What are some of the criteria for when you turn a town into a city?

[Oddible]

I've got the money / income, I assume that one of the main criteria for keeping the town is, does the town's specialization provide something that a city can't. What are some of the others?

 

[PauI]

production in the town basically (status quo & potential) & if takes >7 turns to grow even with 50% bonus

 

[StargazingDog]

The game recommends a 1:1 ratio. That seems a good rule of thumb. I factor what rural yields are remaining in the town, how much a specialized town would help, and how badly my main cities need food from the towns.