Buying the wrong amount of paint is one of the most common DIY mistakes — too little means a half-finished room and a color match gamble, too much means money sitting on a shelf. The reliable method takes about ten minutes with a tape measure.
The method in one line
Every paint estimate — whether you do it by hand or with the Paint Calculator — is this same chain of five small calculations. Do them in order and you will land on the same number a paint store would quote you.
Step 1: Calculate gross wall area
For a standard four-wall room, measure the room's length and width, and the wall height (floor to ceiling, not to the top of furniture). Then:
A 14 × 12 ft room with 8 ft ceilings has a gross wall area of 2 × (14 + 12) × 8 = 416 square feet. The 2 accounts for opposite wall pairs; the perimeter (length + width) times 2 walks around all four walls.
For rooms that break the four-wall assumption — L-shapes, partial walls, accent walls — measure each wall individually and add the areas: width × height per wall. The Paint Calculator has an individual-walls mode for exactly this.
Step 2: Deduct doors and windows
You do not paint over glass or hardware, so openings come off the total:
- Standard interior door: roughly 21 sq ft (3 × 7 ft)
- Standard window: roughly 12 sq ft (3 × 4 ft)
- Custom openings: width × height, measured
In the example room, two doors and three windows deduct 2 × 21 + 3 × 12 = 78 sq ft, leaving a paintable area of 416 − 78 = 338 sq ft.
If you are painting trim and doors the same color as the walls, skip the deduction for those openings and simply buy a little extra — the door faces you skip come back as door edges and trim.
Step 3: Multiply by coats
Most repaints need two coats; one coat rarely hides evenly, especially over repairs or color changes. A dark color over a light wall can take three. Multiply the paintable area by your coat count: 338 × 2 = 676 sq ft of coverage needed in the example.
Step 4: Add a waste allowance
Some paint never reaches the wall: tray residue, brush and roller build-up, drips, and the touch-ups every project needs. A 5–15% allowance is standard, with 10% a sensible default. In the example: 676 × 1.10 = 744 sq ft.
Go toward the top of the range for textured walls, many corners, or bare drywall; see How Much Extra Material Should You Buy? for how to choose.
Step 5: Divide by coverage and round up
Coverage is printed on the can — most interior paints fall between 350 and 400 sq ft per gallon, but products genuinely differ, so check your own can. In the example: 744 ÷ 350 = 2.12 gallons. Paint is sold in sealed containers, so buy 3 gallons.
At a sample price of $42 per gallon, that is 3 × $42 = $126. Enter your own price in the Paint Calculator for the same math with your numbers.
Ceilings: a separate decision
The calculation above covers walls only. Ceiling paint is its own estimate — ceiling area is length × width (14 × 12 = 168 sq ft in the example), usually painted white with its own coverage. If you are painting walls and ceiling the same product, tick "Include ceiling" in the calculator and it folds the ceiling area into the total; the result always states which scope it used.
Common mistakes
- Forgetting the second coat. The single most common cause of running out mid-room.
- Measuring height wrong. Measure to the ceiling; baseboards and moulding barely change the area, but a foot of missed wall is a foot of missed wall.
- Trusting a generic coverage number. Texture, porosity, and color contrast all eat coverage. Cheap paint often lists optimistic coverage too.
- Counting the same window multiple times instead of using a quantity.
- Assuming one gallon covers a room. Most average bedrooms need two to three gallons for two coats.
FAQ
Does round-number buying help?
Yes — after rounding up, most rooms land neatly at 2, 3, or 4 gallons anyway. Where the math says 3.05, buying 3 and a quart is a reasonable middle ground for touch-ups.
Should I mix leftover cans to get a full gallon?
Only if they are the same product and batch. "Boxing" paint from different batches is a real practice for avoiding batch variation, but do it before you start, in a clean container, and mix thoroughly.
Does this method work for exterior walls?
The arithmetic does, but exterior surfaces rarely have clean four-wall geometry, and masonry or rough siding consumes far more paint than the label assumes. Measure each surface individually and lean toward higher waste.