Regular contributions
How monthly contributions grow over time
With regular contributions, the ending balance comes from two sources: money deposited and growth assumed on the balance while each deposit remains invested.
Example: $100 deposited each month
The table uses a $0 starting balance, $100 deposited at the end of every month, a fixed 6% nominal annual rate, and monthly compounding. Values are rounded to cents.
| Time | Deposited | Projected interest | Ending balance |
|---|---|---|---|
| 10 years | $12,000.00 | $4,387.93 | $16,387.93 |
| 20 years | $24,000.00 | $22,204.09 | $46,204.09 |
| 30 years | $36,000.00 | $64,451.50 | $100,451.50 |
The 30-year balance is not three times the 10-year balance. Earlier deposits have more periods in which to compound, so time changes the proportion coming from projected interest. Investor.gov similarly describes regular investing and time as separate drivers of long-term growth.
Beginning versus end-of-month deposits
A beginning-of-month contribution receives one additional monthly growth period compared with an end-of-month contribution. The difference is usually modest for one deposit, but it accumulates across many deposits. Use the timing that matches the scenario instead of choosing the option with the larger output.
The calculator lets you switch between beginning and end timing while holding every other input constant.
Contribution frequency is not compounding frequency
Contribution frequency says how often you add money. Compounding frequency says how often interest is added. A person may contribute monthly to an account that compounds daily, for example. Treating the two settings separately makes the assumptions visible.
Test your own contribution schedule
Open the calculator, enter the amount you plan to add each period, and compare the total deposits with projected interest. The SEC's Investor.gov calculator also includes monthly contributions for an independent comparison.
Compare $100 and $200 per month over the same twenty years
Use a hypothetical fixed 6% nominal annual rate, monthly compounding and deposits at the end of each month. Start from $0 and hold the horizon at 20 years in both scenarios.
- Enter $100 for each monthly deposit, calculate, then choose Use as scenario A.
- Change only the monthly deposit to $200 and calculate again. A remains pinned while the page is open.
- Read the comparison's separate differences for deposits and projected interest, rather than treating the entire balance increase as earnings.
| Component | A: $100/month | B: $200/month | B minus A |
|---|---|---|---|
| Total deposited | $24,000.00 | $48,000.00 | $24,000.00 |
| Projected interest | $22,204.09 | $44,408.18 | $22,204.09 |
| Ending balance | $46,204.09 | $92,408.18 | $46,204.09 |
The extra $46,204.09 in the projected balance includes $24,000 of your own additional deposits. The remaining $22,204.09 is additional projected interest under these fixed assumptions. This is not a forecast of investment returns.
Keep the horizon unchanged for this exercise. If you change it, the calculator flags that the ending balances belong to different dates. If you need a target amount by a deadline instead, use the savings goal planner.
Reproduce the monthly deposit comparison →