The amount of shares outstanding in a company will often change due to a company issuing new shares, repurchasing and retiring existing shares, and other financial instruments such as employee options being converted into shares.
The weighted average of outstanding shares is a calculation that incorporates any changes in the amount of outstanding shares over a reporting period. It is an important number, as it is used to calculate key financial measures such as earnings per share (EPS) for the time period.
Let's look at an example:
Say a company has 100,000 shares outstanding at the start of the year. Halfway through the year, it issues an additional 100,000 shares, so the total amount of shares outstanding increases to 200,000. If at the end of the year the company reports earnings of $200,000, which amount of shares should be used to calculate EPS: 100,000 or 200,000? If the 200,000 shares were used, the EPS would be $1, and if 100,000 shares were used, the EPS would be $2  this is quite a large range!
This potentially large range is the reason why a weighted average is used, as it ensures that financial calculations will be as accurate as possible in the event that the amount of a company's shares changes over time. The weighted average number of shares is calculated by taking the number of outstanding shares and multiplying the portion of the reporting period those shares covered, doing this for each portion and, finally, summing the total. The weighted average number of outstanding shares in our example would be 150,000 shares.
The earnings per share calculation for the year would then be calculated as earnings divided by the weighted average number of shares ($200,000/150,000), which is equal to $1.33 per share.
To read more, see The Basics of Outstanding Shares and the Float and The Dangers of Stock Dilution.

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