Evaluating Return on Investment

[Account Closed]

My wife and I currently own 4 investment properties - one duplex, two triplex's and a single family home.  We are looking to calculate the return on investment for each property to objectively evaluate our portfolio.  Are there a few formulas or key metrics you recommend we use to properly evaluate our properties (i.e. home value plus rental income, cap rate, etc)?  If so, would you mind sharing the formula?  Thanks!