Pro painters in Roanoke, VA use a smart phone app, that measures (if you have the right tape app) the room, calculates the sq footage and ceiling area, add in the doors, windows, casings, and so on, number of workers expected to work, color and maker of the paint and then it spits out an estimate.
In December 2019, several painters used this to provide me an estimate of over $12K and all of their estimates were within a couple of hundred dollars. 3,000 sq ft home.
The old timers (6 guys, two provided by my realtor) came in just around $4K and were within a couple hundred of each other.
The guy we chose did it for $3800 and included paint. And he did a fantastic job, had one guy helping here and there. Less than a week to complete the job, cleaned up and damp mopped.