A calculator is only as good as the numbers behind it. This page lists the sources for every default value on the site, the checks each calculator passes before it is published, and the rules we follow when a result turns out to be wrong.
Every calculator opens with a realistic scenario already loaded, so the page answers a question before you type anything. Those defaults follow three rules. First, they come from a named public source, not from a guess. Second, they describe the typical case rather than a favorable one, which is why the loan calculators use average market rates instead of the best advertised rate. Third, the page states its rounding, so a figure like $697.57 can be reproduced by hand.
Tax and payroll pages use the current-year federal figures published by the agencies that set them. Savings and debt pages use published national averages and rate data. Housing pages use the indexes that lenders and economists actually cite. The table below maps each group.
| Calculator group | Primary sources |
|---|---|
| Take-home pay, W-4 withholding, bonus tax, charitable giving | IRS tax tables and standard deductions (Rev. Proc. 2025-32); Social Security Administration wage base announcement for the current year |
| Compound interest, savings goal, CD interest, APY vs APR | FDIC national deposit rate data; standard compound interest and effective annual rate formulas |
| Loan, mortgage payoff, amortization, auto loan | Freddie Mac Primary Mortgage Market Survey; Experian State of the Automotive Finance Market; standard amortization formula |
| Inflation, net worth, retirement withdrawal, FIRE | Bureau of Labor Statistics CPI-U; Federal Reserve Survey of Consumer Finances; long-run real return assumptions stated on each page |
| 50/30/20 budget, holiday budget, rent vs buy | The original 50/30/20 allocation framework; National Retail Federation holiday spending data; housing cost indexes from the Federal Housing Finance Agency |
| Credit card minimum payment, debt payoff, hourly to salary | Issuer disclosure style minimum payment formulas; Federal Reserve consumer credit data; Fair Labor Standards Act definitions for the hourly conversion |
Before a page goes live, its default scenario is recalculated from scratch outside the browser. The formula is implemented a second time in Python, and the JavaScript that ships on the page is executed and compared against it. The published page is only released when both agree to the cent. Edge cases get the same treatment, including zero interest, zero down payment, a single payment term, and the extremes of each input range.
Where a page presents a comparison, such as a term table on the amortization calculator or the credit tier spread on the auto loan calculator, every row in that table is part of the check. Numbers in the explanatory text come from the same verified run, which is why a figure quoted in a paragraph always matches the figure in the table below it.
Results are estimates built on assumptions that are stated on the page. Federal tax calculations ignore state and local tax, most credits, and phase-outs, so a real paycheck can land lower. Loan calculations assume a fixed rate and no late payments. Investment projections show a constant return, while real markets deliver a sequence of good and bad years that can change the outcome for the same average. Treat any multi-decade figure as a scenario, not a forecast.
When a formula, a default, or a tax figure is wrong, we fix it and update the review date at the top of the affected page. Reported errors are welcome at [email protected] or through the contact page. If you find a result that disagrees with your own arithmetic, send the inputs you used and we will publish the correction if we are wrong. The full rules for sourcing, review, and correction, including what this site will not publish, are in the editorial policy.
Tax parameters are refreshed when the IRS and the Social Security Administration publish the following year's figures, usually in the fall. Rate-based default values are reviewed against their source each quarter. Every calculator page carries a visible review date, so you can judge how current it is before you rely on it. The CD interest calculator and the APY vs APR calculator are the two pages most sensitive to rate changes and are checked first each cycle.
The site is built and maintained by Yongwen Wu, who has worked in data and systems for more than twenty years. The about page has the short version. Calculators are built with the same habit that governed that work: write down the assumption, show the formula, and check the arithmetic twice before anyone relies on it.