How much paint a room takes, coat by coat
Wall face less every opening, times the coats, over the square feet a gallon covers. The third number is on the tin, and this page will not invent it.
Three numbers, and only one of them is on the wall
The first is geometry and this calculator works it out. The second is a decision you make standing in the room. The third is a property of a product — it is printed on the can, it is different for a primer, and it is different again over bare board — so it is a box you fill in rather than a figure anybody here can supply.
Every paint calculator that quietly assumes a coverage rate is putting a made-up number underneath somebody’s material order. This one asks.
Worked, on a room with the openings taken out
Take the room 12' × 14' with a 8' ceiling, one 3' × 6' 8" door and one 3' × 5' window, and say the tin says 350 square feet to the gallon — read your own off the can rather than using that one.
| Square feet | |
|---|---|
| Wall face, less the door and the window | 381.0 |
| One coat | 381.0 |
| Two coats | 762.0 |
Roll the ceiling in as well and the two coats become 1098.0 sq ft, 3.14 gallons, 4 to buy. Ceiling paint is usually a different product at a different coverage, so on a real job it is a second sum rather than a bigger version of the first one.
What the openings do, and what they do not
Doors, windows and cased openings all come out of the wall face, exactly as they do for board — a hole in a wall takes no paint. What they add back is cutting-in time, and that is labour rather than material: a room with six windows takes about the same paint as a room with one and considerably longer.
This calculator counts material. It says nothing about hours, because how long a room takes depends on the crew, the finish, the colour change and whether anybody has to mask a floor — and none of those is something a room’s geometry knows.
Primer is its own sum
Primer covers at its own rate and often at a worse one, especially over new board or over a stain being blocked. Run the calculator twice: once at the primer’s coverage for one coat, once at the finish’s for however many coats you are putting on. Adding a coat at the finish’s coverage rate and calling it primer is short by whatever the gap between the two products is.
Where the wall face comes from
Painting is priced off the same wall face as drywall, which means it is only as good as the perimeter and the ceiling height behind it. Trueline takes both off the room itself and keeps every length’s provenance visible — scanned, drawn or measured — right onto the document a client reads, so a paint number nobody has taped says so on its face.
Questions people ask
- How many square feet does a gallon of paint cover?
- Read it off the can. It is a property of the product, it differs between primer and finish, and it differs again over bare drywall. This calculator asks you for the figure instead of assuming one, because an assumed coverage rate is an invented number sitting underneath a material order.
- Do you subtract doors and windows when estimating paint?
- Yes, from the material. A hole in a wall takes no paint, so every door, window and cased opening comes out of the wall face. What openings add is cutting-in time, and that belongs in the labour line rather than the gallons.
- Should the ceiling be in the same calculation as the walls?
- Only if it is the same paint. Ceiling paint is usually a different product at a different coverage rate, so run it as a second sum. The checkbox above adds the ceiling area to the walls for the case where it genuinely is one product.
- How do I allow for primer?
- Run it separately, at the primer’s own coverage, for the number of primer coats. Adding a coat at the finish coat’s coverage rate leaves you short by whatever the difference between the two products is, which over a whole house is gallons.
This is the app’s arithmetic, not a copy of it
The figures above are not worked out by anything written for this page. The form hands what you typed to Trueline’s own measuring engine — the same modules the iPhone app runs — and prints what comes back. Lengths are held as whole nanometres and money as whole cents, so there is no floating-point number anywhere in a measurement, here or in the app.
The one thing a web form cannot do is know whether anybody put a tape on the wall. That is the whole of the difference between this page and the app it comes out of: Trueline carries where every length came from — scanned, drawn or measured — onto the drawing, the takeoff and the proposal a client reads. You can also take the blank forms and use them with whatever you already have.