About InvestorMath

InvestorMath is a small set of free calculators for beginner-to-intermediate long-term investors. The goal is simple: correct math, shown openly, so you can understand the numbers and check them yourself.

How we keep the math right

• One engine. Every calculator uses the same JavaScript module (/assets/js/calc.js). The page you see and the test suite run identical code.
• Automated tests check hand-computed results, closed-form formulas (such as the future value of an annuity), edge cases (0% rates, 0 years, very large values) and input validation.
• External cross-checks. The compound interest calculator matches the SEC's investor.gov calculator to the cent in every scenario we ran (table on the calculator page). The fee calculator reproduces the SEC's published fee example.
• Independent re-implementation. The dividend reinvestment, tax-loss, Roth IRA, millionaire, FIRE, rule of 72, CAGR and savings goal calculators are recomputed by separate Python programs (decimal arithmetic; for tax-loss, the IRS carryover worksheet line by line), and the tests require both to agree.
• Transparency. Each page lists its formula, every assumption we make, and worked examples you can load with one click.

What these calculators are not

They are educational tools that do arithmetic on the assumptions you provide. They don't predict returns, recommend investments, or give tax advice. Real markets don't grow at constant rates, and real tax returns involve many rules a simple estimate leaves out.

Privacy

All calculations run in your browser. Nothing you enter is sent to a server. Your inputs appear in the page URL only so you can share or bookmark a scenario. The only thing stored on your device is your light/dark theme choice (in localStorage). There are no analytics, ads or tracking cookies on this version of the site. The newsletter box is a placeholder: it isn't connected to anything and saves nothing.

Corrections

Accuracy matters more than anything else here. If you think a number is wrong, please tell us; contact details are coming soon.

Disclaimer: InvestorMath calculators give educational estimates, not financial, investment or tax advice. Results depend entirely on the assumptions you enter; real returns vary and are not guaranteed, and past performance does not predict future results. Check important decisions with a qualified professional.