Compound Interest Calculator
With starting balance 10000 usd, monthly contribution 500 usd, annual return 7 percent, years 20 years and 2 more fields, compound interest comes to $300,850.72 — balance at the end. It is reached in 8 steps, the last of which is 10000 * 4.0387388 + 500 * ((4.0387388 - 1) / 0.0058333), and each one is printed on the page with its numbers filled in. The formula is the one published by SEC Investor.gov, not an approximation fitted to it.
Future value of a starting balance plus monthly contributions, separating what you put in from what the interest earned.
Formula and sources checked · How we check
Starting balance 10000, Monthly contribution 500, Annual return 7, Years 20 years
$300,850.72
Balance at the end for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Effective annual yield
(1.0722901 - 1) * 1007.229 %- Balance at the end
10000 * 4.0387388 + 500 * ((4.0387388 - 1) / 0.0058333)300,850.718- Total you contributed
10000 + 500 * 240130,000- Earned by compounding
300850.72 - 130000170,850.718- Balance in today's money
300850.72 / pow(1 + 3 / 100, 20)$166,574- Tax on the gain, if it is taxable
170850.72 * 0 / 100$0- Balance after that tax
300850.72 - 0$300,851- After tax, in today's money
300850.72 / pow(1 + 3 / 100, 20)$166,574
Ask about this in the chatCompare: 20 years against 30Compound Growth Chart
Where the final balance came from
Balance year by year
Contributions are a straight line; the balance is not. The gap between them is compounding, and it widens every year.
| Year | Contributed | Interest | Balance |
|---|---|---|---|
| Year 1 | $6,000.00 | $919.19 | $16,919.19 |
| Year 2 | $6,000.00 | $1,419.38 | $24,338.58 |
| Year 3 | $6,000.00 | $1,955.73 | $32,294.31 |
| Year 4 | $6,000.00 | $2,530.85 | $40,825.16 |
| Year 5 | $6,000.00 | $3,147.55 | $49,972.70 |
| Year 6 | $6,000.00 | $3,808.82 | $59,781.53 |
| Year 7 | $6,000.00 | $4,517.90 | $70,299.43 |
| Year 8 | $6,000.00 | $5,278.24 | $81,577.68 |
| Year 9 | $6,000.00 | $6,093.55 | $93,671.22 |
| Year 10 | $6,000.00 | $6,967.79 | $106,639.02 |
| Year 11 | $6,000.00 | $7,905.24 | $120,544.25 |
| Year 12 | $6,000.00 | $8,910.45 | $135,454.70 |
| Year 13 | $6,000.00 | $9,988.32 | $151,443.02 |
| Year 14 | $6,000.00 | $11,144.12 | $168,587.14 |
| Year 15 | $6,000.00 | $12,383.47 | $186,970.62 |
| Year 16 | $6,000.00 | $13,712.41 | $206,683.03 |
| Year 17 | $6,000.00 | $15,137.43 | $227,820.45 |
| Year 18 | $6,000.00 | $16,665.45 | $250,485.91 |
| Year 19 | $6,000.00 | $18,303.94 | $274,789.85 |
| Year 20 | $6,000.00 | $20,060.87 | $300,850.72 |
The table above groups the 240 months into 20. The amortisation schedule prints every one of them, with the split between interest and principal.
Worked example
$10,000 plus $500/mo at 7% reaches $300,851 in 20 years. You contributed $130,000 of that; the other $170,851 is compounding. Extend to 30 years and it more than doubles again to $691,151, while contributions only rise to $190,000.
How to work it out yourself
- 1.Start with the balance you have and the amount you will add each month.
- 2.Divide the annual return by 1,200 for the monthly rate, and multiply the years by 12 for the number of periods.
- 3.The starting balance grows by (1 + rate)^periods; the contributions grow by ((1 + rate)^periods − 1) ÷ rate.
- 4.Subtract everything you put in from the final balance to see what compounding actually contributed.
$10,000 plus $500 a month at 7% over time
| Years (years) | Balance at the end | Total you contributed | Earned by compounding |
|---|---|---|---|
| 5 | $49,972.70 | 40,000 | 9,972.7 |
| 10 | $106,639.02 | 70,000 | 36,639.02 |
| 15 | $186,970.62 | 100,000 | 86,970.62 |
| 20 | $300,850.72 | 130,000 | 170,850.72 |
| 25 | $462,290.03 | 160,000 | 302,290.03 |
| 30 | $691,150.47 | 190,000 | 501,150.47 |
| 35 | $1,015,588.82 | 220,000 | 795,588.82 |
| 40 | $1,475,520.81 | 250,000 | 1,225,520.81 |
Contributions rise in a straight line; the interest column is where the curve is.
The formula
- Effective annual yield
(1.0722901 - 1) * 100 - Balance at the end
10000 * 4.0387388 + 500 * ((4.0387388 - 1) / 0.0058333) - Total you contributed
10000 + 500 * 240 - Earned by compounding
300850.72 - 130000 - Balance in today's money
300850.72 / pow(1 + 3 / 100, 20) - Tax on the gain, if it is taxable
170850.72 * 0 / 100 - Balance after that tax
300850.72 - 0 - After tax, in today's money
300850.72 / pow(1 + 3 / 100, 20)
Source: SEC Investor.gov — compound interest, Federal Reserve — savings and investing data
Questions people actually ask
- Is 7% a realistic return?
- It is the common shorthand for long-run US equity returns after inflation — roughly 10% nominal minus 3% inflation. It is an average across decades, not a rate any single year delivers.
- Why does the last decade matter more than the first?
- Compounding is exponential, so growth is proportional to the balance. In the example above, years 21–30 add $390,000 to the balance while years 1–10 add $97,000, from identical contributions.
- Does this account for taxes and inflation?
- No. It returns a nominal, pre-tax figure. For purchasing power in today’s dollars, enter a real return — the nominal rate minus expected inflation.
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