Enter your starting amount, monthly contribution and expected return — then watch compounding do the heavy lifting, year by year. Results update live as you type.
Compounding frequency: monthly. Results update as you type.
| Year | Balance | Interest earned |
|---|
Compound interest means you earn returns not just on the money you put in, but on the returns those returns already generated. In year one, a 7% return on $10,000 adds $700. By year twenty, that same 7% applies to a balance swollen by two decades of growth — adding thousands per year without you lifting a finger.
With monthly contributions, the future value is:
FV = P(1+i)N + PMT × (((1+i)N − 1) / i)
Where P is your initial principal, i the monthly rate (annual ÷ 12), N the total number of months, and PMT your monthly contribution. This calculator compounds monthly, the most common frequency for savings accounts and investment plans.
Want to do it by hand once? See how to calculate compound interest manually — formula, worked examples, and the Rule of 72.
Try it: $500/month for 30 years at 7% grows to roughly $566,000. Start 10 years later with the same $500/month and you reach only about $244,000 — less than half, despite contributing for 20 years instead of 30. Time is the most powerful input in this formula, which is why the best day to start was ten years ago and the second-best day is today.
Compound interest is interest calculated on the initial principal plus all previously accumulated interest. It causes wealth to grow exponentially rather than linearly over time.
Future value = P(1+i)^N + PMT×(((1+i)^N − 1)/i), where P is initial principal, i is the monthly interest rate, N is total months, and PMT is the monthly contribution.
Starting early almost always wins. Because compounding is exponential, money invested 10 years earlier can roughly double the final result even with smaller monthly contributions.
For long-term stock investing, 7% annual (after inflation) is a common planning assumption. For savings accounts, use 2–4%. Always run a conservative scenario alongside an optimistic one.