(Price you bought your house- value of land)/27.5
for example: you bought your house $ 200K. the value of land is $50K
depreciation value=(200K-50K) / 27.5=$5454.55
every year you can deduct $5454.55 from your rent's profit when you full your tax return.
But when you sell your house, you have to pay back all your deduction in past years.