For a plain 12 × 15 ft bedroom with 9 ft walls, one interior door, two windows and two coats of a 350 sq ft per gallon latex, the math works out to about 2.49 gallons. Rounding up to whole gallons gives 3 US gallons. This estimator assumes a whole-gallon purchase; interior paint is also sold in 1-quart cans at major US retailers, and availability of quart sizes varies by product and color.
The shape of a paint estimate: a wall area, minus the openings, times the number of coats, divided by the coverage printed on the can. The round-up to whole gallons is a convenience assumption, not a legal requirement.
The direct answer
Follow this once and you can do it for any room:
- Perimeter × height — add the two sides of the room, double it, multiply by the wall height. For 12 × 15 ft with 9 ft walls: 2 × (12 + 15) × 9 = 486 sq ft of gross wall.
- Subtract openings — this estimator models an interior door as 3 × 7 ft (21 sq ft) and a window as 3 × 5 ft (15 sq ft). These are assumptions, not universal specs; measure your actual openings when they differ. One door and two windows take out 51 sq ft, leaving 435 sq ft of wall to paint.
- Multiply by coats — a true color change needs 2 coats on primed drywall, so 870 sq ft of painted surface.
- Divide by coverage — a good interior latex is often rated 350 sq ft per gallon. 870 ÷ 350 ≈ 2.49 gallons.
- Round to the purchase size you plan to buy — rounding up to whole gallons gives 3 US gallons at $42 each, or $126. If the product is offered in quarts, 2 gallons plus 2 quarts can be closer to the true 2.49 figure for some lines.
Open the paint calculator to run this example with every step shown.
The reasoning
The paint can tells the truth about coverage, not the internet. "350 sq ft per gallon" is the figure for a good quality interior latex rolled on primed drywall at a normal temperature. On a rougher surface (textured ceiling, raw masonry) or in hot, dry weather, that figure drops. Thicker products like a stain-blocking primer run 200–300 sq ft per gallon. Thin exterior stains can be 150 or less. Enter whichever number is on the label and the estimate matches the actual product you are buying.
The 21 sq ft per door and 15 sq ft per window are model assumptions, not universal dimensions. If the room has a double-door opening, a picture window, or non-standard sizes, measure the openings and use the actual square footage instead of rounding them into fictitious door- or window-counts. The point is to subtract the openings from the painted surface.
Coats are where estimates go wrong. "One-coat" paints work when you repaint the same color on an already-painted smooth surface. A real color change — dark to light, light to dark, saturated color on new drywall — needs two coats to look even, and sometimes three. Multiply the painted area by the number of coats you plan to put down, not by one.
Common mistakes
- Counting the ceiling by habit. Ceiling paint is usually a different flat finish bought separately; leave the ceiling checkbox off unless you really are rolling the same product up top.
- Trusting "coverage" from a reviewer. Websites and reviewers quote the figure that worked for them on a specific surface. The can is calibrated to your product.
- Ignoring primer. A new drywall or raw wood job needs a primer coat at its own coverage (often 200–300 sq ft/gal). Run the paint calculator a second time with the primer's coverage figure and 1 coat.
- Treating whole-gallon rounding as the only option. This estimator rounds up to whole gallons as a convenience assumption. Many interior lines are also tinted in 1-quart cans; check the product page for available sizes before deciding how to round.
Where this fits with the floor
If you are also redoing the floor, the same room dimensions feed the square footage calculator for area and the flooring calculator for boxes to order. One measuring trip, three estimates.
The US gallon used throughout is 231 cubic inches, from NIST's unit definitions; it is not the imperial gallon used in the UK. "Gallons to buy" in this guide is always a US gallons figure. The unit definition fixes the volume of a US gallon, not the packaging a given paint line is sold in.