Compound Interest Calculator
| Start Amount ($) | Annual Investment ($) | Annual Rate (%) | Profit ($) in 10 Years | Total ($) |
|---|---|---|---|---|
| 26,955.64 | 46,955.64 |
All Assets Per Year
| Year | Start Amount | Total Contribution | Profit per Year | Accumulated Profit | Total |
|---|---|---|---|---|---|
| 1 | $10,000 | $1,000 | $1,210 | $1,210 | $12,210 |
| 2 | $12,210 | $2,000 | $1,453 | $2,663 | $14,663 |
| 3 | $14,663 | $3,000 | $1,723 | $4,386 | $17,386 |
| 4 | $17,386 | $4,000 | $2,022 | $6,409 | $20,409 |
| 5 | $20,409 | $5,000 | $2,355 | $8,763 | $23,763 |
| 6 | $23,763 | $6,000 | $2,724 | $11,487 | $27,487 |
| 7 | $27,487 | $7,000 | $3,134 | $14,621 | $31,621 |
| 8 | $31,621 | $8,000 | $3,588 | $18,209 | $36,209 |
| 9 | $36,209 | $9,000 | $4,093 | $22,302 | $41,302 |
| 10 | $41,302 | $10,000 | $4,653 | $26,956 | $46,956 |
How This Compound Interest Calculator Works
Compound interest is what happens when the returns your money earns start earning returns of their own. Each year, the calculator adds your annual investment to the running balance, then applies your annual rate to the whole balance — principal, past contributions, and past gains together. Over enough years, the gains from growth outpace the gains from your own contributions, which is why long time horizons matter so much more than most people expect.
Enter a start amount, how much you plan to add each year, and an expected annual return, and the table above updates instantly to show your profit and total balance for any number of years from 1 to 500.
The Formula
For a lump sum with no further contributions, compound interest follows:
Total = Start Amount × (1 + Annual Rate) ^ Years
When you add a fixed amount every year, each contribution compounds for a different number of years depending on when it was added — a contribution made in year 1 has far longer to grow than one made in year 19. This calculator runs that year-by-year math for you instead of relying on a simplified formula.
Example
Starting with $10,000, adding $1,000 a year, at an 11% annual return: after 10 years the balance is roughly what you see in the table above when you set Years to 10 — try it and compare how much of the total came from contributions versus growth. Push the timeline out to 20 or 30 years and the growth portion starts to dwarf the amount you actually put in.
Frequently Asked Questions
What counts as a realistic annual rate?
The S&P 500 has historically returned around 10% annually before inflation over long periods, though any single year can vary widely. Bonds and savings accounts typically return far less. Use our S&P 500 calculator to see rates derived from actual historical prices instead of guessing.
Does this calculator account for taxes or inflation?
No — the numbers shown are nominal, pre-tax figures. To see the effect of inflation on your purchasing power over time, use the portfolio calculator, which has an inflation-adjustment option.
Why does the profit grow so much faster in later years?
Because interest is earned on interest. In early years, most of your balance is money you contributed. In later years, a growing share is unrealized gains from prior years compounding on themselves — this is the "hockey stick" curve you'll see if you push the Years slider higher.
How is this different from simple interest?
Simple interest is calculated only on the original principal every period, so growth is linear. Compound interest is calculated on the principal plus all previously earned interest, so growth accelerates over time — that difference is the entire point of investing early.