When coverage is difficult to estimate, add more rather than less when doing DIY wall painting. You can always pour the leftover back into cans. For large jobs, use the bucket and a roller screen rather than a roller tray. It’s much faster to load your roller with the screen than to use a roller pan. Simply dunk the roller into the paint bucket, then roll it along the screen until it stops dripping.
My wife and I just painted the interior of our house with about 6-8 gallons, of $30+ per gallon (meaning the good stuff, non-diluted) with absolutely fantastic results. However we just paid an average of $5 per gallon. Reason...all big box stores have paint, set aside, that has been mixed but not picked up by the customer. They need to sell it quick and if you're not in a hurry (you know well in advance that a room or two need painting and it's not like the roof leaking and needs an immediate fix) you can go to each store when you need other supplies or food, like Walmart (when convenient, driving 20 miles to each is not worth it) and over the course of a month or two, pick out some very nice colors of quality paint. We found perfect colors...not saying they were our first choice but when we opened the can, very nice and some even better than our original picks. Cost to paint the entire house was about $100, with all materials included, period. We had it on the market for a few months to sell, didn't sell, painted the rooms, got 2 offers the day after we finished, took the best one and never looked back.
If you are going to hire a contractor (professional painter) give a room by room punch list of the fixing of holes small or big, have it identified for the painter to tell you if you need a drywall finisher or if he does this type of work good. Wall repairs can be 3-5 steps to do repairs ( one per day for good drying of patching material and a good sanding) this is what gives you a good paint job only using high quality paint.
Can't speak highly enough of the service we received from James and his team. I did my due diligence and met with 3 painters. If I'd have met with James first, I might have skipped the other meetings. He talked me out of some extra work that wouldn't have come out the way i'd envisioned and showed me a packet full of successful projects, happy clients that dated back to the 1970s.