It is mostly done by sqft, for example in my area the average rent $/sqft is $0.75 so for my 2020 sqft rental I should charge around $1515/month. By talking with local realtors, agents, craigslist, and property managers you should easily be able to find out what is average in your area. What ever the number is is obviously a ballpark, and can be adjusted both up and down for other factors such as schools, upgrades (granite, fixtures etc), number of bedrooms, covered patios, and so forth, but it will give you a good starting point. I generally list mine below market value so that I have lower turnover.
Hope this helps