The S&P 500 is an index, a list of shares with rules for how much of each to hold, covering about 500 large US companies. SPY (the S&P 500 exchange-traded fund, or ETF) holds those shares and trades on a stock exchange like a single share. Twenty years of $500 a month into SPY means $120,000 paid in. From October 2006 to September 2026, that money grew to $568,039, one of the best results in SPY's record. The figure counts dividends, the cash that companies pay their shareholders, as extra shares bought.
A plan can start in any month. The record allows 165 start months. The median plan, the middle result when all 165 are ranked, ended at $323,817. The lowest ended at $241,763.
What if you invest $500 a month for 20 years? A plan like this has 240 monthly purchases and pays in $120,000. What it becomes depends on the market over those 20 years, which cannot be known in advance.
From October 2006 to September 2026, $500 a month in SPY (the S&P 500 ETF) grew to $568,039, one of the best results in its record. The figure counts dividends, the cash companies pay their shareholders.
A plan can start in any month. Across the 165 start months in SPY's price record, the lowest 20-year result was $241,763 and the middle result, the median, was $323,817. None ended below the $120,000 paid in.
The start date mattered. Only two of the 165 start months ended higher than the October 2006 plan.
What investing $500 a month means
Investing $500 a month means buying $500 of a fund on a set day each month, whatever its price. Over 20 years, that is 240 purchases and $120,000 paid in.
When the price is low, $500 buys more of the fund. When it is high, $500 buys less. This habit is called dollar-cost averaging.
The sums below are arithmetic on past prices. They are not a forecast. Our guides explain what the S&P 500 is and what an ETF is. The rules of the test:
Buy day: the first trading day of each month, meaning the first day the stock exchange is open. The price used is that day's closing price, the last price of the day.
Dividends: reinvested on the day they are paid, meaning used to buy more of the fund. Our price data already counts them.
Fractions of a share: allowed, so all $500 is invested each time.
Valuation: the day the holding's worth is measured, the last trading day of the 20th year.
Costs: SPY's yearly charge of 0.0945% is already taken out of its price. Dealing charges and taxes are left out.
How $500 a month grew from 2006 to 2026
Source: YX Insights
Chart 1 follows the plan from the first purchase on 2 October 2006 to 30 September 2026. The grey line is the money paid in. It climbs by $500 a month to $120,000. The blue line is what the holding was worth.
At the end of February 2009, $14,500 paid in was worth $8,482. On the last trading day of 30 months between January 2008 and August 2010, the value was below the money paid in.
Then the value pulled ahead of the money paid in. At the end of 2020, $85,500 paid in was worth $232,444.
On 30 September 2026, the 240 purchases were worth $568,039. That is $448,039 more than was paid in. The final value is 4.73 times the $120,000.
How 20-year plans ended, for every start month in SPY's record
Source: YX Insights
Chart 2 runs the same plan from every start month that SPY's price record allows. The record begins in January 1993, so the first plan started in February 1993, the first month with a full set of purchase dates.
The last plan that fits started in October 2006. That gives 165 plans, each valued at the end of its 20th year.
No plan ended below the $120,000 paid in. The lowest ended at $241,763, from a February 1993 start. The median was $323,817. The highest, from a June 2006 start, was $575,591.
The October 2006 plan ranked third of the 165. Only the June 2006 start ($575,591) and the September 2006 start ($573,631) ended higher.
A plan that started in March 2000 began in the month SPY reached its highest price to that date. It still ended at $318,186, well above the $120,000 paid in.
These 165 plans overlap, so they are not 165 separate tests. They cover one stretch of history. Decades before 1993 are not in this price record. No figure from elsewhere fills the gap.
A worse 20 years: the plan that began in February 1993
Source: YX Insights
Chart 3 follows the lowest result. Purchases ran from 1 February 1993 to 2 January 2013. The holding was valued on 31 January 2013. The $120,000 paid in became $241,763, or 2.01 times.
As a yearly rate, that plan earned 6.5%. The October 2006 plan earned 13.7%. Each is a money-weighted return: the single yearly rate that fits the whole series of purchases, counting each $500 from the day it went in.
This plan also had falls. On the last trading day of four months, the value sat below the money paid in. Three were in 1993 and 1994. The fourth was February 2009, when $96,500 paid in was worth $95,352.
What would have changed the result
The start date changed it most. The same $500 a month ended at $241,763 from a 1993 start and at $568,039 from an October 2006 start.
The timing of the payments changed it too. A lump sum is the whole amount paid in at once. This is hindsight arithmetic on past prices, not a forecast.
Prices rose over each of these 20-year spans. In all 165 of them, a $120,000 lump sum on the first purchase date would have ended above the monthly plan. For the October 2006 start, that is $991,834 against $568,039. For the February 1993 start, it is $587,252 against $241,763.
A fund's charge changes it as well. The Investment Company Institute (ICI), the US fund industry body, reports a 2025 average of 0.40% a year for equity mutual funds. A charge of 0.40% in place of SPY's 0.0945% would have left the October 2006 plan at $545,305. That is $22,734 less.
What the figures leave out: inflation and tax
The figures are in dollars of the day. US prices rose 67.0% from October 2006 to September 2026, measured by the Consumer Price Index (CPI), which tracks what everyday goods and services cost. So the first $500 paid in equals about $835 in September 2026 prices. Our guide to how inflation erodes savings shows the effect on cash.
Tax depends on the account. We explain how dividends are taxed in the UK and US in How Are Dividends Taxed?
How to read a $500-a-month calculation
Three checks help with any monthly-investing figure, including this one:
The span. The result depends on when the plan starts and when it is valued. Dividends, charges and taxes also change it.
The kind of return. SPY's price, with dividends counted, rose 11.1% a year from 2 October 2006 to 30 September 2026 for one sum paid at the start. The plan's 13.7% is a money-weighted return on purchases made at different dates. The two rates measure different things, so they need not match.
The record. These are 165 overlapping plans from one stretch of history. Our guide to investing $100 a month for 10 years runs the ten-year version on a longer data series, over every span since 1871.
Paying $500 a month for 20 years means paying in $120,000, which became $568,039 in SPY from 2006 to 2026. The lowest of the 165 plans in SPY's record ended at $241,763, so the result depends on the 20 years the plan runs through.
Common questions about investing $500 a month
How much is $500 a month for 20 years worth?
The answer depends on returns, which cannot be known in advance. Paying $500 a month for 20 years means paying in $120,000. In SPY (the S&P 500 ETF), the plan from October 2006 to September 2026 grew to $568,039 with dividends counted. The lowest of 165 start months ended at $241,763.
How much do you pay in if you invest $500 a month for 20 years?
You pay in $120,000. That is $6,000 a year, across 240 monthly purchases. Gains add to that sum. Falls in value reduce what it is worth. In the lowest 20-year plan in SPY's record, the $120,000 paid in was worth $241,763 at the end.
Has a 20-year monthly plan in the S&P 500 ever lost money?
No 20-year plan in SPY's price record ended below the money paid in, before inflation, dealing charges and tax. SPY began in January 1993, so 165 plans fit, starting from February 1993 to October 2006. Decades before 1993 are not in that record, so it cannot say what happened then.
What is $500 a month for 20 years worth in today's money?
The figures above are not adjusted for inflation, so no single figure is given. US consumer prices rose 67.0% from October 2006 to September 2026, so the first $500 paid in equals about $835 in September 2026 prices. Our guide to how inflation erodes savings explains the effect.
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DISCLAIMER: This article is strictly educational. Any information or analysis in this note is not an offer to sell or the solicitation of an offer to buy any securities. Nothing in this note is intended to be investment advice and nor should it be relied upon to make investment decisions. Any opinions, analyses, or probabilities expressed in this note are those of the author as of the note's date of publication and are subject to change without notice.