How to use the paint calculator
- Add a row for each wall with its width and height in feet, and the number of doors and windows on that wall.
- Tick Also paint the ceiling if needed and enter the ceiling area (length x width of the room).
- Set the number of coats and check the coverage per gallon against the label on your paint. Edit the door and window areas if yours are not standard.
- Read the gallons to buy and the gallons-plus-quarts option. Add a price per gallon for a cost estimate.
Formula
Wall area = width x height
Paintable wall area = wall area - (doors x door area) - (windows x window area)
Gallons (exact) = paintable area x coats / coverage per gallon
Gallons with waste = exact gallons x (1 + waste percent)
Gallons to buy = gallons with waste rounded up to a whole gallon
The door area (20 sq ft), window area (15 sq ft) and coverage (350 sq ft per gallon) are starting assumptions. Every one of them is an editable input.
Worked example: a 14 ft by 12 ft room with 8 ft walls
Two walls are 14 x 8 = 112 sq ft and two are 12 x 8 = 96 sq ft, so the walls total 416 sq ft.
The room has one door (20 sq ft) and two windows (2 x 15 = 30 sq ft). Deducting 50 sq ft leaves 366 sq ft to paint.
Two coats at 350 sq ft per gallon need 366 x 2 / 350 = 2.09 gallons. With a 10% allowance that is 2.30 gallons, so you buy 3 gallons, or 2 gallons plus 2 quarts if your paint is sold in quarts.
If you also paint the 168 sq ft ceiling, the area is 534 sq ft and the total with waste is 3.36 gallons, so you buy 4 gallons (or 3 gallons plus 2 quarts).
How coverage really works
The coverage number on a paint can is a best-case figure for a smooth, previously painted surface. Fresh drywall, textured walls, bare wood and dramatic color changes all drink more paint, and a rough surface can need noticeably more than the label suggests. That is why coverage is an input here and not a hidden constant. If your project is a hard surface, lower the coverage number.
A primer coat is separate from the finish coats. If you need primer, run the calculator again with one coat and the primer label coverage, then add the results.
Doors, windows and trim
Deducting openings makes the answer more accurate, but it is a judgment call. Many painters do not deduct small windows because the trim around them still needs paint and the savings are tiny. If you prefer that approach, set the door and window area inputs to 0. Baseboards, crown molding and doors themselves are not calculated here, so budget a quart or more for trim.
Buying gallons and quarts
The calculator rounds up to whole gallons so you are never short, and it also shows the gallon-plus-quart combination when the exact amount ends just past a gallon. Not every brand or finish is sold in quarts, and a quart often costs more per ounce than a gallon, so compare prices before choosing. Keep leftover paint labeled with the room and date for touch-ups.
Assumptions and limits
- Coverage per gallon is an assumption (350 sq ft by default). Use the figure on your paint can and lower it for rough or porous surfaces.
- Each door is assumed to be 20 sq ft and each window 15 sq ft unless you edit them. Real openings vary, and the deduction is only as good as that estimate.
- Each coat uses the same coverage. A first coat on bare or dark surfaces usually uses more.
- Trim, baseboards, doors, closets and cabinets are not included unless you add them as wall rows.
- This is a planning estimate. Confirm quantities with your paint supplier and follow the manufacturer instructions for your product.
Frequently asked questions
How much paint do I need for a 12 by 12 room?
With 8 ft walls the four walls total 384 sq ft before doors and windows. Two coats at 350 sq ft per gallon need about 2.2 gallons, so most people buy 3 gallons. Enter your actual walls for a closer figure.
Do I need to subtract doors and windows?
It is optional. Subtracting gives a tighter estimate, while leaving them in adds a built-in cushion. Set the door and window areas to 0 to ignore them.
How many coats of paint should I plan for?
Two coats is a common plan for a color change, with one coat often enough for a touch-up in the same color. Follow the instructions on your product, which may call for primer first.
Is a ceiling painted with the same coverage as walls?
This tool uses the same coverage for both. Ceiling paint is often a separate product with its own label, so you can run the ceiling alone with its coverage and add the gallons.
Why does the result round up to whole gallons?
Paint is sold in fixed container sizes and you cannot buy 2.3 gallons. Rounding up avoids a second trip and a possible color mismatch between batches.