How to use it
- Enter the room's length, width and ceiling height in feet.
- Add the number and size of doors and windows.
- Set the coats, coverage per gallon and waste allowance.
- Pick the can sizes you can buy and read the result.
How the calculator works
Wall area = perimeter × ceiling height = 2 × (length + width) × height. Doors and windows are subtracted (count × width × height), and the ceiling (length × width) is added if you tick it.
Paint needed = area to paint × coats × (1 + waste %) ÷ coverage per gallon.
Example: a 12 × 12 ft room with 8 ft ceilings has 2 × 24 × 8 = 384 sq ft of wall. Subtract one 3 × 7 ft door (21 sq ft) and two 3 × 4 ft windows (24 sq ft): 339 sq ft. Add the 144 sq ft ceiling: 483 sq ft. Two coats at 350 sq ft per gallon with 10% waste: 483 × 2 × 1.1 ÷ 350 ≈ 3.04 gallons. The calculator then suggests which cans to buy and shows what's left over: with all sizes ticked that's one 5-gallon bucket, without the bucket 3 gallons plus 1 quart.
Coverage: what to enter
Coverage is printed on the can, usually as a range such as 250–400 sq ft per gallon per coat; use the middle of the range. Typical values:
| Surface | Coverage per gallon |
|---|---|
| Smooth, primed drywall | 350–400 sq ft |
| Previously painted wall, same color | 350–400 sq ft |
| Textured walls, popcorn ceilings | 250–300 sq ft |
| New drywall, bare plaster | 200–300 sq ft (prime first) |
| Primer | 200–300 sq ft |
Dark-to-light color changes, fresh drywall and rough surfaces can easily take 20–30% more. When in doubt, plan for two coats.
Choosing can sizes
Tick the sizes your store sells: quart (0.25 gallon), gallon and 5-gallon bucket. The calculator looks for the combination that covers what you need with little left over and not too many cans, because several small cans usually cost more per gallon than one big one. It lists the cans, the total and the expected leftover.
If you're painting several rooms the same color, calculate each room and add up the gallons before buying, so the 5-gallon bucket can pay off. Buy all the paint for one color at the same time; tints can vary slightly between batches.
Primer, ceilings and cost
Tick Also calculate one coat of primer to see how much primer you need, with its own coverage (300 sq ft per gallon by default). Prime new drywall, patched areas, stains and strong color changes.
The ceiling is calculated with the same paint here. If you use a separate ceiling paint, untick the ceiling and work it out by hand: length × width × coats ÷ coverage per gallon. Enter the price per gallon to see the cost of the cans you'll buy.
Tips before you buy
- Measure each wall rather than guessing, and round dimensions up to the next half foot.
- A standard interior door is about 3 × 7 ft (21 sq ft); a typical window about 3 × 4 ft (12 sq ft).
- Trim, doors and cabinets need separate, usually semi-gloss paint: roughly 1 quart covers the trim of an average room.
- Keep the leftover paint, labeled with the room, for touch-ups.
Frequently asked questions
How much paint do I need for a 12x12 room?
About 2 gallons for two coats on the walls of a 12 × 12 ft room with 8 ft ceilings, one door and two windows. With the ceiling included, plan on about 3 gallons.
How many square feet does a gallon of paint cover?
Typically 250–400 sq ft per coat; 350 sq ft is a common figure for smooth, primed walls. Check the label.
Do I need two coats of paint?
Usually yes, especially for color changes, light colors over dark, or new drywall. One coat may do for a fresh coat in the same color with high-quality paint.
Should I subtract doors and windows?
Yes, for an accurate estimate. The waste allowance covers cutting in around them.
How much primer do I need?
One coat over the same area; primer typically covers 200–300 sq ft per gallon.
Last updated: November 12, 2026