How to use the paint calculator
- Enter the room length, width and wall height.
- Enter the number of doors and windows.
- Choose the number of coats. Two is usual for a colour change.
- Enter the coverage from the paint can. It is printed on the label and varies by product and surface.
Buy all the paint for one room at the same time so the colour matches from can to can, a habit Decoratoradvice recommends for every project.
Paint formula
Wall area = 2 × (length + width) × height
Paintable area = wall area − 20 sq ft per door − 15 sq ft per window
Paint = paintable area × coats ÷ coverage per gallon
The door and window allowances assume typical sizes (about 3 × 7 ft and 3 × 5 ft). For a large patio door or picture window, measure and subtract the real area.
Worked example
A 14 × 12 ft living room with 8 ft walls, one door, two windows, two coats, 350 sq ft per gallon:
Walls: 2 × (14 + 12) × 8 = 416 sq ft
Openings: 20 + (2 × 15) = 50 sq ft
Paintable: 366 sq ft
Paint: 366 × 2 ÷ 350 = 2.09 gallons
That is 2 gallons and 1 quart. If the remainder would need three quarts or more, the calculator rounds up to another gallon instead.
Ceilings and trim
This calculator covers walls only.
- Ceiling: area = length × width. For the room above, 14 × 12 = 168 sq ft; at 350 sq ft per gallon, one coat needs about half a gallon.
- Trim and doors: usually a quart or two of trim paint per room, depending on how much there is. The baseboard calculator gives the length of trim in a room.
- For a sloped or irregular ceiling, add up its parts with the square footage calculator.
Tips
- New drywall and big colour changes often need a primer coat; count it as a coat with its own coverage.
- Textured or porous walls absorb more paint. Lower the coverage figure to be safe.
- Mix all cans of the same colour in a large bucket ("boxing") so slight batch differences do not show.
- Darker colours absorb more light, so a room may need brighter lighting after a repaint; the room lighting calculator estimates lumens.
- Patch and sand first. Fresh drywall from the drywall calculator project needs to be primed before the finish coats.
Common mistakes
- Using floor area instead of wall area. Paint goes on walls; floor area is much smaller.
- Forgetting the second coat. One coat rarely covers a colour change evenly.
- Assuming every paint covers the same. Use the coverage printed on your product.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door, one window and two coats at 350 sq ft per gallon: 384 − 35 = 349 sq ft × 2 ÷ 350 = 2.0 gallons.
How many square feet does a gallon of paint cover?
It depends on the product and surface. The can states the coverage; use that number in the calculator.
Do I need two coats of paint?
Usually, for even colour, especially when changing colour. Some one-coat products cover in a single coat over a similar colour.
Should I subtract doors and windows?
Yes. The calculator removes 20 sq ft per door and 15 sq ft per window. Measure large openings and subtract their actual size.
How much paint do I need for an accent wall?
Multiply the wall's width by its height, multiply by coats and divide by coverage. A 12 × 8 ft wall with two coats at 350 sq ft per gallon needs about 0.55 gallons. That is just over 2 quarts, so you will need 3 quarts or a gallon. The calculator rounds 3 quarts or more up to a gallon.
Can I use this for wallpaper?
No. Wallpaper is counted in rolls and full-height strips; use the wallpaper calculator.
Key takeaways
- Paint = (wall area − openings) × coats ÷ coverage.
- Use the coverage printed on your paint can.
- Two coats are normal for a colour change; primer counts as a coat.
- Ceilings and trim are separate calculations.