How the calculator works it out
For each room it finds the wall area from the perimeter and the height, then takes off the doors and windows. Ceilings are length times width. The paint you need is that area times the number of coats, divided by how far one gallon goes. Walls, ceilings, trim and primer are kept apart because they are usually different products.
Wall area = 2 × (length + width) × height − doors × 21 − windows × 15
Ceiling area = length × width
Gallons = area × coats ÷ coverage per gallon
Primer gallons = (walls + ceilings) ÷ primer coverage, one coat
Trim is estimated from the room: baseboard runs round the perimeter less 3 ft for each doorway, door casing is 17 ft per door (7 + 7 + 3) and window casing 16 ft per window (5 + 5 + 3 + 3). That length times the trim width gives the trim area.
Worked example: a 12 × 14 ft bedroom
With 8 ft walls, one door and one window, the walls come to 2 × 26 × 8 = 416 square feet, less 21 and 15 for the openings: 380 square feet. Two coats at 350 square feet per gallon is 760 ÷ 350 = 2.17 gallons, so 3 gallons. If gallons are $45 and a 5-gallon pail is $190, three single gallons ($135) are cheaper than a pail. The ceiling is 168 square feet; two coats is 0.96 gallons, so 1 gallon of ceiling paint. Priming walls and ceiling together is 548 ÷ 200 = 2.74, so 3 gallons of primer. The trim is 82 linear feet at 3½ inches, 24 square feet, which two coats turn into 0.14 gallons: a quart is enough. This bedroom is loaded when the page opens.
Picking the cheapest cans
Paint comes in single gallons and 5-gallon pails, and the price per gallon of a pail is usually lower. That does not make a pail the right buy for 2 gallons. The calculator rounds what you need up to whole gallons, then tries every number of pails from none upwards and fills the rest with single gallons. It keeps the cheapest combination, and if two cost the same it keeps the one with less left over. Without prices it simply uses a pail for every full 5 gallons.
Coverage at a glance
| Product | Coverage per gallon | Gallons per 1,000 sq ft, one coat |
|---|---|---|
| Paint | 350–400 sq ft | 2.5–2.9 |
| Primer | 200–300 sq ft | 3.3–5 |
What the numbers are based on
- Coverage. The Sherwin-Williams paint calculator gives about 350 to 400 square feet per gallon of paint and 200 to 300 for primer. Your can's label is the final word: some one-coat and specialty paints quote less.
- Door and window deductions. 21 square feet is a 3 × 7 ft door and 15 square feet a 3 × 5 ft window. These are the calculator's assumptions, not a standard, and you can change both.
- Trim. The casing lengths assume one face of a standard door and window. Painting both sides of an interior door's casing roughly doubles that part.
Before you buy
- Buy all the paint for one color at the same time, and box it (mix the cans together) so the shade is even from wall to wall.
- A textured or popcorn ceiling can take noticeably more than a flat one; lower the coverage to 250 to 300 to be safe.
- Keep a little for touch-ups and write the color code on the lid.
Questions people ask
How much does a gallon of paint cover?
Sherwin-Williams puts one gallon of paint at about 350 to 400 square feet, and one gallon of primer at about 200 to 300 square feet. This calculator starts at the low end of both ranges, 350 and 200, so you are less likely to run short. Rough, porous or very dark surfaces drink more paint, so if your wall is textured or you are covering a strong color, lower the coverage figure.
How much paint do I need for a 12 × 14 bedroom?
With 8 ft walls, one door and one window, the walls are 2 × (12 + 14) × 8 = 416 square feet less 21 for the door and 15 for the window, so 380 square feet. Two coats at 350 square feet a gallon is 760 ÷ 350 = 2.17 gallons, so buy 3 gallons. The ceiling (168 square feet, two coats) needs a little under 1 gallon.
Do I need two coats?
Plan on two. A single coat rarely hides the old color evenly, especially when you change color or paint over patched drywall. One coat can be enough when you repaint the same color in good condition. The calculator multiplies the area by the number of coats you choose.
When should I use primer?
Prime new drywall, patched areas, bare wood, stains, glossy surfaces and big color changes such as dark to light. Primer is one coat and covers less per gallon than paint, about 200 to 300 square feet according to Sherwin-Williams. Tick the primer box and the calculator works it out for the walls and any ceilings you included.
Is a 5-gallon bucket cheaper than single gallons?
Often, but not always. It depends on the two prices at your store and how much you need. Enter both prices and the calculator tries every mix of 5-gallon pails and single gallons and picks the cheapest that covers the job. For 12.2 gallons at $45 a gallon and $190 a pail, two pails plus three gallons ($515) beats thirteen gallons ($585) and three pails ($570).
How much should I deduct for doors and windows?
This calculator takes 21 square feet off for each door (3 × 7 ft) and 15 for each window (3 × 5 ft). Change those figures under "Door and window sizes" if yours are different, such as a 6 ft patio door or a large picture window. Small closets and narrow windows are often better left in, since cutting in around them uses paint anyway.