Paint Calculator
Calculate gallons of paint by room size - with door and window deductions, number of coats, and optional ceiling and trim.
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Wall perimeter is calculated automatically from length and width.
Trim = baseboards plus door and window casing (not the door slabs).
Each door deducts 21 ft² from the wall area.
Each window deducts 15 ft² from the wall area.
Your Paint Estimate
- Enter room length and width - the calculator finds the wall perimeter automatically.
- Enter ceiling height - 8 ft is standard; taller walls need proportionally more paint.
- Choose coats - two coats is standard; use three for a dark or drastic color change.
- Pick the surface - smooth repaint covers ~350 ft²/gal; new drywall and texture cover less.
- Add ceiling, trim, and openings (Advanced) - toggle the ceiling and trim, and enter doors and windows to deduct.
One gallon of paint covers about 350 square feet per coat - roughly the four walls of a 10 x 10 ft room in one coat. Most jobs need two coats, so a 10 x 10 room takes about 2 gallons and a 12 x 12 room about 2 to 3 gallons. To estimate any room: wall area (perimeter x ceiling height), minus doors and windows, times coats, divided by 350. Enter your room size above for the exact number of gallons to buy.
How Much Paint Do I Need?
Paint is sold by the gallon, and one gallon covers roughly 350 square feet of smooth, primed wall per coat. Manufacturers often quote 400 square feet, but that figure assumes ideal conditions - 350 is the realistic working number this calculator uses, so you are less likely to come up short mid-wall.
To estimate a room, find the wall area (perimeter × ceiling height), subtract the doors and windows you are not painting, multiply by the number of coats, and divide by the coverage per gallon. This calculator does all of that, and lets you add the ceiling and trim and round up to whole gallons to buy.
Most jobs need two coats. A single coat only works for a same-color touch-up; covering a dark color with a light one, or painting bare drywall, can take three.
How Wall Area Is Measured
Paint Coverage by Surface
Coverage per gallon drops as the surface gets more porous or textured - pick the matching option in the calculator:
| Surface | Coverage per gallon | Notes |
|---|---|---|
| Smooth, primed or repaint | ~350 ft² | The calculator default |
| New / bare drywall | ~320 ft² | First coat soaks in; prime first if possible |
| Light texture (orange peel) | ~300 ft² | Knockdown and light splatter finishes |
| Heavy texture / popcorn | ~250 ft² | Deep texture and masonry drink paint |
| Primer | ~200-300 ft² | Coverage varies; check the can |
Gallons by Room Size (2 coats, walls only)
| Room (ft) | Wall area | Gallons (2 coats) | Buy |
|---|---|---|---|
| 10 × 10, 8 ft | 320 ft² | 1.8 gal | 2 gal |
| 10 × 12, 8 ft | 352 ft² | 2.0 gal | 2 gal |
| 12 × 12, 8 ft | 384 ft² | 2.2 gal | 3 gal |
| 12 × 15, 8 ft | 432 ft² | 2.5 gal | 3 gal |
| 15 × 20, 9 ft | 630 ft² | 3.6 gal | 4 gal |
| 20 × 20, 9 ft | 720 ft² | 4.1 gal | 5 gal |
Before door/window deductions, at 350 ft²/gal. Subtract about 21 ft² per door and 15 ft² per window for a tighter estimate.
Painting the Ceiling
A ceiling is just length x width - the same as the floor below it. A 12 x 12 ceiling is 144 square feet, so at two coats it takes well under a gallon (about 0.8), meaning one gallon of ceiling paint covers a typical bedroom ceiling with some to spare. Larger rooms scale up from there: a 15 x 20 ceiling is 300 square feet and still fits in two gallons for two coats.
Ceilings are usually painted with flat white ceiling paint, which hides roller marks and minor imperfections better than wall sheens. Turn on the ceiling option in the calculator to fold it into your total, or paint it as its own small job - one gallon handles most rooms.
Paint Formulas
The room-based method:
Less openings = − 21 ft² per door − 15 ft² per window
Ceiling (opt.) = length × width
Gallons = (paintable area × coats) ÷ coverage per gallon
Buy = round gallons up to the next whole can
Trim (baseboards and casing) is estimated from the room perimeter when enabled. Doors and windows are deducted from the wall area but not from trim.
Worked Examples
Standard bedroom (12 x 12, 8 ft). Walls are 384 square feet. Two coats at 350 per gallon is 768 / 350 = 2.2 gallons - buy 3 one-gallon cans, or 2 gallons plus a quart. Add one door and one window and the wall area drops to about 348 square feet, still 2 gallons for two coats.
Open space (1,000 sq ft of wall). Painting 1,000 square feet of wall takes about 3 gallons per coat (1,000 / 350), or roughly 6 gallons for two coats. At that size, 5-gallon buckets are usually cheaper per gallon than singles.
Whole room with ceiling (15 x 20, 9 ft). Walls are 630 square feet plus a 300-square-foot ceiling = 930 square feet. Two coats is about 5.3 gallons - buy 6, or a 5-gallon bucket plus a single.
How to Get an Accurate Paint Estimate
Measure the room, not just one wall - the calculator turns length and width into the full perimeter for you. Enter the real ceiling height; 9- and 10-foot ceilings add noticeably more area than the standard 8.
Deduct the openings. A standard door is about 21 square feet and an average window about 15, so a room with two doors and three windows loses roughly 87 square feet of wall - close to a quarter-gallon per coat. Skipping the deduction is the most common reason people over-buy.
Buy all your paint at once and, for large jobs, "box" it - combine the cans in one bucket and stir - so the color is identical wall to wall. Keep the leftover labeled for touch-ups.
Before and After Painting
Paint is one step in finishing a room. The usual order:
Related Calculators
Paint is one way to finish a wall - see the full surface calculator collection for wallpaper, tile, and flooring.
Frequently Asked Questions
How much paint do I need for a 12x12 room?
A 12×12 room with 8-foot ceilings has about 384 square feet of wall. At two coats and 350 square feet per gallon, that is roughly 2.2 gallons - buy 3 one-gallon cans, or 2 gallons plus a quart. Deduct about 21 square feet per door and 15 per window for a tighter number.
Does one gallon really cover 400 square feet?
Only in the best case - one coat on a smooth, primed, sealed surface. Real walls, second coats, and any texture bring it down, so this calculator uses 350 square feet per gallon as its working default and lets you drop lower for new drywall or texture. Using 350 keeps you from running out partway through the second coat.
How many coats of paint do I need?
Two coats is standard and gives even color and durability. One coat only suffices for a same-color touch-up. Three coats may be needed when covering a dark color with a much lighter one, painting over bare or patched drywall, or using a hard-to-cover color like a deep red.
Should I subtract doors and windows?
Yes, if you want an accurate number. A standard door is about 21 square feet and an average window about 15. The deductions add up - a few openings can save you a quarter-gallon or more per coat - which is why this calculator subtracts them when you enter the counts in Advanced.
How much paint do I need for 1000 square feet?
For 1,000 square feet of wall, plan on about 3 gallons per coat at 350 square feet per gallon, or roughly 6 gallons for two coats. At that scale a 5-gallon bucket is usually cheaper per gallon than buying singles. Subtract doors and windows for a tighter number.
How much paint for a 10x10 room?
A 10x10 room with 8-foot ceilings is about 320 square feet of wall, close to what one gallon covers in a single coat. For the usual two coats, buy 2 gallons. Add the ceiling (100 square feet) and you still stay within 2 to 3 gallons total.
How much paint for the ceiling and trim?
Turn on the ceiling option to add length × width to the estimate; ceilings usually take the same two coats. Trim - baseboards plus door and window casing - is estimated from the room perimeter when enabled and is usually small enough to buy as a quart unless the room is large.