Methodology

Principles

• One clear question per calculator. Each calculator answers a specific question, and its page explains the formula, assumptions, a worked example and its limitations.
• Estimates are labeled as estimates. Results depend on the inputs and on stated assumptions. Real contracts, tax rules and lender practices can differ.
• Facts are sourced. Definitions, legal rules and time-sensitive data come from official or primary sources. The page names the country or region they apply to and shows when they were checked.
• Nothing is invented. Calcendium does not publish made-up rates, statistics, ratings or testimonials. Where a value varies, such as a local sales tax rate, you enter it.

Exact decimal arithmetic

Ordinary computer arithmetic stores numbers in binary floating point, so 0.1 + 0.2 does not come out as exactly 0.3. That is harmless for many tasks but wrong for money. Calcendium's financial calculations use decimal arithmetic with 40 significant digits, and money is rounded to whole cents only where the conventions below say so.

Loan conventions

The loan calculators share one amortization engine with these rules:

• Rate. The APR you enter is treated as a nominal annual rate. The monthly rate is APR ÷ 12.
• Timing. Payments are made at the end of each monthly period, and interest compounds monthly.
• Interest each month. Interest is the opening balance × the monthly rate, rounded to the cent (half-up: 0.005 rounds up). The rest of the payment reduces principal.
• Scheduled payment. Calculated with the standard annuity formula and rounded to the cent. On very long terms, it is kept at least one cent above the first month's interest so the loan always amortizes.
• Final payment. The last payment is whatever clears the remaining balance plus that month's interest. It may be smaller than usual (after extra payments) or a few cents larger (from rounding the scheduled payment).
• Extra payments. Extra amounts are applied in full to principal in the month they are paid.
• Safety limits. A payment you enter that does not exceed the first month's interest is rejected, because the balance would never go down. Schedules are capped at 1,200 months.

Lenders may use daily interest accrual, different day-count rules or other rounding. Your contract can therefore differ from a Calcendium estimate by small amounts.

Testing

Every financial formula has automated tests. They check results against well-known reference values, such as the standard payment on a 30-year loan, and against an independent implementation. They also cover edge cases: zero interest, zero principal, very high rates, invalid terms, final partial payments, payments too small to repay a loan, negative trade-in equity, financed fees and rounding behavior. The tests run on every change before it can be published.

Sources and review dates

Calculator pages list the sources behind their definitions and rules, with the jurisdiction and the date each source was checked. Each page also shows when the calculator and its explanation were last reviewed. Pages that depend on data that changes over time, such as tax rates or thresholds, will state the country and year of the data. They will be added only with a defined update process.

Languages and regions

Language, country, currency and units are treated as separate settings. A translated page is published only after its terms, formulas and regional assumptions have been reviewed for that audience. The currency selector on a calculator changes how amounts are displayed, not the math.

Privacy of your inputs

Calculations run entirely in your browser. Calcendium does not receive, store or analyze the values you enter. See the privacy policy.

Corrections

If you believe a result, formula or source is wrong, please write to [email protected] with the calculator, your inputs and what you expected. Confirmed errors are corrected and the page's review date is updated.