How we calculate

They give estimates for planning and comparing. They are not financial, tax or legal advice, and they don't replace a lender's, the tax authority's or your employer's official figures. For a loan, the APR and the Loan Estimate or credit agreement from your lender are what count.

• Fixed monthly payment: M = P × r × (1 + r)^n / ((1 + r)^n − 1), where P is the loan amount, r the monthly rate (annual rate / 12) and n the number of payments. At 0% interest, M = P / n.
• Amortization schedule: each month, interest is the remaining balance × the monthly rate, and the rest of the payment reduces principal. Each row is rounded to the cent and the final payment is adjusted to the remaining balance, so the schedule ends at exactly zero.
• Extra payments: extra amounts go entirely to principal and the regular payment stays the same, so the loan ends earlier. If your lender re-amortizes and lowers the payment instead, results will differ.
• Mortgage PITI: principal and interest plus property tax, homeowners insurance, PMI and HOA dues entered by you. PMI drops off automatically when the balance reaches 78% of the original home price, as the Homeowners Protection Act provides for most conventional loans.
• APR: the nominal annual percentage rate under the Truth in Lending Act (Regulation Z): the monthly internal rate of return that equates the amount you actually receive (after prepaid fees) with your payments, multiplied by 12. We find it numerically by bisection.

• Credit card payoff: interest accrues monthly at APR / 12 on the balance carried. Real issuers use a daily periodic rate on the average daily balance, so actual interest can differ by a few dollars a month.
• Savings and retirement: contributions are added at the end of each month and growth is compounded monthly from the annual return you enter. Your income goal is entered in today's dollars and grown with inflation, while projected savings are shown in future dollars at your retirement age. Returns are never guaranteed; we don't assume any market return for you.

Sales tax and VAT calculators use the rate you enter or pick. The listed VAT standard rates come from the European Commission and HMRC; local sales tax rates vary by city and county, so check your state revenue department for the exact combined rate.

Weighted grades are Σ(score × weight) / Σ(weight). GPA uses the common 4.0 scale (A = 4.0, A− = 3.7, B+ = 3.3 …). Schools differ: some give A+ a 4.3, some weight honors and AP classes. Your school's published scale is always the one that counts.

Money is rounded to the cent. Where an official method rounds each item (for example each month of a schedule), we do the same, so totals can differ by a cent or two from rounding only the final figure. Percentages are shown with up to four decimal places.

Each calculation engine has automated tests: published examples from regulators and lenders, hand-checked cases and edge cases (0% interest, very short or very long terms). A change only ships when every test passes.

• Last reviewed: the day we checked the page's calculations item by item against its sources. We only show it when that check really happened, and we say what was checked.
• Last updated: the last meaningful change to the page's text or calculation, taken from our version history. We show this when there is no review date.

We never show a date that isn't backed by a real review or change.

Tell us on the contact page: the tool name, the values you entered and what you expected. We fix reported errors, and the date of the fix appears on the page.

Kitpebble is built and maintained by METE ONLINE Kft.. We don't name an outside expert reviewer because we don't have one; instead, this public methodology and the sources listed on each page let anyone check the math.