Withdrawing money from your investment account complicates the return on investment (ROI) calculation because you may not simply calculate the overall change in value. As an example, say you started off with $5,000, withdrew $2,000, and ended with $4,000. Simple ROI calculations would give you a 20 percent decrease, although there was obviously $1,000 in growth at some point. To compensate for withdrawals, you must separate the investment into periods between withdrawals and calculate the period returns separately before calculating the overall return.

1. Separate the investment into periods between withdrawals. Note the beginning and ending balances of each period. Referencing the investment's balance sheet will give you the figures you need. In the example, assume the statement showed a $5,500 balance immediately before the withdrawal, meaning the first period began with $5,000 and ended at $5,500. The withdrawal created a starting balance of $3,500 for the second period, after which it grew to $4,000.

2. Divide each period's ending balance by the starting balance to calculate the period's growth factor. In the example, the first period had a growth factor of 1.10, because $5,500 divided by $5,000 is 1.10. The second period had a 1.14 growth factor.

3. Multiply each period's growth factor to calculate the total growth factor. In the example, 1.10 times 1.14 gives you 1.25.

4. Subtract 1.0 from this figure to calculate the ROI. In the example, your investment would have experienced a ROI of 0.25, or 25 percent.