The formula behind the calculator
Paint is sold by coverage: a gallon covers a stated number of square feet per coat. So the question "how much paint do I need" is really three smaller ones: how many square feet of wall, how many coats, and how far a gallon goes on your surface.
Gallons = (wall area - doors and windows) x coats / coverage per gallon
Wall area is the perimeter of the room times the wall height. A 12 by 14 foot room with 8 foot walls has a perimeter of 52 feet and 416 square feet of wall. Take off a door (20 sq ft) and two windows (30 sq ft) and 366 square feet remain. Two coats at 350 square feet per gallon is 2.1 gallons. Buy two gallons and a quart.
How far a gallon really goes
The can usually says 350 to 400 square feet. That figure is for a smooth, sealed, previously painted wall. Real walls vary:
| Surface | Coverage per gallon |
|---|---|
| Smooth drywall, previously painted, similar colour | 375 to 400 sq ft |
| Typical interior wall | 350 sq ft |
| Lightly textured wall (orange peel) | 300 to 325 sq ft |
| Bare, unprimed drywall | 250 to 300 sq ft (first coat soaks in) |
| Heavy texture, popcorn ceiling, brick, stucco | 200 to 250 sq ft |
| Exterior wood siding | 250 to 350 sq ft |
Rollers also matter. A thick-nap roller on texture holds more paint and spreads less area per load. Spraying uses 20 to 30 percent more paint than rolling because of overspray, though it is far faster on exteriors and ceilings.
How many coats
- One coat works when you are repainting the same colour in the same sheen with a quality paint-and-primer product on a sound wall.
- Two coats is the standard for any colour change. The first coat covers, the second evens the sheen and hides roller marks. Most paint warranties assume two coats.
- Three coats for deep reds, bright yellows and oranges, which have weak pigments, and for going from a dark colour to a pale one without tinted primer.
When you need primer
Primer is not an extra coat of paint; it seals the surface so the paint sits on top instead of soaking in, and it blocks stains. Use it on bare drywall and joint compound, patched areas, glossy surfaces that have been sanded, water or smoke stains, wood that may bleed tannin, and any big colour change. One coat of primer at normal coverage is enough. Skip it when repainting a sound, matte wall in a similar colour; the money is better spent on a second coat of the finish paint.
Ceilings and trim
The ceiling is simply length times width. Flat ceiling paint covers like wall paint, and one coat is often enough if the ceiling is already white; two if it is stained or you are changing shade.
Trim is measured by length. Baseboards, door casings, window casings and crown moulding are typically 3 to 6 inches wide, so a room with 50 feet of baseboard and the casings for a door and two windows has around 60 square feet of trim. Trim paint is usually semi-gloss, goes on in two coats and is bought by the quart. The calculator rounds this up because trim is fiddly and wastes paint on the brush.
Quick reference: paint per room
Two coats on walls, 8 foot ceilings, one door, two windows, 350 sq ft per gallon.
| Room | Wall area | Wall paint | Ceiling paint |
|---|---|---|---|
| Small bathroom, 6 x 8 | 174 sq ft | 1 gal | 1 qt |
| Bedroom, 10 x 12 | 302 sq ft | 1 gal + 3 qt | 1 qt to 2 qt |
| Bedroom, 12 x 12 | 334 sq ft | 2 gal | 2 qt |
| Living room, 14 x 18 | 462 sq ft | 2 gal + 3 qt | 3 qt |
| Open-plan, 20 x 24 | 654 sq ft | 3 gal + 3 qt | 1 gal + 2 qt |
Mistakes that waste paint or money
- Buying gallons for every colour. For an accent wall a quart often does one coat; two quarts cost less than a gallon.
- Mixing batches. Tinted cans vary slightly. If a job needs three gallons, have the store box them (pour together) or buy a 5-gallon bucket.
- Skipping the 10 percent extra. Touch-ups a year later need the same batch, and a roller holds paint you never get back.
- Painting over gloss without sanding or primer. The new paint peels in sheets.
Doing it yourself or hiring a painter
A room is a weekend for a careful amateur and a day for a crew. Painters buy at trade prices, own the ladders and sprayers, and the finish on trim and ceilings shows the difference. If you are painting more than two rooms or any exterior, get a quote; painters on ProBusiness101 list their services and photos of finished work so you can compare before you call.