Compound Interest Calculator
See how a starting balance plus regular contributions grows with compound interest over time.
Want to understand the concept, not just the number? Read The Compound Interest Guide and more below.
| Year | Start | Contributions | Growth | End |
|---|---|---|---|---|
| 1 | $10,000 | $2,400 | $801 | $13,201 |
| 2 | $13,201 | $2,400 | $1,033 | $16,634 |
| 3 | $16,634 | $2,400 | $1,281 | $20,315 |
| 4 | $20,315 | $2,400 | $1,547 | $24,262 |
| 5 | $24,262 | $2,400 | $1,832 | $28,495 |
| 6 | $28,495 | $2,400 | $2,138 | $33,033 |
| 7 | $33,033 | $2,400 | $2,466 | $37,900 |
| 8 | $37,900 | $2,400 | $2,818 | $43,118 |
| 9 | $43,118 | $2,400 | $3,196 | $48,714 |
| 10 | $48,714 | $2,400 | $3,600 | $54,714 |
| 11 | $54,714 | $2,400 | $4,034 | $61,147 |
| 12 | $61,147 | $2,400 | $4,499 | $68,046 |
| 13 | $68,046 | $2,400 | $4,998 | $75,444 |
| 14 | $75,444 | $2,400 | $5,532 | $83,376 |
| 15 | $83,376 | $2,400 | $6,106 | $91,882 |
| 16 | $91,882 | $2,400 | $6,721 | $101,003 |
| 17 | $101,003 | $2,400 | $7,380 | $110,783 |
| 18 | $110,783 | $2,400 | $8,087 | $121,270 |
| 19 | $121,270 | $2,400 | $8,845 | $132,515 |
| 20 | $132,515 | $2,400 | $9,658 | $144,573 |
Range of outcomes
The projection above uses a single steady return, so it draws one smooth line. Real markets rise and fall from year to year, and the order of those swings changes where you end up. This simulation runs 500 separate paths, giving each year a random return centered on your expected rate, then sorts the results from worst to best.
The three figures below are the outcomes at the 10th, 50th, and 90th percentiles. The median is the middle result, with half of the paths above it and half below. It usually lands under the smooth projection, because market swings drag on compound growth. Past performance never guarantees future returns, so treat this as a range to plan around rather than a promise.
Simulating a range of outcomes...
How it's calculated
Each month your balance earns one twelfth of the annual rate, and then your contribution is added. The next month compounds on the larger balance, so growth builds on growth. Over long periods this is what separates compound interest from a simple straight line.
As an example, $10,000 left to grow at a 7% annual return with monthly compounding becomes $20,096.61 after 10 years. It roughly doubles with no new money added. Put in a monthly contribution as well and the curve bends up faster, because every dollar you add starts compounding from the day it lands.
The longer your time horizon, the more of your ending balance comes from growth rather than the amount you put in. Small changes to the rate or the number of years move the final number a lot, so it is worth trying a few combinations.
Assumptions
- Interest compounds monthly, and contributions are added at month end.
Last updated: 2026-08-07
These assumptions follow our general methodology.
Frequently asked questions
What is the difference from simple interest?
Simple interest is paid only on the original principal. Compound interest is paid on the principal plus all previously earned interest, which is why the curve steepens over time.
How often does interest compound here?
Monthly, which is the default across this site. More frequent compounding produces a slightly higher balance than annual compounding, and it is closer to how most real accounts credit growth.
What rate should I use?
For a savings account, use its stated APY. For long-term investing, many people model a diversified return of around 7% a year after inflation, but past performance does not guarantee future results, so try a range.
What does the Monte Carlo panel add?
The main result uses one steady rate, so it draws a smooth curve. The Monte Carlo panel runs hundreds of paths where each year gets a random return around your rate, then shows the 10th, 50th, and 90th percentile balances. It turns a single number into a realistic range, which matters more the longer you invest.
Does inflation reduce these returns?
It does. A 7% return with 3% inflation is closer to 4% in real buying power. To see what a future balance is worth in today money, use the Inflation calculator, and consider entering an inflation-adjusted return here.