How we calculate

Last updated

Every SureCalcs calculator follows the same principles: standard, published formulas; official data; clearly stated assumptions; and automated tests that check results against independently verified values.

Formulas

Loan and mortgage payments use the standard amortization formula. Savings and investment growth use compound-interest and annuity formulas, simulated period by period so that contributions, compounding frequency, taxes and inflation are handled exactly. Time-value-of-money functions follow the same conventions as Excel and Google Sheets (PV, FV, PMT, NPER and RATE), and interest rates are solved numerically to a precision far beyond one cent. Each calculator page shows its formula and a worked example.

Rounding

Calculations are carried out at full precision and only rounded for display, usually to the cent or to the whole dollar. Lenders and payroll systems round at each step, so their figures can differ from ours by a few cents.

Tax calculations

Income tax and paycheck estimates apply the 2026 federal tax brackets, standard deductions, credits and payroll-tax rules published by the IRS and the Social Security Administration, and each state’s 2026 brackets and deductions. They are estimates: they don’t cover every credit, deduction or local tax, and your actual withholding depends on your Form W-4.

Data sources

Testing and review

  • Each calculator has unit tests with reference values derived independently (closed-form results, published examples or a second implementation).
  • Each page is reviewed for accuracy against primary sources, and every number quoted in the text is generated from the same code that powers the calculator, so text and results always agree.
  • Tax brackets, contribution limits and other annual figures are updated when the IRS, SSA and states publish new values.

Limitations

Calculators can only be as accurate as their inputs and assumptions. Future interest rates, investment returns, inflation and tax laws are uncertain. Use the results as estimates and confirm important decisions with your lender, tax professional or financial adviser. If you spot an error, email hello@surecalcs.com.