How to calculate
Simply put, an unrealized gain or loss is the difference between an investment's value now, and its value at a certain point in the past. The calculation can be done for any time period, such as the unrealized gain over the past month, but the most useful unrealized gain/loss is calculated from the time at which the investment was originally made. And, the calculation is rather simple.
First, figure out the investment's current market value. For example, if you own 100 shares of a certain stock, and its current value is $70 per share; your investment is worth $7,000.