Gross pay is the wrong number, and lenders use it anyway
Every affordability rule in circulation is stated against gross pay. The familiar pair is twenty-eight and thirty-six: housing should take no more than twenty-eight per cent of gross monthly income, and all debt payments together no more than thirty-six. Underwriters apply something close to this, so it is a real constraint on what you will be offered.
It is also a number nobody lives on. Between gross pay and the money that reaches your account sit federal income tax, payroll contributions and, depending on where you work, state and local income tax as well. The gap is not small and it is not the same everywhere: two people on identical salaries, one in a state with no wage income tax and one in a state with a high one, take home noticeably different amounts and can afford noticeably different houses.
This calculator applies all three rules at once. The first two are the conventional gross-income pair, so you can see what a lender is likely to say. The third tests the same payment against take-home pay for the state you actually work in, using the same tax engine as our salary pages. The answer is the smallest of the three, because a buyer has to satisfy every constraint rather than the friendliest one, and the page names which rule is doing the binding.
The three ratios are conventions, not law
No agency publishes twenty-eight and thirty-six. They come from decades of underwriting practice, they vary between lenders and loan programmes, and government-backed programmes work to different figures again. Treat them as a starting point and change them: all three are ordinary form fields.
The third ratio has no standard value at all, because nobody else applies it. Thirty-five per cent of take-home pay is a defensible starting point and nothing more. What matters is not the number but the comparison: when the take-home rule is the one that binds, the gross-income rules were quietly assuming you keep more of your salary than you do.
What the price includes
The monthly budget each rule produces has to cover everything the house costs, not just the loan. That means the instalment, property tax, home insurance, any association fee, and mortgage insurance whenever the loan would start above eighty per cent of the price. That last condition is a real constraint on the answer rather than a footnote: the search stops at whatever price the budget covers, and if the premium is what tips it over, the price you see is the largest house that stays on the cheap side of the line.
Property tax is asked for as a rate rather than an amount here, because the price is the thing being solved for and the tax follows it. Your county publishes the rate; we do not guess it, for the same reason we leave it empty on the mortgage page. Insurance and association fees are entered as amounts, since neither scales with the purchase price in any way that can be checked.