Calculate the property's annual gross income. For most investment properties, the gross income is the rent, however, some properties generate cash from other sources, such as coin operated washing machines. For example, an apartment rents at $1,200 per month, therefore, the expected gross income is $14,400 per year.
Add up the projected annual expenses associated with the property. All rental units come with operating costs. These include property taxes, hazard insurance, maintenance bills and property management fees. For example, suppose you pay $850 in maintenance, $750 in taxes and $800 in insurance per year for a rental unit. The total annual expenses are $2,400.
Subtract the annual expenses from the gross annual income. This gives you the property's net income. For example, with an annual income of $14,400 and expenses of $2,400, a rental unit has net income of $12,000.
Divide the net income by the property's original purchase price. Suppose you bought the apartment for $150,000. Dividing the net income of $12,000 by a $150,000 purchase price yields .06.
Convert the decimal figure that results from dividing net income by purchase price to a percentage. In this example the cap rate of .06 is equivalent to a cap rate of 6 percent.