Calculator Methodology

Formulas

Each calculator uses published, peer-reviewable formulas — for example, the standard fully-amortizing mortgage payment formula, the future-value-of-an-annuity formula for contributions, and the present-value formula for retirement-need estimates. The exact formula and variables for each tool are summarized on its results page.

Inputs

All meaningful inputs are user-controlled. Where a calculator pre-fills a default (for example, an inflation assumption), the default is labeled and editable.

Assumptions

• Annual rates compound according to the convention disclosed on each tool (e.g. monthly for mortgages, annually for long-horizon investment projections).
• Inflation and return defaults are deliberately moderate; aggressive defaults can mislead.
• Taxes are simplified and may not reflect your full bracket, deductions, credits or state-level rules.
• Fees, spreads, loads, sequence-of-returns risk and behavioral factors are usually excluded unless explicitly modeled.

Rounding & precision

Calculations are performed in double-precision floating point and rounded for display. Displayed values may differ from internally computed values by a small fraction of a cent or basis point.

Limitations

Calculator outputs are educational estimates. Real-world results will differ — sometimes materially — due to taxes, fees, market behavior, regulatory changes and unmodeled personal factors.

Data sources

When external data (tax brackets, contribution limits, historical reference rates) appears, we cite primary U.S. sources such as the IRS, SSA, Federal Reserve and BLS. See Data Sources for the full list.