Compound Interest Calculator
See what your savings turn into over time — with regular contributions, any compounding frequency, and an honest year-by-year view of how much of the growth is interest.
Fill in the fields to see your result.
Year-by-year growth
What you add each year, what the interest adds, and the balance at the end of every year. Watch the interest column overtake your contributions.
What compound interest actually is
Simple interest pays you on the money you put in. Compound interest pays you on the money you put in and on the interest you've already earned. That second part is small at first and enormous later, which is why the year-by-year table above looks almost flat for the first few years and then starts to climb steeply.
Put another way: the interest starts earning its own interest. Give it enough years and the growth stops being about what you contribute and starts being about what the balance is already generating on its own.
The formula
For a lump sum with no contributions:
Adding regular contributions turns it into an annuity calculation on top. This calculator works it out period by period instead, which handles any combination of contribution frequency and compounding frequency — and lets it show you the real year-by-year table rather than just a final figure.
A worked example
5,000 to start, 300 a month added, at 7% a year compounded monthly, for 20 years:
- Your 5,000 grows to about 20,194
- Your contributions grow to about 156,278
- Final balance: roughly 176,472
- You put in 77,000 — so about 99,472 is interest
Interest is 56% of the final balance. More than half of that money was never yours to begin with; it was earned by money that was itself earned.
Why starting early beats saving more
Time is the variable compound interest is most sensitive to, and it isn't close. Run these two in the calculator:
| Saver | Contributes | For | Roughly ends with (7%) |
|---|---|---|---|
| Starts at 25 | 200 a month | 40 years | about 525,000 |
| Starts at 35 | 400 a month | 30 years | about 490,000 |
The second saver puts in twice as much every month and still ends up behind. The ten extra years did more work than doubling the contribution. This is the single most useful thing a compound interest calculator can show you — and you can check both of those rows yourself in the tool above.
Compounding frequency matters less than you'd think
Daily compounding sounds much better than annual, and it is better — but only slightly. On 10,000 at 5% for 10 years, annual compounding gives about 16,289 and daily gives about 16,487. That's a difference of roughly 1.2% over a decade. The rate and the number of years matter far more, so don't chase compounding frequency at the expense of a better rate.
Things this calculator doesn't know about
- Inflation. The figures above are in today's numbers, not adjusted for what money will be worth later. A rough guide: subtract your local inflation rate from the interest rate to see the result in real terms.
- Tax. Interest and investment returns are taxable in most countries, though often not inside a pension or a tax-free savings wrapper.
- Fees. An investment platform charging 1% a year takes a bigger bite than it sounds like — try running your rate with 1% subtracted and compare.
- Variable returns. Real investments don't return a smooth 7% every year. This shows an average; the actual path will be bumpy.
Frequently asked questions
What interest rate should I use?
For a savings account, use the AER or APY your bank quotes — that figure already accounts for compounding. For investments there's no guaranteed rate, so people usually model a long-run average: something in the 5–8% range before inflation is a common assumption for a diversified stock portfolio. Whatever you choose, treat it as a projection, not a promise.
Is APY the same as the interest rate?
Not quite. The nominal rate is the headline figure; the APY (or AER) is what you actually earn once compounding is accounted for, so it's slightly higher. If your bank gave you an APY, enter it and set compounding to 'once a year' — that avoids counting the compounding twice.
When are my contributions added?
At the end of each period, after any interest for that period has been credited. That's the conservative assumption, and it matches how most banks and pension schemes actually work. If yours credits contributions at the start of the period, your real balance will be slightly higher than shown here.
Does this account for inflation or tax?
No — the numbers are in today's money and before tax. For a rough real-terms view, subtract your country's inflation rate from the interest rate and run it again. Tax depends entirely on where you are and what kind of account you're using, so it's left out deliberately rather than guessed at.
Why does the balance barely move in the early years?
Because in year one the interest is earned on a small balance, and there's no accumulated interest yet to earn interest of its own. Compounding is exponential, and exponential curves look almost flat at the start. The year-by-year table shows exactly when the interest column starts outgrowing what you add — and for most realistic inputs that's a genuinely surprising moment.
Is any of this financial advice?
No. This is a maths tool, not a recommendation. It shows what a given rate would produce over a given time — it can't tell you whether a product is suitable for you, what risk you're taking, or what the returns will actually be. For decisions that matter, talk to a qualified adviser.
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Please note: CalcuLane gives estimates for general information only and is not financial advice. Lenders round differently, and fees, insurance and payment-date rules can change what you actually pay. Always confirm figures with your lender before committing to a loan.