Quick answer: how much can monthly investing grow?
In this fixed 6% return scenario, investing $500 at the beginning of each month for 20 years grows to $227,887. You contribute $120,000, and the remaining $107,887 comes from the assumed investment growth.
This example uses annual compounding with monthly contributions applied according to the selected contribution timing.
The result changes when you adjust the monthly amount, return, timeline, contribution timing, starting balance, or inflation assumption. It is not a prediction of future S&P 500 returns.
$100, $500, and $1,000 monthly investment examples
This table starts at $0 and uses a fixed 6% annual return, beginning-of-month contributions, annual compounding, and no taxes or fees. Monthly contributions are applied within each year based on the selected timing.
| Monthly investment | 10 years | 20 years | 30 years |
|---|
| $100/month | $16,331 | $45,577 | $97,953 |
| $500/month | $81,655 | $227,887 | $489,765 |
| $1000/month | $163,310 | $455,773 | $979,531 |
Contributions vs investment gains
In the early years, your own contributions usually make up most of the balance. Over longer periods, compounding can become more important. This table uses $500 per month, a fixed 6% annual return, beginning-of-month contributions, and a $0 starting balance.
| Period | Total contributed | Investment gains | Ending value |
|---|
| 10 years | $60,000 | $21,655 | $81,655 |
| 20 years | $120,000 | $107,887 | $227,887 |
| 30 years | $180,000 | $309,765 | $489,765 |
Starting from $0 vs starting with a lump sum
Monthly contributions build the balance over time. Adding an existing $10,000 starting balance gives that money the full period to compound alongside the same $500 monthly contribution.
| Period | $0 + $500/month | $10,000 + $500/month |
|---|
| 10 years | $81,655 | $99,564 |
| 20 years | $227,887 | $259,958 |
| 30 years | $489,765 | $547,200 |
Beginning vs End-of-Month Contributions: A Small Timing Detail That Compounds
A monthly calculator should state when each contribution is invested. Beginning-of-month contributions have slightly more time to earn the assumed return than end-of-month contributions.
This is one reason two monthly investment calculators may show slightly different results even when the monthly amount and return rate are the same.
This comparison isolates contribution timing. It uses $500 per month, a fixed 6% annual return, annual compounding, and no starting balance.
| Period | Beginning of month | End of month |
|---|
| 10 years | $81,655 | $81,260 |
| 20 years | $227,887 | $226,783 |
| 30 years | $489,765 | $487,394 |
Before inflation vs after inflation
Nominal results show the future dollar amount before accounting for rising prices. This table estimates today's purchasing power using a fixed 3% inflation rate over 20 years.
| Monthly investment | Nominal value | After 3% inflation |
|---|
| $100/month | $45,577 | $25,235 |
| $500/month | $227,887 | $126,175 |
| $1000/month | $455,773 | $252,351 |
This is not a historical backtest
The examples use a fixed 6% annual return. They do not represent actual S&P 500 returns in any specific historical period. Real returns are uneven, and some years can be negative.
The S&P 500 index itself is commonly quoted as a price index, which does not include dividends. If you want to model total return, use a return assumption that already includes reinvested dividends.
Formula used
For monthly contributions:
Each monthly contribution is added according to the selected timing. Beginning-of-month contributions are treated as invested earlier in the period than end-of-month contributions.
This calculator uses the selected annual return and compounding setting. If annual compounding is selected, monthly contributions are applied within the year based on their timing, then growth is applied using the annual return assumption.
For investment gains:
Investment gains = ending value − starting amount − total contributions
For inflation adjustment:
Inflation-adjusted value = nominal future value ÷ (1 + inflation rate)years
What this calculator does not include
- Actual year-to-year market volatility
- A changing sequence of gains and losses
- Taxes or account-specific tax rules
- Fund fees, platform fees, or brokerage costs
- Exchange rates or FX fees
- Guaranteed or predicted returns
FAQ
Are contributions added at the beginning or end of the month?
You can select either timing. Beginning-of-month contributions have slightly more time to compound than end-of-month contributions.
Does the result separate contributions from investment gains?
Yes. The calculator shows how much you contributed and how much of the result comes from the fixed return assumption.
Does this include dividends?
Only if your return assumption already includes dividends. The calculator does not separately model dividend payments or reinvestment. If you want to model total return, use an annual return assumption that already includes reinvested dividends.
Does this include inflation?
Yes. Set an inflation assumption to compare the nominal future value with an estimated value in today's purchasing power. The inflation adjustment is simplified and uses the same inflation rate for the full period.
Is this a prediction?
No. This is a fixed-assumption scenario calculator. It does not predict future S&P 500 returns, market crashes, recoveries, or the order of gains and losses.
Does this include taxes or fees?
No. The calculator does not model taxes, fund fees, platform fees, brokerage costs, or account-specific rules. These costs can reduce the final result.
Why do monthly investment calculators show different results?
Different calculators may use different assumptions for contribution timing, compounding frequency, inflation, dividends, taxes, and fees. For example, beginning-of-month contributions usually produce a slightly higher result than end-of-month contributions because the money is invested earlier. Read why S&P 500 calculators give different results.
Related S&P 500 calculators
This calculator is for educational scenario planning only. It is not financial advice. Results are hypothetical and do not include taxes, fees, exchange rates, brokerage costs, or actual market volatility.