If the point of hiring a well established, experienced, reputable painting contractor is to secure the professionalism and trust suggested to be inherent with that choice, then I would EXPECT that professionalism and experience to include the ability to make the proper and correct calculations for labor and materials for a fixed price quote, and there should be NO reason for the contractor to put the cost burden of their miscalculation on the consumer.
Pros prefer 5-gallon buckets with a roller grid to roller pans. They hold more paint than pans and, says Doherty, "It's tougher to tip over a bucket." A bucket also lets you box, or mix, two or three cans of paint to avoid color discrepancies. To use a bucket and grid, dip the roller a quarter of the way into the paint and run it over the ramp to work the paint into the nap.

Basic labor to paint home with favorable site conditions. Prep up to 3 hr per 100 SF - clean, scrape, sand and patch up to 4 defects per 100 SF. Caulk gaps and spot prime. Roll / brush 2 coats of paint over lightly textured ceiling/walls. Paint up to 2 doors and door trim per room. Includes planning, equipment and material acquisition, area preparation and protection, setup and cleanup.	129 square feet	$93.64	$126.68

The speed with which a painter can complete the job will determine its final cost, but time is difficult to estimate. Some painters have more experience and cover a wall faster, but some are more methodical and take more time. Most painters should be able to cover about 100 to 120 square feet of flat surface in an hour, unless they are working on a very large wall. Wood or plaster might reduce that amount to 80 to 100 square feet. You should also consider the time required for a first coat to dry before a second coat may be applied. This will add time as well -- anywhere from one to 48 hours depending on the paint.
For first timers, as well as seasoned DIY’ers, picking out paint can be both exciting and daunting. So many options exist from choosing the perfect shade of grey for your in-home office, all the way to the which relaxing hue you’ll be using for your family room. Picking out colors and finishes that compliment each other could consume entire days in the paint aisle before you even consider picking up a brush.
Payment. Every client is different. Every job has it's own unique set of circumstances. I always start off the discussion with my clients saying that I'm flexible on how they would like to proceed with payments but that I prefer half down. I refuse to change order. Change orders are my last resort. Instead I ask questions at our initial meeting and try to be sure to cover all bases before I submit my estimate.
I did go with the highest bidder and it did not matter. They did a shoddy job and threatened to take me to court when I pointed out mistakes that they made based on their work standards stated on the quote. I have not paid them yet, just filed a complaint with the BBB. No money down asked, no contract signed. The boss stated I have to high of expectations just because I live in a 121 year old house!
To get basic measurements, measure the length and width of each wall you want to paint in inches. Multiply the length and height together, and divide by 144. This will give you the square footage for each wall. A gallon of paint typically covers around 400 square feet. In some cases, you may need a primer 2 first, then a single coat of paint, meaning that you may need one gallon of primer and one gallon of paint to cover 400 square feet. However, if the wall requires multiple coats, this will increase the total amount of square footage required, and therefore the amount of paint.

House Interior Painting Design Co