Free money tools
Compound Interest Calculator
See what a small, steady habit turns into over the years. Pick an amount to set aside each week, choose how long you'll keep it up, and watch compound interest do the heavy lifting.
Free to use. Everything runs in your browser — nothing you type is sent anywhere.
How it works
Compound interest is what happens when your money earns a return — and then that return starts earning a return too. Each year you don't just grow your contributions; you grow the growth from every year before it. Given enough time, that snowball gets surprisingly large.
This calculator works with three numbers:
- How much you save each week — a small, repeatable amount you can actually stick to, like the price of a couple of coffees.
- How long you keep it up — your time horizon. Compounding rewards patience, so the longer the runway, the more dramatic the curve.
- Your estimated annual return — the average yearly growth rate you expect. A long-term, broadly diversified stock portfolio has historically averaged somewhere around 6–7%, though real returns bounce around year to year and the future isn't guaranteed.
The result splits into two parts: the money you actually put in, and the growth stacked on top. The further out you look, the bigger that growth slice becomes — that's the whole point of starting early. Saving as a team? Pool what the two of you can set aside each week and run it together — Merger is built for two people sharing a life, and it works just as well on your own.
A worked example
Say you tuck away $25 a week — about $108 a month — and earn a steady 5% a year. After 10 years you'd have contributed roughly $13,000 of your own money, but the balance could grow to around $17,000. Stretch the same habit to 30 years and your ~$39,000 of contributions could grow to well over $90,000 — most of that being growth, not deposits. That's compounding rewarding you for showing up consistently.
Key terms
- Compound interest
- Earning a return on both your original contributions and on the returns they've already produced. Because last year's growth earns its own growth this year, your balance accelerates over time rather than growing in a straight line.
- Rate of return
- The average yearly growth rate you expect on your money, shown as a percentage. It's an estimate, not a promise — real-world returns rise and fall from year to year, so treat it as a planning assumption.
- Contribution
- The amount you add yourself — here, the steady weekly sum you set aside. Your total contributions are the deposits you made; everything above that is growth.
- Time horizon
- How long you let the money grow before you need it. Time is compounding's most powerful ingredient: a few extra years near the end can add more than the early years did.
Frequently asked questions
How does compound interest actually grow my money?
Each year my balance earns a return, and the next year that return earns a return too. So I'm not just growing my deposits — I'm growing the growth from every previous year. Early on it feels slow, but over time the curve gets steeper because there's more money working for me.
What return should I assume in the calculator?
It depends on where my money sits. Cash and savings accounts earn far less than a diversified stock portfolio, which has historically averaged around 6–7% a year over the long run. I'd pick a rate that matches my plan, knowing real returns bounce around year to year and nothing is guaranteed.
Is saving a small amount each week even worth it?
Yes — that's the whole point. A modest, consistent habit I can actually keep beats a big amount I can't sustain. Because compounding rewards time more than size, starting now with a little usually beats waiting until I can save a lot.
Why is starting early such a big deal?
The money I invest today has the most years to compound, so it grows the most. Waiting even a few years can cost me more than it seems, because I lose the years where the snowball would have been biggest. Time is the one input I can't get back later.
Does this account for inflation or taxes?
No — it shows the nominal future value before inflation and taxes. Inflation means a future dollar buys a bit less than it does today, and taxes can take a slice of the growth depending on the account. I treat the result as an optimistic, simplified estimate rather than an exact forecast.
Can my partner and I plan this together?
Definitely. We can pool what we're each able to set aside and run it as one weekly habit, or each of us can try our own number. Merger is built for two people sharing a life — and it works just as well if I'm doing this solo.
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