Paint Calculator: Gallons, Coats and a Material List
Perimeter times height, less the doors and windows, times the number of coats, divided by real coverage. The default coverage is 350 square feet a gallon, which is what you get on a smooth primed wall and not what you get on bare drywall.
When the figures look right, the paint material list downloads as a real PDF with your business details on it, built inside your own browser. Nothing you type is uploaded anywhere.
Worked example
These are the figures the calculator opens with and the answer it gives. Change anything above and every number below moves with it.
- Room perimeter, add all walls48 ft
- Wall height9 ft
- Doors2
- Windows3
- Coats2
- Coverage per gallon350 sq ft
Paint needed2 gallons
| Gross wall area | 432 sq ft |
|---|---|
| Less doors and windows | -87 sq ft |
| Net wall area | 345 sq ft |
| Area over 2 coats | 690 sq ft |
| Gallons, exact | 1.97 |
| Buy | 2 gallonsor 1 five gallon pail |
The material list this produces
| Paint, gallons | 2 |
|---|---|
| Primer, gallons (if bare or color change) | 1 |
| Painter's tape, rolls | 1 |
What this calculator assumes
- A door is taken as 21 sq ft and a window as 15 sq ft. Deducting them matters on a small room and barely moves a big one.
- 350 sq ft per gallon is the usual figure on a smooth primed wall. Bare drywall, textured walls and deep color changes all drink more, so drop it to 300 if any of those apply.
- Ceilings and trim are not included. Run the ceiling as its own area.
Coverage is a marketing number
The 350 to 400 square feet on the can assumes a smooth, sealed, primed surface and a roller in the hands of someone not in a hurry. Reality moves in one direction only.
Bare drywall soaks the first coat and can drop effective coverage to 250. Textured walls, knockdown or orange peel, have more surface area than their footprint suggests and lose 10 to 20 percent. Spraying puts more on the wall and more in the air than rolling. A deep color over a light one takes three coats whatever the can says.
Do the deductions honestly
A standard door opening is about 21 square feet and an average window about 15. On a small bathroom with a door and a window those deductions are a real slice of the wall. On a 30 by 20 great room with nine foot ceilings they barely register.
The one deduction never worth making is a small deduction that leaves you buying three gallons when four would have finished it. Leftover paint is the repair kit for the callback, and a mismatched touch up two years later costs more than the gallon did.
Ceilings and trim are separate jobs
This calculator does walls. A ceiling is its own area, length times width, usually one or two coats of a flat ceiling white. Run it as a second area with the ceiling dimensions as the perimeter and height.
Trim is bought by the quart more often than the gallon, and a gallon of trim enamel goes a long way, typically 300 to 400 linear feet of standard casing and base.
Questions people ask
How many gallons of paint for a 12x12 room?
A 12 by 12 room with 9 foot ceilings has a 48 foot perimeter and 432 square feet of wall. Less one door and one window, that is about 396, and at two coats and 350 sq ft a gallon you need 2.26 gallons, so buy three or a five gallon pail.
Do I need primer?
The primer line on this page's material list is for bare surfaces or a color change.
Painting estimate template and Painting invoice template, free, no signup, PDF Word and Excel.
Common sizes, worked out
The same calculator with the numbers already filled in, one page per size, each ending in its own material list.
More sizes, answered
Each row is this calculator run with those numbers when the page was built. Type the same numbers in above to see every line of the working.
| Example | What it assumes | Paint needed |
|---|---|---|
| 5x7 bathroom | A 5 by 7 bathroom with eight foot walls, one door, one window, two coats. | 1 gallon |
| 6x8 bathroom | A 6 by 8 bathroom with eight foot walls, one door, one window, two coats. | 1.25 gallons |
| 8x8 room | An 8 by 8 room with eight foot walls, one door, one window, two coats. | 1.5 gallons |
| 8x10 room | An 8 by 10 room with eight foot walls, one door, one window, two coats. | 1.5 gallons |
| 9x9 room | A 9 by 9 room with eight foot walls, one door, one window, two coats. | 1.5 gallons |
| 9x10 room | A 9 by 10 room with eight foot walls, one door, one window, two coats. | 1.75 gallons |
| 10x10 room | A 10 by 10 room with eight foot walls, one door, one window, two coats. | 1.75 gallons |
| 8x12 room | An 8 by 12 room with eight foot walls, one door, one window, two coats. | 1.75 gallons |
| 9x12 room | A 9 by 12 room with eight foot walls, one door, two windows, two coats. | 1.75 gallons |
| 10x12 room | A 10 by 12 room with eight foot walls, one door, two windows, two coats. | 1.75 gallons |
| 12x16 room | A 12 by 16 room with eight foot walls, one door, two windows, two coats. | 2.5 gallons |
| 14x14 room | A 14 by 14 room with eight foot walls, one door, two windows, two coats. | 2.5 gallons |
| 20x20 room | A 20 by 20 room with nine foot walls, two doors, four windows, two coats. | 3.75 gallons |
| 2000 sq ft house | A 2,000 sq ft house, interior walls taken as 2.8 times the floor area, twelve doors, fourteen windows, two coats. | 29.5 gallons |
| 3000 sq ft house | A 3,000 sq ft house, interior walls taken as 2.8 times the floor area, sixteen doors, eighteen windows, two coats. | 44.75 gallons |