Calculator methodology and assumptions

Fixed-rate payment formula

Mortgage and installment-loan payments use the standard amortizing payment formula: payment = principal × [monthly rate × (1 + monthly rate)^months] ÷ [(1 + monthly rate)^months − 1]. When the interest rate is zero, principal is divided evenly across the term.

Each scheduled payment applies interest to the outstanding balance first; the remainder reduces principal. Rounding can create a very small final-payment difference.

Mortgage and PITI assumptions

Principal and interest come from the fixed-rate formula. Annual property tax and homeowners insurance inputs are divided by 12. PMI is estimated from the annual PMI rate entered by the user and shown as an initial monthly amount; the calculator does not predict a servicer's cancellation date. Cancellation rules, appraisal requirements, FHA mortgage insurance, escrow adjustments, HOA dues, and flood insurance may differ.

Affordability and DTI

The affordability tool uses 28% of gross monthly income as a front-end housing guide and 36% as a back-end total-debt guide. The lower available housing budget controls the estimate. These ratios are planning benchmarks, not universal underwriting limits.

DTI is monthly debt divided by gross monthly income. It does not measure utilities, groceries, childcare, taxes withheld from pay, maintenance, or personal comfort.

Refinance comparisons

Break-even months equal upfront closing costs divided by positive monthly payment savings. Lifetime comparisons use the remaining scheduled cost of the current loan against new scheduled payments plus closing costs, adjusted for cash received. Taxes, insurance, points, opportunity cost, and tax treatment are not automatically modeled.

Quality checks and corrections

We test edge cases including zero interest, short terms, extra principal, low down payments, and invalid inputs. Content and formulas are reviewed when a calculator is changed. To report a suspected error, include the inputs and expected result through our feedback page.

Independent educational use

FinanceCalc is not a lender, broker, bank, tax adviser, or financial adviser. Results are educational estimates and do not represent available rates, approval, or a Loan Estimate. Read our editorial policy and terms for more detail.

Methodology reviewed September 17, 2026.