@Nadia V.
So,
Gross rent = rent rate × 12 months
= gross rent for the year
ROI = Gross rent ÷ total cost of buying the house(including closing costs, inspectors etc) × 100
Now, you can put in the rate for management. If your PM charges 8 or 10 percent, you put that. Check for the average vacancy rates in the area and put that in. Now, add maintenance. If it's a new house, you can put 5 percent or if it's old, you can put in 8 or 10. Add insurance.
Now, subtract all these "expenses" from your gross rent. This will give you the net rent.
Now, ÷ the net rent with the total cost of acquiring the property. This will give you NOI(Net return on investment).
Now, if you're financing, get COC = NOI ÷ down payment × 100
P.S. If you're planning to keep the property for a longer time, consider CapEx for roof, HVAC, boilers or what not.
Furthermore, this calculation does not consider mortgage because that is separate, and deducted from the remainder rent. You can subtract mortgage from the net rent and that will give you cash flow.