If you put money aside every month, adding up your contributions is straightforward. Working out how those deposits could grow takes another step: each deposit has a different amount of time to earn a return.
Here is how to calculate compound growth with monthly contributions, using $300 a month over 10 years as an example. We will separate the money you contribute from the estimated gains, then compare different return assumptions.
Start with five inputs
You need a starting balance, monthly contribution, time frame, assumed annual return, and contribution timing. Money you already have belongs in the starting balance. The amount you plan to add belongs in the monthly contribution.
Our example starts at $0, adds $300 at the end of each month, and runs for 120 months. We assume a constant 6% effective annual return. Contributions and returns stay the same throughout; taxes, fees, and inflation are excluded.
What could $300 a month become in 10 years?
Your contributions total $300 × 120 = $36,000. The first deposit arrives at the end of month one and has 119 months left to grow. The last deposit has no time to earn a return yet. Treating all $36,000 as though it had been invested on day one would overstate the result.
Under these assumptions, the balance after 120 months is $48,742.03.
That consists of $36,000 contributed and $12,742.03 in calculated gains. It is an illustration of the inputs, not a forecast of investment performance.
Compare different annual returns
Keep the deposits, time frame, and contribution timing unchanged, then vary only the return. The rates below are comparison scenarios, not recommendations or predictions.
| Assumed annual return | Contributions | Estimated gains | Final balance |
|---|---|---|---|
| 0% | $36,000.00 | $0.00 | $36,000.00 |
| 3% | $36,000.00 | $5,834.40 | $41,834.40 |
| 6% | $36,000.00 | $12,742.03 | $48,742.03 |
| 9% | $36,000.00 | $20,915.60 | $56,915.60 |
At 0%, the final balance is simply your contributions. With a constant positive return, earlier deposits and their accumulated gains continue to grow. A negative return can leave you with less than you contributed.
Convert the annual return to a monthly rate
This calculator uses an effective annual return: the total percentage increase over one year. The equivalent monthly rate at 6% is (1.06)^(1/12) − 1, or about 0.4868%.
Simply dividing 6% by 12 gives 0.5% per month. Compounded for 12 months, that produces an effective annual return of about 6.17%. If two calculators give different answers, check how they interpret the annual rate and when they add contributions.
What changes if you contribute at the start of the month?
With a positive return, each beginning-of-month deposit gets one additional month of growth. In our 6% example, end-of-month contributions produce $48,742.03; beginning-of-month contributions produce $48,979.29.
At 0%, the two results are equal. With a negative return, earlier deposits experience an additional month of losses. The timing comparison does not mean investing earlier always produces a better real-world outcome.
Try your own monthly contribution
The calculator opens with this article’s example as its default. Change the monthly contribution and time frame to match your scenario, then try different return assumptions. You can also switch between beginning- and end-of-month deposits.
Calculate with monthly contributions →
If you are investing a starting amount without adding more, use our lump-sum compound growth calculator. Our guide to doubling your money and the Rule of 72 explains how the return affects the time needed.
Why your actual balance may differ
This example assumes the same return every month. Real investment returns vary. Savings accounts can use daily balances, different interest-crediting schedules, and changing rates. Taxes and fees can reduce what you keep, while inflation affects its purchasing power.
Use the result to understand your assumptions and compare scenarios, rather than treating it as a promised future balance.