Estimate the US gallons of paint to buy for a room from its walls, doors and windows, the number of coats and the coverage printed on the can. See square footage for odd-shaped rooms.
Gallons are rounded up to the next whole US gallon (231 cubic inches), the US retail standard.
How we calculated this
The calculator takes the perimeter of the room (2 × (length + width)) and multiplies it by the wall height to get the gross wall area. It subtracts 21 sq ft per door and 15 sq ft per window — model assumptions based on a 3 × 7 ft door and a 3 × 5 ft window; measure your openings if they differ. It then multiplies by the number of coats, divides by the sq ft per gallon on the can, and rounds up to the next whole US gallon.
gross wall = 2 × (length + width) × wall height
wall area = gross wall − 21 × doors − 15 × windows
painted = (wall area + optional ceiling) × coats
gallons = ceil(painted ÷ sq ft per gallon) [US gallons]
The US gallon is defined as 231 cubic inches, and interior paint is commonly sold in 1-gallon and 1-quart cans at major US retailers. The calculator figures the wall area actually painted, subtracts assumed door and window openings, multiplies by the number of coats, divides by the coverage printed on the can, and rounds up to whole gallons. Whole-gallon rounding is a convenience assumption; quart availability varies by product.
Why coverage varies so much
The square feet per gallon printed on the can is the number that matters, and it varies with the paint. A high-quality interior latex is often rated for 350–400 sq ft per gallon, a thick primer for 200–250, a thin exterior stain for 150. Pricier 'one-coat' paints assume you are repainting the same color on a smooth surface; a true color change still needs two coats to look even, which doubles the gallons.
The calculator does not try to pick a number for you. Enter the coverage printed on the can, so the estimate matches the actual product you are buying.
Doors, windows and ceilings
This estimator models an interior door as 3 × 7 ft (21 sq ft removed) and a window as 3 × 5 ft (15 sq ft). These are model assumptions, not universal specs. If your room has a double-door opening, a picture window, or non-standard sizes, measure the openings and reflect the actual area rather than inventing extra door- or window-counts. Trim and baseboards are counted inside the wall area because they are usually painted along with the walls.
The ceiling checkbox adds the floor area as a second painted surface. Keep it off when the ceiling is already finished or will be painted separately with ceiling paint.
Worked example: a 15 × 12 ft bedroom
A 15 × 12 ft bedroom with a 9 ft ceiling has a wall perimeter of 54 ft and a gross wall area of 486 sq ft. Subtract one door (21 sq ft) and two windows (30 sq ft) and you are painting 435 sq ft. Two coats make 870 sq ft. At 350 sq ft per gallon that is about 2.49 gallons, so you buy 3 US gallons. At $42 per gallon the paint costs $126. If the ceiling were painted too, the painted area climbs to (435 + 180) × 2 = 1,230 sq ft, which rounds up to 4 gallons.
Limits and assumptions
The estimate rounds up to the next whole US gallon so the number is 'what to buy.' It assumes rectangular rooms with uniform wall height, standard door and window sizes, and the same color throughout. Trim, cabinets, closets and accent walls are counted inside the wall area; if you are painting them in a different color, run the calculator a second time for the trim or accent area with its own coverage figure. Primer, if needed, is a separate calculation at its own coverage rate.
Key terms
US gallon
A US gallon is exactly 231 cubic inches (about 3.785 liters), different from the UK imperial gallon. The unit definition fixes volume, not the packaging a paint line is sold in; interior paint is also offered in 1-quart cans by many brands.
Coverage
The square feet of surface one gallon covers in one coat. Printed on the can; depends on the paint, the surface and whether you are rolling or spraying.
Coat
One pass of paint from edge to edge, allowed to dry before the next. A true color change usually needs two coats; dark over light sometimes three.
Frequently asked questions
How many gallons do I need for a 12 × 14 ft room?
With 9 ft walls, 2 coats, a 350 sq ft/gal paint, 1 door and 2 windows, the painted area is (468 − 51) × 2 = 834 sq ft, which rounds up to 3 US gallons.
Why does the calculator round up to a whole gallon?
Rounding up to whole gallons is a convenience assumption this estimator uses because gallon cans are the common purchase size. Many interior paint lines are also tinted in 1-quart cans, so if you need slightly over 2 gallons, 2 gallons plus 1–2 quarts can be a closer match. Availability of quarts varies by product and color.
Does one gallon really cover 350 square feet?
That is the typical label figure for a good interior latex rolled on primed drywall. Rough surfaces, hot weather and dark-over-light color changes all drop coverage; use the number printed on the can you are buying.
Should I include the ceiling?
Only if you are painting it with the same paint. Ceiling paint is usually a different flat finish bought separately; leave the checkbox off and figure it on its own.
What about primer?
Primer has its own coverage (often 200–300 sq ft per gallon) and usually needs one coat. Run the calculator a second time with the primer's coverage figure and 1 coat.
Figure out the US gallons of paint to buy for a room from its walls, doors, windows, number of coats and the coverage printed on the can, with a worked example.
Split an L-shaped, bay-windowed or many-angled room into simple rectangles, measure each one, and add the areas for a clean square-footage figure to order material from.
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