When calculating property ROI, how do I estimate rent per month?

[Jimmy Lieu]

Hi. I am starting out in real estate and I have been learning a lot. I've found a few properties but I am having trouble calculating ROI or cash flow because I don't know how to estimate rent per month on properties?

How do I estimate a good rent per month for the property I am looking at? Is there a website or tool that helps me find this out? Or is it a formula? Or do I just ask my realtor? 

If I don't know how to estimate rent per month, how can I calculate cash flow or ROI? Thanks in advance for the help.