For a rental you plan to hold, start with the rental cash flow calculator: it tells you whether the deal pays you every month. Buying a flip, start with the ARV calculator to find your ceiling, then use the PITI calculator once you have a specific loan quote to check.
As accurate as the inputs you give them. Rent, comps, and rehab costs are the three numbers that make or break a deal, and none of them can be guessed reliably from a spreadsheet formula alone. Real comps and rent data, pulled per listing, are what DealBumble's full app adds on top of the math here.