How We Calculate These Numbers
Every figure on this site comes from a formula you can check. This page documents which formulas, which assumptions, and where the limits are — because a calculator that doesn't tell you what it assumed isn't giving you a number you can rely on.
Who writes this
Ground Figures is written and built by Kulasekar Mittapalli, a licensed Mortgage Loan Officer and software engineer. The perspective here comes from originating loans, not from summarizing other websites — which matters most in the places where published guidance and underwriting practice diverge.
This site does not originate loans, take applications, or receive compensation from lenders for referrals. Nothing here is a loan offer or a commitment to lend.
The core formulas
Monthly principal and interest uses the standard amortizing payment formula:
Amortization schedules apply interest to the outstanding balance each period, subtract the interest portion from the payment, and reduce the balance by the remainder. Totals shown are the sum of those periods, not an approximation.
Debt-to-income divides monthly obligations by gross monthly income. Affordability inverts the 28% and 36% guidelines to solve for a supportable payment. Refinance breakeven divides total costs by monthly savings. Reserve ratios divide annual reserve contribution by annual assessment income. None of this is proprietary — the value is in stating the assumptions alongside the output.
What the assumptions are, and where they break
You supply them
We don't fetch live rates or predict them. The rate you enter is the rate we use.
Often reassessed at sale
A seller's current bill can understate a buyer's future bill considerably in some jurisdictions.
Estimated, not quoted
Premiums vary by roof age, construction, claims history, and location. Get a real quote.
Priced individually
Depends on credit score and loan-to-value. Our defaults are approximations.
Two structural limits worth stating plainly. First, these tools model the loan, not the transaction — closing costs, prepaids, and seller concessions shape what you actually bring to closing and aren't reflected in a monthly payment figure. Second, guidelines are not decisions: automated underwriting weighs DTI, credit, reserves, and loan-to-value together, so a ratio that looks marginal in isolation may clear comfortably in context, and vice versa.
Nothing you type leaves your browser
Every calculation on this site runs client-side in JavaScript. No inputs are transmitted to a server, logged, or stored. There are no accounts, and none of the tools ask for identifying information.
The one exception to be aware of is scenario sharing: the "copy link" feature encodes your inputs into the URL so the page can reproduce them. That link contains those values, so treat it accordingly before sending it to anyone.
How we handle corrections
Pages carry a "last reviewed" date rather than only a publication date, because mortgage rules change and a 2024 article about 2026 requirements is worse than no article. When guidance changes materially, we update the page and move the review date rather than publishing a near-duplicate.
If you find an error — a formula, a figure, or a rule stated incorrectly — tell us. Corrections to substantive errors are made to the page directly, and we'd rather be told than be wrong quietly.
What we don't do
We don't predict rates. We don't publish "best lender" rankings, because we're not compensated by lenders and have no basis for ranking them. We don't republish press releases as articles. And we don't present a calculator output as a decision — the tools are there to make a tradeoff visible, not to tell you what to do with it.