Quick Summary
The 28/36 rule says one thing, lender DTI says another, and the actual cost of ownership says a third. A spreadsheet that runs all three and the gap between them.
Quick answer. Three different numbers answer “how much house can I afford,” and they rarely agree. The 28/36 rule sets a conservative ceiling on housing and total debt. The lender’s DTI approval can run 15 to 20 points higher. The true cost of ownership, once maintenance and reserves are in, lands below both. A spreadsheet that runs all three side by side shows the spread rather than hiding it. Our Home Affordability Calculator is built to surface those three numbers from the same inputs.
Most affordability calculators give a single answer, and that single number is almost always the lender’s. The lender’s math is the cleanest to code and the most generous to display.
The lender’s number is not optimizing for the borrower’s monthly comfort. It is optimizing for default risk on a mortgage portfolio. Different question, different answer.
The three numbers and why they disagree
The same buyer with the same income and debts can be told they can afford three different homes depending on who is asked.
| Source | What it measures | Typical answer |
|---|---|---|
| 28/36 rule | A budgeting heuristic: housing under 28% of gross, total debt under 36% | Conservative, often the lowest of the three |
| Lender DTI | Risk model: what the loan program will approve | Often 25 to 45% higher than the 28/36 number |
| True cost of ownership | What the home actually costs to run once owned | Lands between the two, often below 28/36 once maintenance is in |
The 28/36 figure comes from decades-old underwriting norms, when housing was cheaper relative to income. The lender DTI figure comes from current loan program guidelines, which allow much higher ratios. The cost-of-ownership figure comes from the actual line items in a homeowner budget - most of which are missing from the lender’s calculation.
Run all three from one set of inputs and the spread becomes the actual question. Some buyers land near the lender ceiling, others closer to the cost-of-ownership floor, with the 28/36 number as a reference between them. The math does not decide; it clears the fog.
The 28/36 rule and where it came from
The 28/36 rule is two thresholds:
- Front-end ratio: housing costs (principal, interest, property tax, insurance, HOA) under 28% of gross monthly income.
- Back-end ratio: all debt payments combined (housing plus car, student loans, credit cards, etc.) under 36% of gross monthly income.
The rule originated in conventional mortgage underwriting in the 1970s and 1980s, when lenders used it as a hard cap. It is no longer a hard cap at most lenders, but it remains a useful budgeting reference because it bakes in some breathing room.
A worked example, used throughout this article: a household with $120,000 in gross annual income ($10,000/month) and $400/month in existing debt payments.
| Rule | Calculation | Result |
|---|---|---|
| Front-end (28%) | $10,000 x 28% | $2,800/month for all housing costs |
| Back-end (36%) | $10,000 x 36% - $400 existing debt | $3,200/month for housing |
The binding constraint is the lower of the two, which is the front-end at $2,800. That is the 28/36 ceiling for monthly housing.
Lender DTI and what gets approved
Lenders look at debt-to-income ratios that often run well above 36%. Conventional loans backed by Fannie Mae or Freddie Mac typically allow a back-end DTI up to 45%. Automated underwriting may approve up to 50% with strong compensating factors: high credit score, large reserves, and a low loan-to-value ratio. FHA loans can go higher still - manually underwritten FHA files have been approved with back-end DTI around 50 to 57% in some scenarios.
The same $120,000 household at a 45% back-end DTI:
| Step | Math | Result |
|---|---|---|
| Maximum total debt | $10,000 x 45% | $4,500/month |
| Less existing debt | $4,500 - $400 | $4,100/month available for housing |
That is $4,100/month for housing - 46% higher than the 28/36 ceiling of $2,800.
The lender is not being reckless on a portfolio basis. Default rates at higher DTI bands are manageable in aggregate. For an individual borrower, the $1,300/month gap between $2,800 and $4,100 is the size of a car payment or the line item that absorbs an annual property-tax reassessment. It is real money inside a budget, even if it is not a risk metric on a portfolio.
The framing problem with single-number affordability calculators is right there. The number they show is the approval number, not the comfort number. The two answer different questions, and a buyer is the one who has to decide which one they are asking.
The true cost of ownership
The lender’s monthly payment quote covers PITI - principal, interest, property tax, insurance. The 28/36 rule covers PITI plus HOA. Neither covers what it actually takes to run a home.
The lines that get omitted, with rough monthly ranges for a $500,000 home:
| Line | Why it’s missing from PITI | Typical monthly |
|---|---|---|
| Maintenance | Not a lender concern | $300 to $1,200 |
| HOA dues (where applicable) | Sometimes included, often forgotten | $0 to $800 |
| Utilities (whole-house) | Lenders ignore them | $200 to $500 |
| Lawn / pest / cleaning | Out of scope | $50 to $300 |
| Reserve for capital replacement | Not a monthly bill | $150 to $600 |
The maintenance reserve is the largest hidden line. Common rules of thumb run 1 to 3 percent of home value annually for routine maintenance and capital replacement, with newer homes at the low end and older homes at the high. On a $500,000 home, that is $5,000 to $15,000 per year, or roughly $417 to $1,250 per month smoothed across the year. The Homeowner’s Budget Template breakdown walks through the full category list and the timing problem with annual bills.
Add these lines back to PITI and the “comfortable” affordability figure typically lands 10 to 30 percent below the lender’s number, near the 28/36 ceiling or slightly below it. The math is straightforward; the lender just does not include it.
The worked example: $120,000 household across three metros
Same buyer, same income, same down payment. The differences come from property tax, insurance, and HOA. Assumptions:
- Gross income: $120,000/year, $10,000/month
- Existing debt: $400/month
- Down payment: 20% (so no PMI)
- Mortgage rate: 6.4% (close to the Freddie Mac May 2026 PMMS reading of 6.37%)
- 30-year fixed
- Homeowners insurance: $1,800/year ($150/month) baseline, adjusted per metro
- Maintenance reserve: 1.5% of home value annually
The variable is property tax rate, which varies enormously by state and county. Three illustrative metros:
| Metro | Property tax rate | Insurance assumption |
|---|---|---|
| Austin, TX (Travis County) | ~1.8% of value | $2,400/year (hail/wind exposure) |
| Phoenix, AZ (Maricopa County) | ~0.6% of value | $1,800/year |
| Newark, NJ (Essex County) | ~2.7% of value | $1,500/year |
Property tax data ranges from the Tax Foundation property tax dataset and county assessor publications; insurance figures are illustrative ranges.
Holding the 28/36 ceiling of $2,800/month for total housing costs constant, the maximum home price shifts by metro:
| Metro | Max home price (28/36 ceiling) | Approximate monthly breakdown |
|---|---|---|
| Phoenix | ~$390,000 | $1,952 P&I, $195 tax, $150 ins, $488 maintenance reserve |
| Austin | ~$335,000 | $1,677 P&I, $503 tax, $200 ins, $419 maintenance reserve |
| Newark | ~$315,000 | $1,576 P&I, $709 tax, $125 ins, $394 maintenance reserve |
Same income, same rule, home prices ranging from $315,000 to $390,000. A $75,000 swing driven almost entirely by property tax. The lender ceiling sits 40 to 50 percent higher in each metro; the cost-of-ownership-aware ceiling lands slightly below the 28/36 number once maintenance is in.
This is the kind of contrast a single-number calculator hides. The Cost of Living Comparison Spreadsheet covers similar metro-level math for households weighing locations.
The down payment question
Down payment percentage shifts the math through three channels:
- Loan-to-value (LTV): Below 80% LTV (20%+ down on conventional), the loan typically clears PMI. PMI runs roughly 0.3 to 1.5 percent of the loan amount annually, adding $100 to $400/month on a typical loan.
- Loan amount: A larger down payment means a smaller mortgage, lower P&I, lower DTI ratios, more headroom for the same home price.
- Reserve liquidity: A bigger down payment means less in liquid reserves for closing costs, the maintenance reserve, and the emergency fund. That trade-off lives outside the affordability formula but inside the comfort calculation.
A worked illustration for the Austin scenario at $335,000:
| Down payment | Loan amount | Monthly P&I | PMI | Total monthly (PITI + maint) |
|---|---|---|---|---|
| 5% ($16,750) | $318,250 | $1,990 | ~$130 | $3,240 |
| 10% ($33,500) | $301,500 | $1,886 | ~$125 | $3,135 |
| 20% ($67,000) | $268,000 | $1,676 | $0 | $2,800 |
Same house, same income. The 5% down household sits above the 28/36 ceiling; the 20% down household sits just below. The 5% household keeps more cash on hand but pays for that liquidity in PMI and a larger monthly carry. The spreadsheet surfaces the trade so the decision sits on numbers rather than on whichever scenario the lender quoted last.
Interest rate sensitivity
A one-percentage-point shift in mortgage rates moves monthly P&I on a $300,000 loan by about $200, or roughly $40,000 in home-price affordability at constant monthly payment. Assuming a 20% down payment and roughly $840/month of non-P&I housing costs (tax, insurance, maintenance) inside a $2,800/month ceiling:
| Rate | Monthly P&I on $300K, 30-year | Approximate home price at $2,800/mo total housing |
|---|---|---|
| 5.5% | $1,703 | ~$430,000 |
| 6.5% | $1,896 | ~$385,000 |
| 7.5% | $2,098 | ~$350,000 |
A swing from 5.5 to 7.5 percent reduces the affordable price by roughly 19 percent for the same income and the same rule. Rate environment is the largest single variable in cross-time comparison. The FHFA 2026 conforming loan limit announcement shows national price levels rising 3.26% year over year, but borrower purchasing power moves with rates more than with home-price drift.
How the spreadsheet runs all three numbers
The input side is short:
- Gross household income (monthly)
- Existing monthly debt payments
- Down payment available (dollar amount)
- Mortgage rate
- Property tax rate (or county average)
- Insurance estimate
- Maintenance percentage (1 to 3%)
- HOA estimate
The output side has three rows side by side: the 28/36 ceiling, the lender ceiling at a chosen DTI (often 43% or 45%), and the cost-of-ownership-aware “comfort” figure. Each row shows max home price, monthly housing payment, and effective DTI.
Three patterns turn up most often:
- The three numbers cluster within 10 percent of each other - usually a sign of low existing debt and a moderate maintenance assumption. The decision space is narrow.
- The lender number is 30 to 50 percent above the others. Common when income is solid but housing markets are expensive. The household has room to decide where in the band to sit.
- The cost-of-ownership number lands below the 28/36 number. Common for older homes, high-HOA condos, or high property-tax metros. The maintenance reserve is doing real work.
Reading the three together is the value of the spreadsheet. A single number, no matter which one, hides the trade-offs.
When the spreadsheet meets the rest of the financial plan
Home affordability is one input to the larger picture. Down payment funds compete with retirement contributions, emergency reserves, and other goals. A 20% down payment on a $400,000 home is $80,000 in cash no longer available for an index fund position, a Roth IRA, or a business runway. The Financial Planning Template sits one layer up, modeling 40-year cash flow and net worth across goals so the home decision fits the broader trajectory.
Renovation costs often surface after affordability is settled. A house at the comfort ceiling with a 10-year-old roof and an HVAC system near end of life carries replacement liabilities a first-year maintenance reserve may not absorb. The Home Renovation Budget Spreadsheet article covers phase-based cost tracking and realistic contingency math for projects above $25,000.
Templates that fit
- “I want all three affordability numbers from one set of inputs.” Home Affordability Calculator. The 28/36 ceiling, the lender DTI ceiling, and a comfort figure that includes maintenance and reserves. Down payment scenarios, PMI math, and tax/insurance lines are inputs, not assumptions.
- “I want the home decision inside a 40-year plan.” Financial Planning Template. Long-horizon cash flow, net worth, and goal tracking; the home purchase is one event in a multi-decade projection.
- “I want a monthly homeowner budget once the house is bought.” The Homeowner’s Budget Template breakdown covers categories and sinking-fund structure for tax, insurance, and maintenance.