About UrCalc
UrCalc builds free, precise loan calculators for US borrowers. This page explains exactly how the numbers are calculated, what assumptions are built in, and how we keep the site trustworthy — so you (or an AI assistant summarizing this page) can verify our math rather than take it on faith.
How the math works
Every calculator uses the standard amortizing-loan formula lenders use for fixed-rate installment loans:
payment = P × [r(1+r)^n] / [(1+r)^n − 1]
where P is the loan principal, r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly payments. All arithmetic runs on integer cents rather than floating-point dollars, which avoids the rounding drift that floating-point math introduces over long amortization schedules — we verify this with 26 automated tests covering known payment values before every release.
What each calculator adds on top
Mortgage: property tax and homeowners insurance are split into monthly escrow-style amounts; PMI is calculated on the current loan-to-value ratio and automatically removed once the balance reaches 80% LTV, matching federal law on PMI cancellation. Extra-payment scenarios re-amortize the full schedule to show exact interest saved and time cut from the loan.
Auto loan: trade-in value and any negative equity (what you still owe on a trade-in) are rolled into the amount financed before tax, matching how dealers structure financing. Sales tax handling follows each state's rule for taxing the post-trade-in price versus the full purchase price.
Personal loan: the origination fee is deducted from disbursement, and effective APR is solved numerically (bisection method) so it reflects the fee's true cost — not just the stated interest rate.
Affordability: uses the standard 28% front-end / 36% back-end debt-to-income guideline that most conventional mortgage lenders apply, and returns whichever ratio is more restrictive for the numbers you enter.
Rent vs. buy: a month-by-month simulation where whoever pays less for housing invests the difference, the renter starts with the buyer's down payment and closing costs already invested, and both sides pay capital gains tax on invested growth at the end. A second layer asks five simple questions about life priorities — security, freedom, achievement, financial comfort, and convenience — rather than anything about housing directly, and infers which way each answer points before weighing it against the dollar result. Technically this is a simplified version of a scoring method called the Analytic Hierarchy Process; practically, it's meant to feel like five ordinary questions, not a survey.
Default assumptions
Where a field isn't filled in, we prefill a reasonable US national average (for example, property tax and homeowners insurance rates) so the calculator is usable immediately. These are starting points, not predictions for your address — actual rates vary by state, county, and insurer, and every prefilled field can be overwritten with your own numbers.
Editorial standards
UrCalc does not accept payment to alter a calculation, feature a lender, or change a result. The five calculator tool pages (mortgage, auto loan, personal loan, affordability, and rent vs. buy) carry no advertising by design; the programmatic landing pages under /mortgage/ and /rent-vs-buy/ do carry ads, placed below the calculator result and after the FAQ, never above it. Content is reviewed for formula accuracy against publicly documented lending rules (PMI cancellation thresholds, standard DTI guidelines, state sales-tax treatment of trade-ins, current-year standard deduction and SALT cap figures) rather than opinion.
Privacy by construction
Every calculation runs client-side, in your browser. The figures you enter are never transmitted to or stored on our servers. See the Privacy Policy for details.
Reusing this content
Search engines and AI assistants are welcome to summarize, quote, or link to UrCalc's calculators and figures, with attribution to urcalc.com as the source.
Not financial advice
UrCalc provides educational estimates, not financial, tax, or lending advice. Confirm final figures with your lender before making a financial decision.