Investor Wisdom Daily’s calculators are educational tools that show how money questions like taxes, compound growth and loan payments are worked out. They are estimates, not tax, legal or financial advice, and they don’t replace a tax return, a loan offer or a professional.
How the calculators compute
You enter a few numbers, the page applies a published formula or table to them, and the result appears straight away.
- Income tax. Your income, minus any pretax contributions and the standard deduction, gives taxable income. That amount is split across the federal brackets, each slice is taxed at its own rate, and the slices are added up. Optional payroll tax applies Social Security tax up to the yearly wage base, plus Medicare tax and Additional Medicare tax above the legal thresholds.
- Compound interest. Each month, that month’s deposit goes in first and then the whole balance grows at the monthly rate. Optional settings take an estimated tax off each year’s gains and show the result in today’s dollars.
- Mortgage. The monthly principal-and-interest payment uses the standard amortization formula. Property tax and insurance are your yearly figures divided by 12. PMI, when it applies, is your PMI rate applied to the original loan.
- Roth IRA. Contributions grow at your assumed yearly return until retirement and are compared with the same saving in a taxable account.
Where a tool offers “Show the math”, it lists each step and its formula so you can check the arithmetic.
Where the figures come from
Tax and limit figures are kept in one data file, organised by tax year, with the sources for each year listed. They come from primary releases:
- IRS. Federal brackets and the standard deduction for 2026 come from Revenue Procedure 2025-32. IRA and 401(k) contribution limits and Roth IRA income phase-out ranges come from the IRS’s annual retirement-limits release (IR-2025-111 for 2026). Social Security and Medicare tax rates come from IRS Topic 751. The IRS publishes new figures each year, usually in October and November.
- Social Security Administration (SSA). The Social Security wage base, the most earnings that are subject to Social Security tax ($184,500 for 2026), is announced once a year in October.
- FRED (Federal Reserve Bank of St. Louis). Live market defaults come from FRED data series:
- 30-Year Fixed Rate Mortgage Average (MORTGAGE30US), from Freddie Mac’s weekly survey. It updates weekly, usually on Thursdays, and is the starting rate in the mortgage calculator.
- Consumer Price Index for All Urban Consumers (CPIAUCSL), from the Bureau of Labor Statistics, monthly. Inflation is the 12-month change in this index, and it is the starting inflation rate in the compound interest calculator.
- The homepage market snapshot also uses the 10-year Treasury yield (DGS10) and S&P 500, Dow and Nasdaq closes (SP500, DJIA, NASDAQCOM), which update daily on business days, and the federal funds rate (FEDFUNDS), which updates monthly.
The site checks FRED for new values twice a day. Each value is range-checked before use. If a fetch fails, the last good value is kept and flagged as stale. The homepage marks a stale value as possibly out of date, and the calculators skip it and use their built-in starting figures instead. Each live default shows its “as of” date.
Under each calculator, a short line lists its sources and the date its figures were last checked.
What is simplified or left out
The main simplifications are also stated on each tool’s page:
- Income tax covers wages under the standard deduction only. It leaves out tax credits (for example the child tax credit and education credits), itemized deductions, capital gains and qualified dividends, self-employment tax, the alternative minimum tax, and the new deductions for tips, overtime and seniors (2025 to 2028). State tax is a single flat rate that you enter, which is a rough placeholder. For married couples, the payroll estimate assumes one earner.
- Compound interest and Roth IRA assume a smooth, steady return, and real markets are not smooth. Tax on a taxable account is approximated as a yearly charge on that year’s gains. In practice, a lot of tax is owed only when you sell, so the comparison is an illustration and not a promise. The Roth IRA tool does not check whether your income allows you to contribute.
- Mortgage counts PMI until the balance reaches 80% of the home price. Under the usual rules, PMI ends automatically at 78% of the original value, and your servicer’s terms are what apply. Actual escrow amounts vary.
- Other calculators disclose their own simplifications. For example, the rent-vs-buy tool does not add PMI to the buying path.
Your numbers stay on your device
All the math runs in your web browser. Nothing you type is sent to us or to anyone else, and nothing is stored on our servers. A shared link or a saved scenario holds your inputs only in that link or in your own browser.
Reporting a wrong number
If a figure looks wrong, please tell us through the Contact page. The “Report a wrong number” link under each calculator goes there as well. Please include the calculator’s name, the numbers you entered and what you expected. We check each report against the original source, correct confirmed errors and log every correction.
How we check the numbers
Each calculator’s results are tested against independent, worked calculations, and those tests must pass before any change goes live. When the IRS or SSA publishes new figures, we update the data file and check each calculator against the new release. The date under each calculator shows when its figures were last checked.
The calculators are not yet reviewed by a named, credentialed financial professional. When a reviewer joins, their name, credentials and the date of their last review will appear here.