Compound interest calculator
What your investment will be worth, with monthly contributions and year-by-year growth.
Final balance
Starting with $10,000.00 and adding $500.00 a month at 10% per year, after 10 years you will have $125,869.35, of which $55,869.35 is interest.
How this is calculated →- Total contributed$70,000.00
- Total interest$55,869.35
- Equivalent monthly rate0.7974%
Does not account for taxes, fees or inflation.
Year-by-year growth
| Year | Total contributed | Total interest | Balance |
|---|---|---|---|
| 1 | $16,000.00 | $1,270.27 | $17,270.27 |
| 2 | $22,000.00 | $3,267.56 | $25,267.56 |
| 3 | $28,000.00 | $6,064.59 | $34,064.59 |
| 4 | $34,000.00 | $9,741.32 | $43,741.32 |
| 5 | $40,000.00 | $14,385.72 | $54,385.72 |
| 6 | $46,000.00 | $20,094.55 | $66,094.55 |
| 7 | $52,000.00 | $26,974.28 | $78,974.28 |
| 8 | $58,000.00 | $35,141.97 | $93,141.97 |
| 9 | $64,000.00 | $44,726.44 | $108,726.44 |
| 10 | $70,000.00 | $55,869.35 | $125,869.35 |
How compound interest is calculated
With compound interest, each month's interest is added to the balance and earns interest too. The future value of a principal P at a monthly rate i over n months is P × (1 + i)ⁿ.
With a monthly contribution A at the end of each month, add A × ((1 + i)ⁿ − 1) ÷ i. The calculator works month by month and shows the balance at the end of each year.
An annual rate is treated as an effective annual rate (APY) and converted to the equivalent monthly rate: (1 + annual rate)^(1/12) − 1. So 12% a year is about 0.9489% a month, not 1%.
Example: $10,000 at 1% a month for 12 months, with no contributions, grows to $11,268.25 — $1,268.25 of interest.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is charged only on the original amount. Compound interest is charged on the accumulated balance, including past interest, so it grows faster over time.
Is the annual rate an APR or an APY?
It is treated as an effective annual rate (APY). If you have a nominal APR compounded monthly, enter APR ÷ 12 as a monthly rate instead.
Are taxes included?
No. The result is before taxes. On taxable accounts, taxes reduce the final return.
When are contributions made?
At the end of each month, starting with the first. The initial amount starts earning in the first month.
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