Run the Numbers Like a Pro
A guided tour of the calculator — what every input actually does, how to read the outputs, and how to stress-test a deal until you trust the answer.
The calculator is the honest core of this whole site. It takes ten inputs and returns one number that matters — your real monthly cost after rent — with every line shown. This guide walks through each control, explains what the outputs mean, and shows the professional move most first-timers skip: stress-testing.
Start at the top: market and program
Before you touch a dollar figure, set the two segmented controls. Market loads honest median defaults so you're not starting from a blank page — or, better, open a specific city in the Market Explorer and click "Run the calculator with these defaults" to arrive pre-filled. Loan program is the single biggest lever: switch between Conventional (5% down), FHA (3.5%), and VA (0%) and watch the down payment, mortgage insurance, and financed fees update the way lenders actually charge them.
The inputs, and which ones move the answer
Not all ten inputs matter equally. In rough order of impact:
Where the leverage is
- Expected rent. The biggest lever after price. This is the rent on the other unit — the one you don't live in. Use a real comp, not a hope.
- Purchase price and interest rate. Together they set the mortgage; a half-point of rate is worth real money.
- Down payment. Toggle between % and $. Going below a program's floor triggers a warning — lenders decline there.
- Taxes & insurance. Quiet but decisive, especially across state lines. The market defaults already reflect local reality.
- Vacancy & maintenance. Leave them on. Five percent vacancy is ~18 days a year; 8% maintenance funds the water heater's eventual death.
Reading the outputs
The headline — your real monthly cost — is all-in cost minus rent after vacancy. When it goes negative, the badge flips to "you'd get paid to live here," and the caption compares you to the median renter in that market.
The gauge is a quick read of coverage: how much of your all-in bill the rent covers. The forest band in the middle is "living for free" (roughly 90–105% coverage); the terracotta zone past it is "getting paid." Beneath it sit four figures worth understanding:
| Output | What it means |
|---|---|
| Monthly cash flow | The real cost with the sign flipped — positive means money in your pocket after the housing bill. |
| Monthly mortgage (P&I) | Principal and interest only — the loan itself, before taxes, insurance, and reserves. |
| Break-even rent | The rent at which the building covers itself entirely after vacancy. The deal's whole posture in one figure. |
| 5-year equity | Down payment plus principal paid down over five years — no appreciation assumed. Equity you build even if prices never move. |
Read break-even rent hardest. If it sits comfortably below what the unit honestly rents for, you have margin. If the deal only works at the listing agent's optimistic "projected rent," you're buying the projection, not the building.
The pro move: stress-test it
Anyone can make a deal look good in the sunshine. The professional habit is to run the gloomy version before the offer, changing one input at a time so you can see which assumption the deal actually depends on — a sensitivity analysis in plain clothes:
Run these four scenarios
- Rate +0.5%. Rates move between pre-approval and closing. Can the deal take it?
- Rent −10%. Your comp might be optimistic, or the market softens. Does it still hold?
- Vacancy 8–10%. Model a rougher tenant year, or an older building between leases.
- Maintenance 10–12%. For anything pre-1970, bump the reserve. Old bones cost money.
A good deal survives all four bruised but standing. A deal that only works in the base case is telling you something.
Keep and share your numbers
Two features make the calculator a working tool rather than a toy. Copy link to these numbers encodes every input into the URL — send it to your agent, your partner, or your future self, and it reopens exactly as you left it. Email my report sends the worksheet as a one-page summary. Neither requires an account, and we don't sell your address.
When your numbers are ready, the last step is a licensed lender — the calculator is an honest estimate, not a quote. But walking in already knowing your payment, your program, and your break-even rent is exactly how a first-timer stops being one.
