Use this free paint calculator to estimate gallons from room dimensions or a known wall area in square feet. Enter coats and the coverage on your paint can. Walls only: ceilings, trim and primer are separate.
How it works
Wall area is the room perimeter times the wall height, minus doors and windows. Multiply by the number of coats and divide by the coverage of one gallon.
Worked example
A 14 x 12 foot room with 8 foot walls has 416 square feet of wall. Two doors (40) and two windows (30) leave 346. Two coats is 692, divided by 350 is 1.98, so buy 2 gallons.
Tips
- Rough or dark surfaces can need more paint or primer.
- Buy a little extra for touch-ups.
- Ceilings and trim are separate.
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What you'll need
A plain checklist. Needs vary by project, so follow the instructions on your materials.
- Primer, if the surface needs it - stained or new surfaces often do.
- Painter's tape and drop cloths - protect trim and floors.
- Roller, extension pole and brushes - a brush for edges, a roller for walls.
- Paint tray and liners - easier cleanup.
- Patching compound and sandpaper - fill and smooth holes first.
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Estimates only. Confirm quantities and product details with your supplier before you order.
Guide: How many gallons of paint do I need for a room?
Paint calculator square feet: what to enter
Choose Known wall area in square feet when you already measured the surfaces you will paint. Enter the net area after subtracting openings, not the floor area of the room. This mode does not subtract door or window counts again. If all you know is the room length and width, use Room dimensions and enter wall height as well.
For a rectangular room, gross wall area is 2 x (length + width) x height. In room mode we deduct 20 square feet per door and 15 per window. Those are the average opening figures in Behr's measuring guide, not measurements of your particular openings. For unusual doors or windows, measure each width x height, subtract the actual areas, and use known-area mode.1
How many square feet does a gallon of paint cover?
Behr states 250 to 400 square feet per gallon for one coat of its interior ULTRA SCUFF DEFENSE or PREMIUM PLUS products. That is product-specific guidance. Read the label for the paint you choose. The calculator starts at 350 as an example and lets you replace it. More coats multiply the paint needed; a can's coverage for one coat is not a two-coat promise.1
| Net wall area | 1 coat at 350 sq ft/gal | 2 coats at 350 sq ft/gal | Whole gallons for 2 coats |
|---|---|---|---|
| 100 sq ft | 0.29 | 0.57 | 1 |
| 250 sq ft | 0.71 | 1.43 | 2 |
| 350 sq ft | 1.00 | 2.00 | 2 |
| 500 sq ft | 1.43 | 2.86 | 3 |
| 750 sq ft | 2.14 | 4.29 | 5 |
| 1000 sq ft | 2.86 | 5.71 | 6 |
This chart uses example coverage of 350 square feet per gallon, no extra allowance, and rounds only the final quantity to whole gallons. If the product is sold in quarts, buying whole gallons is not your only option. Compare the unrounded result with the container sizes sold locally.
Two more checked examples
Known 500-square-foot wall area, two coats: 500 x 2 / 350 = 2.85714 gallons. The result is 2.86 gallons before rounding, or 3 whole gallons. At 250 square feet per gallon instead, the same job needs 4 gallons. Coverage makes a real difference.
10 x 12-foot room, 8-foot walls, one door and one window: gross walls = 2 x (10 + 12) x 8 = 352 square feet. Subtract 20 + 15 = 35 to get 317. Two coats at 400 square feet per gallon need 317 x 2 / 400 = 1.585 gallons, or 1.59 before rounding and 2 whole gallons.
Ceilings, trim and primer
A flat rectangular ceiling has area length x width. For the 14 x 12-foot room above, it is 168 square feet. Calculate ceiling paint separately with its own coats and coverage, using known-area mode. Do not add it to wall paint when it is a different product or finish. Behr also describes measuring trim length and multiplying by trim width to estimate area. Measure your own trim width rather than assuming every piece is the same.1
Primer is a separate product calculation. Behr warns that porous, rough, textured or previously unpainted surfaces and dramatic color changes can need more paint or additional coats. Check the instructions for the actual paint and primer. This calculator does not add a waste percentage or decide whether primer is needed.1
Frequently asked questions
Should I put my room floor area in the square-feet input?
No. Paint goes on the walls, so use the combined paintable wall area. A 12 x 14-foot floor is 168 square feet, but its 8-foot-high walls have 416 square feet before openings. Room mode calculates that wall area for you.
Does two coats mean twice as much paint?
For this estimate, yes: area x 2 / coverage. Actual coverage can differ between coats and surfaces, so follow the product label and check whether the stated coverage is per coat.
Why does the calculator round up?
It shows the exact estimate rounded to two decimal places and a separate whole-gallon purchase quantity. You cannot buy a fraction of a sealed gallon can. If quarts are available, use the unrounded estimate to work out a smaller container combination. No touch-up reserve is included.
Sources
- Behr: Determine How Much Interior Paint to Buy (manufacturer guidance for two named product lines, measuring walls and openings, ceilings and trim; read October 6, 2026)