How to use the paint calculator
- Measure the room. Run a tape along each wall at floor level for length and width, then measure wall height from floor to ceiling. If the room isn't a simple rectangle, switch to Total wall area and add up each wall's width × height yourself.
- Count doors and windows. The calculator subtracts 20 sq ft per door (a standard 3 ft × 6 ft 8 in door) and 15 sq ft per window (about 3 × 5 ft). Change those sizes if you have patio doors, large picture windows or small basement windows. Leave closet doors in the count only if you aren't painting them.
- Decide on the ceiling. Ceilings are usually painted with a separate flat ceiling paint, so the calculator lists wall and ceiling paint separately when you include it.
- Check the coverage on the can. Most interior paint labels quote roughly 250–400 sq ft per gallon per coat. The default of 350 sq ft is a middle-of-the-road figure for smooth, previously painted drywall.
The paint formula
Paintable area = wall area − (doors × 20 sq ft) − (windows × 15 sq ft) [+ ceiling: length × width]
Gallons = paintable area × coats ÷ coverage per gallon
In plain words: find the total surface you'll roll, multiply by the number of coats, and divide by how many square feet one gallon covers. The calculator then turns that into something you can actually buy — whole gallons, plus a single quart when you're only a little over.
Worked example: a 14 × 12 ft bedroom with 8 ft walls
The perimeter is 2 × (14 + 12) = 52 ft, so the walls are 52 × 8 = 416 sq ft. Subtract one door (20 sq ft) and two windows (2 × 15 = 30 sq ft) and you have 366 sq ft to paint.
Two coats at 350 sq ft per gallon: 366 × 2 ÷ 350 = 2.09 gallons. That's just over two gallons, so buy 2 gallons + 1 quart (2.25 gallons). If quarts are poor value at your store, three gallons leaves you plenty for touch-ups.
Adding the 14 × 12 ft ceiling (168 sq ft) at two coats takes another 168 × 2 ÷ 350 = 0.96 gallons. The calculator gives the ceiling its own shopping list, 1 gallon of ceiling paint, alongside the 2 gallons + 1 quart for the walls, because the two are usually different products. Together that's 3.05 gallons of paint.
If the walls need priming first, one coat of primer over the same 366 sq ft is about 366 ÷ 350 = 1.05 gallons, assuming the primer covers about the same area as the paint. Check the primer label, since coverage on bare drywall is often lower.
Typical paint coverage
Interior paint labels usually quote a range such as "250–400 sq ft per gallon" for one coat. Where you land in that range depends mostly on the surface. Use this as a rough guide for choosing a coverage setting, and always check the figure printed on your can.
| Surface | Coverage setting to use | Per liter |
|---|---|---|
| Smooth, previously painted drywall or plaster | Top of the label range, about 350–400 sq ft | 8.5–10 m² |
| Primed new drywall, light texture | Middle of the range, about 300–350 sq ft | 7.5–8.5 m² |
| Heavy texture (knockdown, popcorn) | Bottom of the range, about 250 sq ft | about 6 m² |
| Rough or porous: bare masonry, stucco, rough-sawn wood | Often below the range; see the product's own chart | — |
Rough and porous surfaces soak up paint and have more real surface area than their flat measurement suggests. Lower the coverage setting rather than adding an extra coat — it gives a more honest estimate.
Primer and color changes
Use a primer on new drywall, patched areas, stains, bare wood and glossy surfaces. Primer seals the surface so the finish coats go on evenly and cover at their rated rate. "Paint and primer in one" products can work over previously painted walls in good condition, but they don't replace a dedicated primer on raw drywall joint compound.
Big color changes — dark to light, or a deep red or navy over white — often need a third coat or a tinted primer. A gray-tinted primer under deep colors can save a coat of the more expensive finish paint. If you're going over a dark color with a light one, set coats to 3 and see how much it adds.
Tips for buying the right amount
- Cutting in doesn't need extra paint. Brushing along the ceiling line, corners and trim uses the same paint over the same area; it's slower, not thirstier. What does add up is paint left in the tray and roller — on small jobs, that's a reason to round up rather than down.
- Buy all of one color at once and box it (mix the cans together in a bucket) before you start. Separately tinted cans can differ very slightly.
- Price the list you'll actually buy. Enter the gallon price, and if the shopping list includes a quart, the quart price too, and the cost matches the list. Without a quart price, the cost shown is for buying whole gallons instead (3 gallons in the bedroom example). In metric mode, the cost is for the whole liters listed, at your price per liter.
- Keep the leftover. A labeled quart of the exact color makes touch-ups invisible years later.
- Trim and doors are separate. Doors, casing and baseboards usually get a harder semi-gloss; a quart is often enough for the trim in one room.
- Repairing or hanging new walls first? The drywall calculator estimates sheets, tape and joint compound, and the square footage calculator handles odd-shaped areas.
Frequently asked questions
How much paint do I need for a 12×12 room?
With 8 ft walls, a 12 × 12 ft room has 384 sq ft of wall. Subtract one door and two windows (50 sq ft) to get 334 sq ft. Two coats at 350 sq ft per gallon is 1.91 gallons, so two gallons will do.
How many square feet does a gallon of paint cover?
Most interior paints list about 250–400 sq ft per gallon for one coat, with 350 sq ft a common figure on smooth, previously painted walls. Textured, porous or unprimed surfaces get less.
Do I really need two coats?
Usually, yes. Two coats give even color and sheen and hide roller marks. One coat can work when you're refreshing the same color over a sound surface; a dramatic color change may need three or a tinted primer.
Should I subtract doors and windows?
Yes for an accurate estimate — a door is about 20 sq ft and a typical window about 15 sq ft. In a small room with few openings, skipping the subtraction simply gives you a bit of spare paint.
How much paint for a ceiling?
Multiply the room length by the width. A 14 × 12 ft ceiling is 168 sq ft; two coats at 350 sq ft per gallon is about 0.96 gallons, so one gallon of ceiling paint.
Is it cheaper to buy a 5-gallon bucket?
Often, when you need four or more gallons of one color. Compare the per-gallon price; a bucket also keeps the color consistent across the job.