Compound Interest Calculator
Compounding turns steady contributions into a much larger sum over decades. The table shows how the interest share accelerates.
$691,150 future value
- You contribute
- $190,000
- Interest earned
- $501,150
| Year | Balance | Contributed | Interest |
|---|---|---|---|
| 2 | $24,339 | $22,000 | $2,339 |
| 4 | $40,825 | $34,000 | $6,825 |
| 6 | $59,782 | $46,000 | $13,782 |
| 8 | $81,578 | $58,000 | $23,578 |
| 10 | $106,639 | $70,000 | $36,639 |
| 12 | $135,455 | $82,000 | $53,455 |
| 14 | $168,587 | $94,000 | $74,587 |
| 16 | $206,683 | $106,000 | $100,683 |
| 18 | $250,486 | $118,000 | $132,486 |
| 20 | $300,851 | $130,000 | $170,851 |
| 22 | $358,760 | $142,000 | $216,760 |
| 24 | $425,345 | $154,000 | $271,345 |
| 26 | $501,905 | $166,000 | $335,905 |
| 28 | $589,934 | $178,000 | $411,934 |
| 30 | $691,150 | $190,000 | $501,150 |
How it works
The balance is compounded monthly; each month's contribution is added and then grows for every remaining month. Interest is the ending balance minus everything you put in.
Explain it to a kid
Imagine a money tree that grows a little bigger every single day — and the bigger it gets, the faster it grows. That's compound interest: your savings earn a little extra, and next year that extra earns more too. The earlier you start, the bigger your tree gets by the time you're grown up.
Frequently asked questions
How often does the balance compound?
Monthly. Each month's contribution is added, and the running balance earns 1/12 of the annual rate that month — interest earns interest going forward, which is what makes growth accelerate over time.
Why does the interest share grow so much in later years?
Early on, most of the balance is your own contributions. As years pass, a larger share of the balance is compounded interest on interest, so the same monthly contribution contributes a shrinking fraction of the total — the table's 'total interest' column shows this crossing over.
Does contribution timing matter?
Slightly. A contribution made at the start of the month earns a full month of return; one made at the end doesn't. Over many years the difference is small compared to the rate and time horizon, but it's part of why real-world results can differ a little from a simplified estimate.
What return rate should I use?
For a conservative estimate, many planners use a long-run inflation-adjusted stock market return of roughly 6–7%, or lower for a more cautious plan. Use a lower rate for cash-like savings and a higher one only if your investments genuinely target growth assets — see the Retirement Calculator for a full accumulation-and-drawdown projection.