Every paint estimate uses the same three inputs: the surface area, the number of coats, and the coverage per gallon from the can. This guide shows how to measure each kind of surface, from a simple bedroom to a house with gable ends, and how to turn the result into gallons and quarts. The paint calculator does the arithmetic.
The formula
Gallons = paintable area (ft²) × coats ÷ coverage per gallon (ft²)
Round up to whole quarts: 1 gallon = 4 quarts.
Coverage figures from paint makers:
| Product | Coverage per gallon | Source |
|---|---|---|
| Typical interior paint | about 350–400 ft² | Sherwin-Williams |
| Benjamin Moore exterior house & trim (N402) | 350–450 ft², depending on texture and porosity | Benjamin Moore data sheet |
| Primer | about 200–300 ft² | Sherwin-Williams |
| One quart of paint | about 100 ft² | Sherwin-Williams |
Use the number printed on your product. If you don't have it yet, 350 ft² per gallon is a cautious starting point.
Step 1: Measure the walls
Rectangular rooms
Wall area = perimeter × wall height, where perimeter = 2 × (length + width).
Example: a 12 × 14 ft room with 8 ft walls. 2 × 26 = 52 ft of perimeter, and 52 × 8 = 416 ft².
Rooms that aren't rectangles
Walk around the room and add every wall's width to get the perimeter. Then multiply by the height. Bump-outs, alcoves and closets add wall length, so measure them too.
Walls with a sloped or vaulted top
Treat the triangular part separately: ½ × width × height of the triangle. A 14 ft wall that rises 4 ft above the 8 ft line to a peak adds ½ × 14 × 4 = 28 ft² to its 112 ft² rectangle.
Stairwells
Measure the tallest wall as a rectangle and subtract the triangle below the stair line, or split the wall into a rectangle and a triangle and add them.
Step 2: Subtract doors and windows
Measure each opening (width × height) and add them up.
Example: one 3 × 7 ft door is 21 ft², and two 3 × 5 ft windows are 30 ft². Subtract 51 ft² in total.
If you skip this step, the estimate runs a little high. That's fine for small rooms, but for rooms with patio doors or large windows it can mean an extra gallon you don't need.
Step 3: Add ceilings separately
A flat ceiling has the same area as the floor: length × width. Ceilings are often painted with a dedicated ceiling paint in a flat finish, so estimate them separately with that product's coverage. Textured and popcorn ceilings absorb more paint, so lower the coverage figure. Use the ceiling calculator.
Step 4: Decide on coats and primer
- Same colour, sound surface: often one coat, if the can's directions allow.
- New colour: usually two coats. This is the calculator's default.
- Bare drywall, patches, stains, or a dark-to-light change: primer first, then paint.
For primer, run the numbers again with one coat and the primer's coverage, often lower than paint (Sherwin-Williams gives 200–300 ft² per gallon).
Step 5: Convert to gallons and quarts
Worked example, a 14 × 16 ft bedroom with 9 ft walls:
| Item | Calculation | Result |
|---|---|---|
| Wall area | 2 × (14 + 16) × 9 | 540 ft² |
| Openings | one 3 × 7 door + two 3 × 5 windows | −51 ft² |
| Paintable walls | 540 − 51 | 489 ft² |
| Paint, 2 coats | 489 × 2 ÷ 350 | 2.79 gal → 3 gallons |
| Ceiling, 2 coats | 224 × 2 ÷ 350 | 1.28 gal → 1 gallon + 2 quarts |
| Primer for walls, 1 coat | 489 ÷ 250 | 1.96 gal → 2 gallons |
Note: the primer row uses 250 ft² per gallon, the middle of Sherwin-Williams' 200–300 range. Use your primer's own figure.
Painting a house exterior
Exterior walls follow the same logic at a larger scale.
- Perimeter: measure every wall around the house and add them.
- Wall height: from the foundation line to the eaves.
- Gable ends: each triangle under a roof peak is ½ × gable width × height from the eave line to the peak.
- Openings: subtract doors, windows and garage doors.
- Trim (fascia, window casings, doors) is usually a different paint, so estimate it separately.
Example: a house with a 140 ft perimeter, 10 ft walls, two gables each 30 ft wide and 6 ft tall, and 150 ft² of openings:
- Walls: 140 × 10 = 1,400 ft².
- Gables: 2 × ½ × 30 × 6 = 180 ft².
- Paintable: 1,400 + 180 − 150 = 1,430 ft².
- Two coats at 350 ft²/gal: 2,860 ÷ 350 = 8.17 gal, so 8 gallons + 1 quart.
Benjamin Moore's data sheet notes that coverage depends on surface texture and porosity. Rough siding, stucco and bare wood take more paint than smooth surfaces, so lower the coverage for those. The exterior paint calculator handles gables for you.
Buying tips
- Round up to quarts, then compare prices. Three quarts can cost about the same as one gallon.
- Buy enough for the whole job at once. Benjamin Moore advises estimating the right amount to keep the colour uniform. Paint tinted on a different day can vary slightly.
- Mix cans together ("boxing") on multi-gallon jobs, so every wall gets the same colour.
- Keep a little for touch-ups. Note the colour name and formula on the lid.
Timing between coats
Recoat times are printed on each product. As one example, the Benjamin Moore N402 data sheet lists 1 hour to touch and 4 hours to recoat at 77 °F and 50% relative humidity. It adds that high humidity and cool temperatures lengthen dry and recoat times. Check your own can.
Common mistakes
- Using floor area for walls. A 12 × 12 floor is 144 ft², but its walls are 384 ft² at 8 ft high.
- Forgetting the second coat. It doubles the paint.
- Using a generic coverage figure on rough surfaces. Lower it for texture.
- Mixing primer and paint coverage. Estimate primer separately.
- Counting trim in the wall total. Trim usually gets a different product.
Frequently asked questions
How do I calculate how much paint I need?
Measure the area to paint, multiply by the number of coats, and divide by the coverage per gallon on the can.
How much paint for a 10 × 10 room?
With 8 ft walls, the walls are 320 ft². Minus a 21 ft² door, that's 299 ft². Two coats at 350 ft²/gal is 1.71 gallons, so 1 gallon + 3 quarts. Sherwin-Williams describes one gallon as enough for a 10 × 10 ft room. Two coats on all four walls take more than that.
How much paint do I need for 1,000 square feet?
For two coats at 350 ft² per gallon, 5.71 gallons, so 5 gallons + 3 quarts. At 400 ft², it's 5 gallons.
Should I buy paint in gallons or 5-gallon buckets?
For large jobs, a larger container can be convenient. The calculator gives gallons, so convert using your total.
How long does paint take to dry between coats?
It depends on the product and conditions. See the recoat time on the label. One Benjamin Moore exterior paint lists 4 hours at 77 °F and 50% humidity.
Sources
- Sherwin-Williams, "How to Use Paint Calculator" FAQ: gallon ≈ 350–400 ft², quart ≈ 100 ft², primer ≈ 200–300 ft², and a gallon is described as enough for a 10 × 10 ft room. 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 texture and porosity; colour uniformity; 1 h touch / 4 h recoat at 77 °F, 50% RH. https://media.benjaminmoore.com/WebServices/prod/assets/production/datasheets/TDS_0402/20120820%20N402%20TDS%20US%20OKF.pdf