Compound Interest Calculator
How much a regular saving habit turns into — and the year the interest starts adding more than you do.
Runs on your device — nothing you type is sent anywhereThis shows what a steady rate would produce. Real returns are not steady, and inflation, tax and fees all take a share that this does not model.
How it works
Three steps, no account, and nothing you type leaves your computer.
Put in what you have and what you add
A starting amount, a monthly contribution, or both. Everything recalculates as you type.
Look for the crossover
The table names the year the interest starts adding more than you do. That year is the whole reason to start early.
Take it with you
The year-by-year breakdown downloads as a PDF or a CSV you can open in a spreadsheet.
What people work out here
Whether starting now actually matters
The gap between starting at 25 and at 35 is far larger than ten years of contributions. Running both makes the size of it concrete.
What a small increase does
Fifty more a month looks trivial and compounds into something that is not. Change one figure and watch the end number move.
Comparing a saving rate honestly
A percentage point of return sounds minor. Over thirty years it is rarely minor.
Setting a target
Working backwards from a number you want, by adjusting the monthly figure until the total lands where you need it.
Why compounding feels slow and then does not
Compound interest is famous, badly explained, and genuinely counter-intuitive. The mechanism is simple enough to state in a sentence: interest is added to your balance, and from then on that interest earns interest too. What is hard to hold in your head is what that does over decades.
The shape of it
Put 10,000 in at 7% and add 200 a month for twenty years. You will have paid in 58,000 of your own money and ended with about 145,000. The other 87,000 was never yours to begin with — the balance produced it.
What is more telling is where that 87,000 came from. In the first year the balance earns 816. In the twentieth it earns 9,712 — from the same rate, applied to a balance that has grown. Nothing about the deal improved. The balance did.
This is why the bars in the chart above look almost flat for years and then climb steeply. It is not acceleration in the rate; it is the same percentage of a much larger number.
The crossover
There is a specific year in every saving plan that deserves more attention than it gets: the year the interest adds more than you do.
Before it, you are the engine — your contributions are doing most of the work and the balance is mostly what you have put in. After it, the balance is doing more than you are, and it keeps pulling further ahead for as long as you leave it alone.
On the figures above it happens in year seven — earlier than most people guess, because the starting balance is already working from day one. The calculator names the year for your own numbers, because it is the single most useful thing in the table and invisible in a column of totals.
Why starting early beats saving more
Two savers, both putting away 200 a month at 7%. One starts at 25 and stops at 35 — ten years, 24,000 in total. The other starts at 35 and continues to 65 — thirty years, 72,000, three times as much money.
At 65 the first saver has about 283,000 from 24,000 paid in. The second has about 245,000 from 72,000 — three times the money, thirty-eight thousand less to show for it. The early saver stopped three decades before the other one finished, and still came out ahead, because the last decade of compounding on a large balance outweighs the first decade of contributions to a small one.
The lesson is not that saving more does not help — it plainly does. It is that time is doing something contributions cannot buy back, which is why the advice is always to start, even small, rather than to wait until you can start properly.
How often interest is added matters less than you think
Compounding monthly rather than yearly does earn more, because interest starts earning sooner. It is a smaller effect than the arithmetic's reputation suggests.
On 10,000 at 7% for twenty years with nothing added, yearly compounding gives 38,697 and monthly gives 40,387. Real, worth having, and 4.4% — a rounding error next to the difference an extra percentage point of return, or five more years, would make. Do not agonise over the frequency; the rate and the term are where the money is.
What this does not account for
A calculator applies a steady rate. Nothing in the real world grows steadily, and three things quietly take a share:
- Inflation. Growing at 7% while prices rise 3% is really growing at about 4% in what the money buys. Over thirty years that difference is enormous.
- Tax. Depending on where you are and the account you use, some of the gain is not yours.
- Fees. A 1% annual fee does not cost 1%; it compounds against you exactly as the returns compound for you.
A useful habit is to run the figures twice: once at the return you expect, and once a few points lower. If the plan only works at the optimistic number, it is not really a plan.
This is arithmetic, not advice. It shows what a given set of assumptions implies. Whether those assumptions are reasonable for your situation is a separate question, and for serious sums it is one worth putting to somebody qualified.
Nothing you type is sent anywhere
Balances, contributions and rates are all worked out in your browser. There is no server, no account and no logging, which for a page where people type what they have saved seems like the obvious arrangement.
Common questions
Is anything I type stored or sent anywhere?
No. The calculation, the chart and the PDF are all produced in your browser. There is no server and nothing is logged. Disconnect from the internet after the page loads and it still works.
What is the crossover year?
The year the interest adds more to your balance than your own contributions do. Before it you are doing most of the work; after it the balance is. The calculator names it for your figures because it is the most useful line in the table and invisible in a column of totals.
Does it matter whether interest is added monthly or yearly?
A little. On 10,000 at 7% over twenty years, monthly compounding beats yearly by roughly 4%. Real, but small next to what an extra percentage point of return or five more years would do. The rate and the term matter far more than the frequency.
Why does starting early matter so much?
Because the last decade of compounding happens on the largest balance. Someone who saves for ten years and stops early can finish close to, or ahead of, someone who saves three times as much starting ten years later. Time does something contributions cannot buy back.
Does this account for inflation?
No. It shows nominal growth. If your money grows 7% while prices rise 3%, what it buys is growing at about 4%. Over long periods that gap is enormous, so it is worth running the figures at a lower rate as a sanity check.
What about tax and fees?
Neither is modelled, and both are real. Fees are the sneakier of the two: a 1% annual fee compounds against you exactly as returns compound for you, so over decades it costs far more than 1% of the final balance.
Is this financial advice?
No. It is arithmetic showing what a set of assumptions implies. Whether those assumptions suit your circumstances is a different question, and for large sums it is worth asking somebody qualified.
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