How to use it
- Measure the room's length and width along the floor, and the wall height from floor to ceiling.
- Set the coats and the coverage printed on your paint can. If you haven't bought paint yet, leave the estimate in place.
- Count the doors and windows. Change the sizes if yours are bigger than average, like a double closet door or a picture window.
- Turn on the ceiling, primer or trim sections if they're part of the job.
- Type your store's prices in the cost section to price the materials.
Results update as you type. Copy result gives you a plain-text shopping list, and the page address saves your numbers so you can come back to the job.
The formula
Wall area = 2 × (Length + Width) × Height
Area to paint = Wall area − (Doors × door size) − (Windows × window size)
Gallons = Area to paint × Coats ÷ Coverage per gallon, rounded up to buy
Ceiling area = Length × Width
Trim area = Feet of trim × (Face width in inches ÷ 12)
Coverage on a can is for one coat, so each coat uses the full amount again. Quarts are shown when a job needs three quarts or less. Past that, a gallon makes more sense. The buying line splits big jobs into five-gallon buckets plus single gallons. That's the size buckets come in, so it isn't an input.
Worked example: a 12 ft × 14 ft bedroom with 8 ft walls
- Walls: 2 × (12 + 14) = 52 ft around, × 8 ft = 416 sq ft
- Openings: 1 door at 20 sq ft + 2 windows at 15 sq ft = 50 sq ft, leaving 366 sq ft to paint
- Wall paint, 2 coats at 350 sq ft per gallon: 366 × 2 ÷ 350 = 2.09 gallons, so buy 3 gallons
- Ceiling: 12 × 14 = 168 sq ft, × 2 coats ÷ 350 = 0.96 gallon, so buy 1 gallon
- Primer on the walls, 1 coat at 300 sq ft per gallon: 366 ÷ 300 = 1.22 gallons, so buy 2 gallons
- Trim: about 49 ft of baseboard plus 17 ft of door casing is 66 ft. At 4 in wide that's 66 × 4 ÷ 12 = 22 sq ft. Two coats is 44 sq ft, or 0.13 gallon, so 1 quart
For walls, three gallons leaves almost a gallon over. That's not waste. Keep it, labeled with the room, for touch-ups. In metric, the same room (3.66 × 4.27 m, 2.44 m walls, 8.6 m² per liter) needs 7.92 liters, still 3 gallons to buy.
Why two coats
One coat rarely hides what's under it evenly, especially over a different color, fresh patches or new drywall. The second coat evens out the color and sheen and fills thin spots from the roller. If you're going over the same color on clean, sound walls, one coat may do, so set coats to 1 and see the difference.
When you need primer
Prime new drywall, big patches and skim coats. Bare joint compound soaks up paint and shows through as dull spots, called flashing. Also prime water stains, smoke damage, bare wood and glossy surfaces, and big color changes like dark to light. Clean, sound, already-painted walls usually don't need it. A "paint and primer" product can stand in on some repaints, but over new drywall, a real primer is the safer call.
Sheen and rough surfaces
Flat and matte hide flaws best, which is why they're common on ceilings. Eggshell and satin wipe clean better and suit most living spaces. Semi-gloss stands up to scrubbing and moisture, so it's the usual pick for trim, doors, kitchens and baths, but it shows every dent. Coverage can differ a little by sheen, so check the can.
The number on the can assumes a smooth, primed wall. Heavy texture, popcorn ceilings, block, brick and rough wood all have more surface than their flat size, and they soak up more paint. For those, lower the coverage. Something in the 250–300 sq ft per gallon range is a common starting estimate. Then watch how far your first gallon actually goes.
Common mistakes
- Forgetting the coats. The label rate is for one coat. Two coats needs twice the paint.
- Over-deducting openings. Small windows and closet doors don't save much paint. If in doubt, deduct less.
- Mixing up batches. Two gallons of the same color can look slightly different. On a big job, pour all your gallons into one bucket and mix them, called boxing, so every wall matches.
- Skipping the closets. Add closet walls to your measurements if you're painting them.
Questions people ask
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and one window, that's 349 sq ft to paint. Two coats at 350 sq ft per gallon comes to 1.99 gallons, so 2 gallons. It's close, so if your walls are textured or the color is changing a lot, buy a third.
How much does a five-gallon bucket cover?
At a label rate of 350 sq ft per gallon, about 1,750 sq ft for one coat, or about 875 sq ft for two coats. Buckets often cost less per gallon than singles, so compare prices once you need four or more gallons.
Should I subtract doors and windows?
Yes for a real count, but use honest sizes. A standard door is about 20 sq ft. A window varies a lot, so measure big ones. Deduct garage-size openings and wide archways too.
How much paint do I need for trim?
Less than most people think. Baseboard and casing are only a few inches wide, so a typical room's trim often fits in a quart. Measure the face width, and round up a little for detailed profiles, which have more surface.
How do I figure the cost?
Type the price per gallon for each paint, a lump sum for supplies, and any labor in the cost section. If you're buying a five-gallon bucket, divide its price by five to get the per-gallon price.