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 (DRIP), tax-loss, capital gains tax, Roth IRA, 401(k), millionaire, FIRE, rule of 72, CAGR, savings, stock average and position size calculators (including its risk/reward section) are recomputed by separate Python programs (decimal or exact-fraction arithmetic; for tax-loss, the IRS carryover worksheet line by line; for capital gains, the IRS Qualified Dividends and Capital Gain Tax Worksheet line by line), and the tests require both to agree.
• Exact share counts. The position size calculator rounds shares down using exact decimal arithmetic, so binary rounding can never turn 100 shares into 99.
• Transparency. Each page lists its formula, every assumption we make, and worked examples you can load with one click. The examples show enough digits that you can redo the arithmetic yourself and get the same result to the cent.
• Every published number is re-checked. Before each release an audit script recomputes every worked-example and FAQ figure on the built pages with independent decimal arithmetic and fails the build check if any number on a page doesn't match.

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 and nothing you enter is sent to us. The site uses Cloudflare Web Analytics, a privacy-focused analytics service that doesn't use cookies, to see aggregated visit counts; there are no ads or tracking cookies. The full details are on the privacy page; how the site is (and isn't) paid for is on the disclosure page.

Corrections

Accuracy matters more than anything else here. If you think a number, formula or explanation is wrong, please report it. Reports are checked against the formulas and sources shown on the page.

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.