The first step in calculating returns for your investment portfolio is identifying and gathering the requisite data. Once you have the data prepared, there are several considerations when performing the calculations.
Begin by defining the time period over which you want to calculate returns (daily, weekly, monthly, quarterly or annually). You need to strike a net asset value (NAV) of each position in each portfolio for those time periods and note any cash flows, if applicable.
Holding Period Return
Once you have defined your time periods and summed up the portfolio NAV, you can begin calculations. The simplest method to calculate a basic return is called the holding period return. It simply calculates the percentage difference from period to period of the total portfolio NAV and includes income from dividends or interest.
Adjusting for Cash Flows
If money was deposited or withdrawn from your portfolios, you need to adjust for the timing and amount of cash flows. For example, when calculating a monthly return, if you deposited $100 in your account midmonth, the portfolio endofmonth NAV has an additional $100 that was not due to investment returns. This can be adjusted using various calculations, depending on the circumstances. For example, the modified Dietz method is a popular formula to adjust for cash flows. Using an internal rate of return (IRR) calculation with a financial calculator is also an effective way to adjust returns for cash flows.
Annualizing Returns
For multiperiod returns, a common practice is to annualize returns. This is done to make the returns more comparable across other portfolios or potential investments. It allows for a common denominator when comparing returns.
Example
For example, the sum total of the positions in a brokerage account is $1,000 at the beginning of the year and $1,350 at the end of the year. There was a dividend paid on June 30. The account owner deposited $100 on March 31. The return for the year is 16.3% after adjusting for the $100 cash flow into the portfolio onequarter of the way through the year.

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Annual Return
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Modified Dietz Method
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Relative Return
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Yearly Rate Of Return Method
The yearly rate of return method is the rate of return achieved ... 
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The amount of profit, or return, that an individual can expect ... 
Abnormal Return
A term used to describe the returns generated by a given security ...