About Math for Money

Math for Money is a small, independent website of free personal finance calculators. Each one answers a single money question, shows its working, and runs entirely in your browser.

What you will find here

• Compound Interest Calculator: See how a starting balance and monthly contributions grow over time, with a year-by-year split between what you put in and what you earned.
• Savings Goal Calculator: Find out how much to save each month to reach a target amount by a deadline, given what you have saved so far and the rate you expect to earn.
• FIRE Calculator: Find your FIRE number from your annual spending and withdrawal rate, and see how many years of saving and growth it takes to reach it.
• Investment Fee Calculator: Compare your ending balance with and without an annual fund or advisory fee, and see how much the fee costs you over time.
• CAGR Calculator: Turn a starting value, an ending value and a number of years into a compound annual growth rate, with total return, gain and growth multiple.

Amounts are shown in US dollars, but the math works the same for any currency.

Who it is for

The calculators are for anyone who wants a quick, clear estimate without a spreadsheet: people saving toward a goal, people comparing "what if" scenarios, and people learning how concepts such as compound interest work. You do not need a finance background. Each page explains its terms in plain English.

How the calculators are built and tested

• Published formulas. Every calculator page explains how its result is worked out, and the methodology page lists the exact formulas, assumptions and limitations.
• Automated tests. The calculation code is separate from the page design and is covered by automated tests that compare its output with known formula results and edge cases such as a zero rate.
• One source for every number. Example figures in the text are produced by the same calculation code as the interactive tool, not typed in by hand, so the explanation and the calculator cannot drift apart.
• Runs on your device. The pages are static files. Calculations happen in your browser as you type, and there is no account or sign-up. See the privacy page for details.

Independent and free

The site is free to use. It does not sell financial products, does not recommend specific investments, banks or brokers, and is not affiliated with any financial institution. There is no advertising on the site at present. If advertising is added later to cover running costs, the privacy page will be updated first.

Not financial advice

The results are estimates for education and planning. They assume the inputs you enter hold steady for the whole period, which real savings rates and investment returns do not, and most of them leave out fees, taxes and inflation. They cannot take your personal circumstances into account. For decisions that matter, consider speaking with a qualified financial professional.

Found a mistake?

Corrections are welcome. If a result looks wrong or an explanation is unclear, please get in touch.