Compound Interest Calculator
Compounding is what happens when the interest your money earns starts earning interest of its own. Put a balance in, add to it every month, and this works out what it becomes — and, more usefully, how much of that total you put there yourself and how much the growth did. No account needed, and nothing you type is sent anywhere.
| Starting Balance: | |
| Monthly Contribution: | |
| Annual Return*: |
% |
| Years: |
| Year | You Put In | Growth Earned | Balance |
|---|---|---|---|
| 1 | $3,400 | $151 | $3,551 |
| 2 | $5,800 | $486 | $6,286 |
| 3 | $8,200 | $1,019 | $9,219 |
| 4 | $10,600 | $1,764 | $12,364 |
| 5 | $13,000 | $2,736 | $15,736 |
| 6 | $15,400 | $3,952 | $19,352 |
| 7 | $17,800 | $5,430 | $23,230 |
| 8 | $20,200 | $7,188 | $27,388 |
| 9 | $22,600 | $9,246 | $31,846 |
| 10 | $25,000 | $11,627 | $36,627 |
| 11 | $27,400 | $14,353 | $41,753 |
| 12 | $29,800 | $17,450 | $47,250 |
| 13 | $32,200 | $20,944 | $53,144 |
| 14 | $34,600 | $24,864 | $59,464 |
| 15 | $37,000 | $29,241 | $66,241 |
| 16 | $39,400 | $34,109 | $73,509 |
| 17 | $41,800 | $39,501 | $81,301 |
| 18 | $44,200 | $45,457 | $89,657 |
| 19 | $46,600 | $52,017 | $98,617 |
| 20 | $49,000 | $59,224 | $108,224 |
| 21 | $51,400 | $67,126 | $118,526 |
| 22 | $53,800 | $75,773 | $129,573 |
| 23 | $56,200 | $85,218 | $141,418 |
| 24 | $58,600 | $95,520 | $154,120 |
| 25 | $61,000 | $106,740 | $167,740 |
| 26 | $63,400 | $118,944 | $182,344 |
| 27 | $65,800 | $132,204 | $198,004 |
| 28 | $68,200 | $146,597 | $214,797 |
| 29 | $70,600 | $162,203 | $232,803 |
| 30 | $73,000 | $179,111 | $252,111 |
* Growth compounds monthly at the nominal annual rate; contributions are added at the end of each month. Actual investment returns vary year to year — this projection assumes a steady rate.
How to read the result
The number most people look at first is the ending balance. The one worth looking at is the split beneath it: total contributions against growth. Early on, almost everything in the balance is money you deposited. The point at which growth overtakes contributions is the moment compounding starts doing the work for you, and how soon it arrives depends far more on time than on the rate.
That is the counter-intuitive part. Chasing a slightly higher return moves the outcome much less than starting earlier does, because a contribution made in year one has every remaining year to compound, while one made in the final year has none. It is also why the years you leave the balance alone matter as much as the years you are adding to it.
What this calculator assumes
Interest is compounded monthly at the annual rate you enter, and contributions land at the end of each month — so a month's deposit earns nothing in the month it is made. That is the conservative convention, and the same one the retirement projections use, so the figures here and elsewhere on the site describe the same arithmetic.
The rate you type is a nominal rate, not a real one. Nothing here is deducted for inflation, tax or fund fees, so the ending balance is in future dollars rather than today's. Over a few years that distinction is small; over thirty it is most of the answer. Two other tools cover the pieces this one leaves out:
- Inflation impact — what a future sum is worth in today's money, which is the honest way to read a long projection.
- Retirement planning — the same compounding run in real terms and against randomly drawn returns, rather than one smooth rate every year.
A steady rate every year is also a simplification. Real returns arrive unevenly, and the order they arrive in changes the outcome once you start withdrawing — which is exactly what a single average rate cannot show you.
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