About Compound Interest Calculator

Compound Interest Calculator is a free set of tools for seeing how money grows over time. Enter a starting balance, a monthly contribution, a rate, and a horizon, and every calculator here lands you on a populated result with a chart — not a blank form. The goal is simple: make the single most important idea in personal finance, how compound interest works, concrete enough to act on.

Who runs it

The site is built and maintained by Pond Software, an independent maker of small, focused web tools. It is not a bank, a broker, or a financial-advice service, and it does not sell a product or manage anyone's money. That independence is the point: there is no account to open and nothing being upsold behind the math.

What's here

Beyond the main compound interest calculator, the site covers the specific questions people actually search for — how long until a balance doubles, what it takes to reach $1 million, how a 401(k) or Roth IRA grows, and how inflation quietly eats into a projection. Each tool prefills a realistic scenario so you start from a worked example and adjust from there.

Editorial approach

Every calculator runs standard, transparent finance math — the future value of a series formula for contributions, effective-yield conversions for compounding frequency, and plain time-value-of-money discounting for inflation. Default rates lean conservative: the tools open near a 7% real return (roughly the S&P 500's long-run average after inflation) rather than a flattering headline number.

The written explanations use round, checkable dollar figures rather than invented statistics. Where a claim depends on an assumption — a return rate, an inflation figure, a compounding schedule — that assumption is stated inline so you can change it and see the result move. When we don't have a defensible number, we leave it out.

A note on advice

These tools are for education and planning, not personalized financial advice. Real returns are volatile, and no projection is a promise. For decisions that matter, run the numbers here to understand the shape of the problem, then talk to a qualified professional. See our terms of use for the full disclaimer.

Get in touch

Spotted a bug, a wrong number, or a calculator that would be useful? The contact page has an email link and we read everything that comes in.