This free online calculator will calculate the weighted average for a company that changes its number of outstanding shares during the course of the time period you are interested in.
Unlike most textbooks that simply limit their examples to number of months, this calculator measures durations in number of days.
Plus, after calculating the weighted average the calculator will generate a linebyline summary of each transaction.
Note that the results of this calculator are most commonly used for calculating the Earnings Per Share (EPS).
Weighted average is a calculation used to give more weight to more influential values within a data set, and lower weight to values with less influence.
As it relates to shares of outstanding stock, the weighted average calculation gives greater weight to larger numbers of outstanding shares and to longer durations, and gives less weight to smaller number of shares and to shorter durations.
To illustrate the difference between simple average and weighted average, suppose a company has 10,000 shares outstanding at the beginning of the year and 20,000 shares outstanding at the end of the year. Using a simple average formula, the average number of shares outstanding would be 15,000 (30,000 ÷ 2 = 15,000).
However, if the yearend share increase was due to a stock split that occurred on December 15th of the year, the 15,000 simple average result would not accurately reflect the daytoday average for the entire year.
The following results from the calculator on this page shows how the weighted average calculation more accurately reflects the daytoday average of outstanding shares.
A Date Range 
B Shares 
C Days 
D % 
E Weighted 
1/1/2013  12/15/2013  10,000  348  95.34%  9,534 
12/15/2013  12/31/2013  20,000  16  4.38%  877 
Weighted average outstanding shares »  10,411 
See how weighted average more accurately reflects the daytoday outstanding shares during the year?
Here are the steps to calculate the weighted average of number of shares outstanding based on durations stated in number of days:
Note that if you find yourself trying to calculate weighted average manually, it will be easier  though less accurate  to work with durations stated in months. In that case you would divide the number of months in each change in outstanding shares by the total months in order to arrive at the percentages in column D above.
With that, let's use the Shares Outstanding Calculator to calculate the weighted average number of shares outstanding for up to 12 stock transaction durations.

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