Small, steady deposits plus time — that’s the whole engine. Set your starting amount, your recurring contribution, and a rate, then watch how fast interest starts doing the heavy lifting for you.
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Get it on Gumroad →Compound interest is interest earned on your interest. Your balance grows, then the next round of interest is calculated on that bigger balance — so growth speeds up over time instead of staying flat. It’s the difference between money sitting still and money snowballing: the longer it compounds, the steeper the curve gets.
A high-yield savings account (HYSA) pays a steady APY — around 4.5% in the presets above — with your principal protected and accessible anytime, which makes it a good home for an emergency fund. An index fund invests in the stock market and has historically averaged higher returns (the 8% preset), but the balance swings up and down along the way. Many people keep short-term safety money in an HYSA and invest long-term money in index funds.
Because compounding needs time more than it needs big deposits. Most of the growth in the chart above happens in the later years, when interest is being earned on a large balance. Starting with $50 a month at 25 can beat starting with $100 a month at 35 — every year you wait removes the most valuable compounding years at the end of the curve.
Disclaimer: These projections are estimates for education only and are not financial advice. The calculator assumes a constant rate of return compounded monthly (or weekly, matching your contribution frequency) with contributions made at the end of each period. Real savings account APYs change over time, and investment returns are never guaranteed — actual results will vary. Talk to a qualified financial professional before making decisions with your money.