Compound interest example: what can monthly contributions grow to in 10 years?
A compound interest estimate depends on more than the annual rate. The starting amount, contribution size, time horizon, and when deposits are added all change the result. This example uses the same assumptions as the Toolhaus calculator so you can reproduce it.
Example inputs
- Starting balance: 100,000
- Annual rate: 5%
- Time: 10 years
- Contribution: 5,000 at the end of each month
- Compounding: monthly
Amounts use a neutral currency unit. The calculator formats results using the selected language, but does not apply currency-specific rates.
What the estimate shows
- Estimated future value: 941,112
- Your starting balance plus deposits: 700,000
- Estimated interest: 241,112
There are 120 monthly periods in 10 years. Deposits add 600,000 over that time; together with the initial 100,000, that makes 700,000 contributed. The remaining 241,112 is the modelled growth under these assumptions. Values are rounded to the nearest whole unit; the calculator displays cents or the locale equivalent.
How the monthly deposit changes the estimate
Keep the rate and time fixed, and change only the deposit made at the end of each month:
| Monthly deposit | Future value | Total contributed | Estimated interest |
|---|---|---|---|
| 0 | 164,701 | 100,000 | 64,701 |
| 5,000 | 941,112 | 700,000 | 241,112 |
| 10,000 | 1,717,524 | 1,300,000 | 417,524 |
Why the time horizon matters
With the same 100,000 starting balance, 5% annual rate, and 5,000 monthly deposit:
| Years | Future value | Total contributed | Estimated interest |
|---|---|---|---|
| 5 | 468,366 | 400,000 | 68,366 |
| 10 | 941,112 | 700,000 | 241,112 |
| 20 | 2,326,432 | 1,300,000 | 1,026,432 |
Reproduce this in the calculator
- Open the compound interest calculator.
- Enter 100,000 as the principal, 5 as the annual rate, and 10 years.
- Enter 5,000 for the contribution per period and choose Monthly.
- Compare future value, total contributions, and total interest. Change one input at a time to see which assumption moves the result.
Compounding and contribution timing
These are different concepts: compounding determines when interest is added to the balance, while contribution timing determines when new money is deposited. In this calculator, one frequency selector controls both. Choosing Monthly means monthly compounding and a 5,000 deposit at each month-end. If your deposit schedule differs from the compounding schedule, this tool cannot model that schedule separately.
The formula behind the estimate
FV = P(1 + r/n)^(nt) + C × (((1 + r/n)^(nt) − 1) / (r/n))
P is the starting balance, r is the annual rate as a decimal, n is the number of periods per year, t is years, and C is the deposit at the end of each period. The second term assumes deposits happen at period-end; depositing at the beginning of each month would produce a different result.
Assumptions and limits
- The annual rate stays constant for the full period; real investment returns can rise and fall.
- Deposits happen at the end of each month in this example.
- Taxes, fees, inflation, and changes in contribution amounts are not included.
- This is a scenario estimate for learning and comparison, not a promise of returns or personal financial advice.
For a plain-language explanation of compound interest, see Investor.gov.