Dollar cost averaging calculator: what does investing every month really give?

ChartCompass · Updated Nov 6, 2026 · Recalculated every evening

Short answer$500 a month for 20 years is $120,000 paid in. On the S&P 500 it became between $113,019 and $672,337 depending on the start month, with a median of $285,854 across 946 start months since 1928. Dividends are not included.

Work it out

Every month, $Years

Paid in–

Worst–

Median–

Best–

End value for each start monthPaid in

Why not just one percentage?

An ordinary compound interest calculator uses the same percentage every year. The market doesn't move like that: there is hardly such a thing as a normal year. Here the same amount runs through every real start month since January 1928, and you see the whole range.

How much does time matter?

The table shows $500 a month in the S&P 500, for every start month that can be counted to the end.

Saving for	Paid in	Worst	Median	Best	Worst ended	Ended below paid in
5 years	$30,000	$12,889	$36,866	$58,145	Feb 1933	183 of 1,126
10 years	$60,000	$37,044	$90,459	$165,299	Feb 2009	93 of 1,066
20 years	$120,000	$113,019	$285,854	$672,337	Sep 1974	1 of 946
30 years	$180,000	$294,035	$642,136	$1,607,198	Jul 1982	0 of 826
40 years	$240,000	$662,373	$1,349,783	$2,769,476	Sep 1974	0 of 706

Why does the end matter so much?

Look at when the worst periods ended. The worst over twenty years began in September 1954 and ended in September 1974, with $113,019 from $120,000 paid in. Compare the table's years with every stock market crash since 1928.

The reason is simple. Early on, little money is in, so a fall costs little and next month's amount buys cheaper. Near the end almost all the money is in, so a fall hits the whole pile.

A one-off amount looks different. There the worst twenty-year period was the one that began before the 1929 crash, because everything went in at the top.

How we calculate

You buy for the amount at each month's last close. The value is read at the next month's last close after the final purchase, so 20 years is 240 purchases.
Closing prices up to Nov 6, 2026. The running month is not counted.
The index leaves out dividends, so someone who owned the companies received more. How much is in the study of the market's average return.
The amount stays the same all the way. Inflation is not taken out. Fees and tax are not included.
The S&P 500 index started in 1957. The numbers before that come from its predecessor with 90 companies.
The periods overlap, so 946 start months are not that many independent outcomes.

How would one company have gone? Work it out in If I had invested.