How to use the paint calculator
- Enter the room's length and width in feet, measured along the floor.
- Enter the wall height, floor to ceiling.
- Enter how many doors and windows the room has, then edit the area per door and per window to match your own (measure width × height).
- Tick "Include ceiling" if the ceiling gets the same paint. Many people use a separate ceiling paint; if so, use the ceiling calculator instead.
- Set the coats (default 2) and coverage per gallon (default 350 ft²). Change the coverage to the figure on your can.
- Optionally tick primer (default coverage 250 ft² per gallon, the middle of Sherwin-Williams' 200–300 range) and enter the primer's label coverage, plus prices per gallon if you want a cost estimate.
- Read "To buy": the result rounded up to whole gallons plus quarts, so you don't overbuy.
The paint formula
Wall area = 2 × (length + width) × height − doors and windows
Gallons = area × coats ÷ coverage per gallon
- Coverage: Sherwin-Williams says a gallon "typically covers about 350–400 square feet". A Benjamin Moore exterior house paint data sheet lists 350–450 ft² per gallon, "depending on surface texture and porosity". The calculator defaults to the conservative 350.
- Quarts: 1 gallon = 4 quarts. The calculator converts the remainder to quarts.
- Primer covers less. Sherwin-Williams gives about 200–300 ft² per gallon.
Worked examples
A 12 × 12 ft room with 8 ft walls
- Walls: 2 × (12 + 12) × 8 = 384 ft².
- Openings: one 3 × 7 ft door (21 ft²) and one 3 × 4 ft window (12 ft²) make 33 ft², which leaves 351 ft².
- Two coats at 350 ft²/gal: 351 × 2 ÷ 350 = 2.01 gallons. Rounded up, that's 2 gallons + 1 quart.
- At 400 ft²/gal it would be 1.76 gallons, so 2 gallons. This shows why the coverage on your specific can matters.
A 14 × 16 ft room with 9 ft walls
- Walls: 2 × 30 × 9 = 540 ft², minus one door (21 ft²) and two 3 × 5 ft windows (30 ft²), leaves 489 ft².
- Two coats: 978 ÷ 350 = 2.79 gal, so 3 gallons.
- Including the 224 ft² ceiling in the same paint: 713 ft² × 2 ÷ 350 = 4.07 gal, so 4 gallons + 1 quart.
One accent wall, 12 × 8 ft
- 96 ft² × 2 coats ÷ 350 = 0.55 gal, so 3 quarts.
A 12 × 12 ceiling, one coat
- 144 ÷ 350 = 0.41 gal, so 2 quarts.
Priming a 12 × 12 room before painting
- Primer on 351 ft² at 200–300 ft² per gallon is 1.17–1.76 gallons. Buy 2 gallons to be safe, or 1 gallon + 1 quart if your primer's label says 300.
How many coats do I need?
The calculator assumes two coats, which is common for a colour change. You may need:
- One coat when repainting the same colour over a sound, clean surface, if the can says one coat is enough.
- Two coats for most colour changes.
- Primer plus two coats for bare drywall, patched areas, stains, or big jumps in colour, such as dark to light.
Always follow the directions on your product's label. The calculator just multiplies by whatever number of coats you enter.
Tips and common mistakes
- Use the coverage on your can. Coverage differs by product and by surface. Rough, porous or textured surfaces soak up more paint, so lower the figure for them.
- Measure doors and windows rather than guessing. Small rooms with several doors and closets can lose a lot of wall area.
- Buy enough to finish. Benjamin Moore's data sheet notes that estimating the right amount helps ensure colour uniformity. A second batch mixed later can vary slightly.
- Box your paint. On jobs of several gallons, pour all the cans into one larger bucket and mix them so the colour is even.
- Trim and doors are usually a different paint or sheen. Leave them out of the wall total.
- Quarts add up. A quart is a quarter gallon. If you need 3 quarts, compare their price with one gallon.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and one window, about 351 ft² of wall. Two coats at 350 ft² per gallon is 2.01 gallons, so 2 gallons + 1 quart. At 400 ft²/gal, 2 gallons is enough.
How many square feet does a gallon of paint cover?
Sherwin-Williams says about 350–400 ft². A Benjamin Moore data sheet gives 350–450 ft², depending on texture and porosity. Check your can.
How many square feet does a quart cover?
Sherwin-Williams says a quart covers about 100 ft². At our default 350 ft² per gallon, a quarter of that is about 87.5 ft², which is slightly more conservative.
How much primer do I need?
Sherwin-Williams gives about 200–300 ft² per gallon of primer. Tick Add one coat of primer in the calculator. Its primer coverage defaults to 250 ft² per gallon, the middle of that range, so change it to your primer's label figure.
Do I subtract doors and windows?
Yes, if you want an accurate number. Measure each opening (width × height) and add them up. Leaving them in gives you a small surplus.
Is two gallons enough for a bedroom?
For a 12 × 12 room with 8 ft walls and two coats at 400 ft²/gal, yes. At 350 ft²/gal you'd need a quart more. Larger rooms or higher ceilings need more. Enter your room's measurements to check.
Sources
- Sherwin-Williams, "How to Use Paint Calculator" FAQ: a gallon covers about 350–400 ft², a quart about 100 ft², primer about 200–300 ft². https://www.sherwin-williams.com/en-us/color/color-tools/paint-calculator
- Benjamin Moore technical data sheet (N402, exterior house & trim): 350–450 ft² per gallon depending on surface texture and porosity; estimate enough for colour uniformity. https://media.benjaminmoore.com/WebServices/prod/assets/production/datasheets/TDS_0402/20120820%20N402%20TDS%20US%20OKF.pdf