The S&P 500 is an index of about 500 large US companies. Putting $100 a month for ten years into a fund that tracks it means paying in $12,000. From October 2016 to September 2026, that money grew to $27,488.
That decade was a strong one. Over every ten-year span since 1871, the median plan ended at $19,493. Here is how a monthly plan works, how wide the range of results has been and what one real decade earned.
What if you invest $100 a month for 10 years? You make 120 monthly purchases and pay in $12,000. What that grows to depends on the return over those ten years, which cannot be known in advance.
Since 1871, the median ten-year plan of $100 a month in US stocks ended at $19,493. Only 2.6% ended below the $12,000 paid in. Both figures count dividends, the cash companies pay shareholders, put back in.
From October 2016 to September 2026, the same plan in SPY (the S&P 500 exchange-traded fund, or ETF) grew to $27,488. Only 15% of past ten-year plans did as well.
The start date mattered. Five-year plans in SPY paid in $6,000. Depending on the start month, they ended between $6,942 and $10,071.
What investing $100 a month means
Investing $100 a month means buying $100 of an investment on a set day each month, whatever its price. Over ten years, that is 120 purchases and $12,000 paid in. When the price is low, $100 buys more shares. When it is high, it buys fewer. This habit is known as dollar-cost averaging.
The examples count dividends. Dividends are the cash that companies pay their shareholders. Here, each one is used to buy more of the investment. We explain them in What Is a Dividend?
The 2016 to 2026 test uses SPY, which holds the shares in the S&P 500. An ETF is a fund that trades on a stock exchange like a share. Our guides explain what an ETF is and what the S&P 500 is.
The rules of the test:
Buy day: the first trading day of each month, at that day's closing price.
Dividends: reinvested in the fund. Our price data already counts them.
Fractions of a share: allowed, so all $100 is invested each time.
Costs: SPY's own fee of 0.0945% a year is already taken out of its price, as our guide to index funds explains. Dealing charges and taxes are left out.
What $100 a month did over every ten-year span since 1871
Our price data covers a single decade. A longer record shows how other decades went. Robert Shiller of Yale University publishes monthly US stock prices and dividends back to January 1871. His series tracks a broad index of US shares called the S&P Composite. For the years before that index existed, it uses earlier stock indexes. His prices are monthly averages of daily closes, so results differ slightly from a real fund.
Source: Robert J. Shiller, Yale University; YX Insights
Chart 1 runs the plan, $100 a month with dividends reinvested, for 1,710 ten-year spans. The first started in January 1871, the last in June 2013. The median plan ended at $19,493.
Of the 1,710 plans, 45 ended below the $12,000 paid in. That is 2.6%. Every one ended in one of four stretches: 1896, 1931 to 1935, 1974 to 1975 or 2008 to 2009. The worst started in June 1922 and ended at $6,343 in June 1932. A plan started in March 1999 ended at $8,437 in March 2009. The best started in September 1919 and ended at $49,449 in September 1929.
How much the start date mattered in five-year plans
Source: YX Insights price data
Chart 2 runs five-year plans in SPY: 60 purchases of $100, or $6,000 paid in. Our price data holds 63 of them, started each month from July 2016 to September 2021. Each is valued on the last trading day of its fifth year.
Every one ended above the $6,000 paid in. The results still spread widely. The best plan started in January 2017 and ended at $10,071 in December 2021. The worst started in October 2017 and ended at $6,942 in September 2022, during that year's fall. The median was $8,877.
So the end date matters as well as the start date. A plan valued during a fall shows a lower result, whatever came before.
How $100 a month grew from 2016 to 2026
Source: YX Insights price data
Chart 3 follows the ten-year plan in SPY from the first purchase on 3 October 2016 to 24 September 2026. The grey line is the money paid in. It climbs by $100 a month to $12,000. The blue line is what the holding was worth.
By the end of 2019, the $3,900 paid in was worth $4,965. By the end of 2021, $6,300 paid in was worth $10,798. On 24 September 2026, the 120 purchases had bought 35.83 shares, worth $27,488. That is $15,488 more than was paid in.
On Shiller's record, 251 of the 1,710 ten-year plans, or 15%, ended at $27,488 or more. So this decade was one of the better ones.
As a yearly rate, the plan earned 15.9%. This is a money-weighted return. It is the single yearly rate that turns each $100, from the day it went in, into the final total.
The path had falls. On 24 December 2018, the plan was worth $2,572, below the $2,700 paid in. In 2020, its value fell from $5,428 on 19 February to $3,671 on 23 March. That was below the $4,200 paid in by then. Purchases made near the low, in March and April 2020, got more shares for each $100. In 2022, the value fell from $10,961 on 3 January to $9,062 on 12 October, but stayed above the money paid in.
What the figures leave out: costs, taxes and inflation
Fund fees. SPY charges 0.0945% a year. If it had charged 0.64% a year in place of that fee, the same plan would have ended at $26,594. That is $894 less. 0.64% was the 2025 average for funds whose manager picks the shares, according to the Investment Company Institute.
Tax. Dividends can be taxed each year, even when they are reinvested. Some accounts shelter investments from this tax. We explain how dividends are taxed in the UK and US.
Inflation. The figures are in dollars of the day. US prices rose 38.6% from October 2016 to August 2026, the latest month published. That is measured by the Consumer Price Index, which tracks what everyday goods and services cost. Our guide to how inflation erodes savings shows what that does to cash.
How to read a $100-a-month calculation
A few checks help with any monthly-investing figure, including this one:
The span. The result depends on when the plan starts. It also depends on when it is valued.
What is counted. Dividends, fees, dealing charges and taxes each change the result.
The kind of return. SPY itself returned 15.4% a year over the same span. That figure is for one sum put in at the start. The plan's 15.9% counts each $100 from the day it went in.
One path among many. The 2016 to 2026 result is one decade. On the record since 1871, ten-year results on the same $12,000 ran from $6,343 to $49,449.
Investing $100 a month for ten years means paying in $12,000, which became $27,488 in an S&P 500 fund from 2016 to 2026. The median ten-year plan since 1871 ended at $19,493, so the result depends on the decade the plan runs through.
Learn more with YX Insights
This explainer is part of the YX Insights Academy. Each one takes a single idea and checks it against real data.
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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.