus tax plus insurance, and the two DTI ratios you land at. The arithmetic uses arbitrary-precision Decimal math so the iteration is reproducible to the cent regardless of the magnitude of the inputs.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Consumer Financial Protection Bureau — Qualified Mortgage Rule (Ability-to-Repay), 12 CFR §1026.43
2. [2]Fannie Mae Selling Guide B3-6-02 — Debt-to-Income Ratios (conventional 28/36)
3. [3]FHA Handbook 4000.1, Section II.A.4.b — Qualifying Ratios (FHA 29/41)
4. [4]Freddie Mac Single-Family Seller/Servicer Guide — Underwriting the Borrower (Chapter 5401)
5. [5]HUD — First-Time Homebuyer Resources and FHA Loan Limits
6. [6]Consumer Financial Protection Bureau — Owning a Home: Affordability Worksheet
7. [7]Homeowners Protection Act of 1998 — PMI cancellation rules at 78% LTV