A gallon of quality interior paint covers roughly 350 to 400 square feet. Even on a whole first floor, materials are a small fraction of what you pay.

What you are actually buying is labor, and labor is decided by condition. Two rooms of identical size can differ by a factor of three, and the difference is entirely in what has to happen before a brush comes out.

That is why a contractor who quotes you a per-square-foot number without walking the house is either padding heavily to protect himself or is going to come back with a change order.

Roughly in order of what it costs, cheapest first.

Sound walls, same color family. Wash, minor patching, caulk, one or two coats. This is the low end and it is genuinely quick.

Sound walls, big color change. Add a tinted primer and often a third coat, particularly going dark to light. Add roughly a third to the labor.

Walls with typical age damage. Nail holes, anchor holes, hairline cracking, previous patches that were done badly. Real patching, sanding, and spot priming. This is most Philadelphia rooms.

Walls with failed plaster. Cracks that need cutting back, taping, and bridging. Sections separated from the lath. This is repair work priced on its own.

Walls with wallpaper. A different job entirely. See below.

Adding trim, doors, and baseboards to a job roughly doubles the labor, and it surprises people every time.

Trim is slow. It is all edges and cut lines, and in an older Philadelphia house it usually carries oil-based enamel that has to be sanded, deglossed, and bonding primed before a modern waterborne finish will hold. Skipping that step is why repainted trim in old houses ends up soft enough to dent or peels off in strips when tape comes up.

If your budget is tight, walls and ceilings only is a legitimate way to scope a job. Cutting the trim prep to include trim cheaply is not.

This is where Philadelphia estimates diverge most from what people expect.

A rowhome interior with layered wallpaper over plaster is not a paint job with an extra step. Removing paper properly means scoring or steaming, full adhesive washdown, then repairing whatever the paper was hiding, then skimming the wall flat before any primer goes on. It can take longer than the painting.

The good news is that it is a one-time cost. Once a wall is stripped, repaired, skimmed, and properly primed, the next repaint is an ordinary job. See wallpaper removal and wall restoration for how that work runs.

When one estimate comes in far below the others, it is almost never because that contractor found efficiencies. It is because something is not in the scope.

The usual omissions: one coat instead of two, no primer over patches so repairs flash under a lamp, no furniture moving, no protection beyond a drop cloth in the middle of the room, trim priced as a light scuff sand instead of a proper degloss and prime, and no wall repair at all, just paint over whatever is there.

Ask any low bidder these four questions. How many coats. What primer goes over the patches. Is the trim being deglossed and bonding primed. Who moves the furniture. The answers usually explain the gap.

Get the prep written down separately from the paint. Every reputable contractor will do this, and it is the only way to compare three quotes that look different.

Then compare like for like. If one quote includes trim and another does not, they are not competing. If one includes wall repair and another says walls are assumed sound, they are not competing either.

Ed writes estimates with prep and paint broken out for exactly this reason. You should be able to see what the condition of your house is costing you rather than getting one number and no explanation.

You will find painting websites with a table of per-room prices on them. We do not have one, and it is worth explaining why rather than just leaving it out.

Any number we published would be wrong for most of the houses that read it. A room with sound walls and a small color change, and a room of identical size with two layers of wallpaper over cracked plaster, are not the same job and cannot honestly share a price. Publishing the low number and then quoting the real one at the door is a bait and switch. Publishing the high number scares off the people whose job actually is straightforward.

What we do instead is walk the house, look at what is under the current finish, and put a written number on it with the prep and the paint broken out separately, at no charge and with nobody calling you afterward.

If you want a rough sense of where you stand before booking anything, call and describe the rooms. Ed will tell you honestly which end of the range you are likely in and whether it is worth him coming out.