Paint Calculator
Guessing paint quantity ends in either a mid-project store run or a garage full of half gallons. This paint calculator estimates wall area from your room dimensions — subtracting standard allowances for doors and windows — or takes a total area directly, then converts to gallons for one or two coats. It uses the industry rule of thumb of 350 ft² per gallon and rounds up to the nearest tenth, with a quarts equivalent and primer estimate alongside.
Each door deducts 21 ft² (rule of thumb)
Each window deducts 15 ft² (rule of thumb)
Rule of thumb: 350 ft²/gal on smooth primed walls
Paint needed
1.8gal
- Quarts equivalentHandy for touch-up planning
- 7.2qt
- Wall area to cover
- 301ft²
- Primer (bare drywall)≈ 80% of finish paint; for raw drywall or big color changes
- 1.4gal
How it was calculated
- Wall area = 301 ft²
- Exact = 301 ft² × 2 coat(s) ÷ 350 ft²/gal = 1.72 gal
- Rounded up to nearest 0.1 → 1.8 gal (= 7.2 qt)
How it works
Wall area from room dimensions: 2 × (length + width) × height gives the gross wall area. A standard door deducts 21 ft² and a window 15 ft² — both rules of thumb, since real openings vary.
Paint needed: wall area × coats ÷ coverage per gallon. The default coverage of 350 ft² per gallon is the industry rule of thumb for smooth, primed interior walls.
Gallons are rounded UP to the nearest tenth: gallons = ceil(exact × 10) ÷ 10. Paint cannot be bought in arbitrary fractions, and running short mid-coat risks visible lap marks.
Quarts equivalent = gallons × 4, useful for touch-up planning. Primer for bare drywall runs about 80% of the finish-paint quantity.
Worked examples
12 × 10 ft bedroom, 8 ft ceilings, 2 coats
Gross wall area: 2 × (12 + 10) × 8 = 352 ft². Subtract 1 door (21 ft²) and 2 windows (30 ft²): 352 − 51 = 301 ft². Two coats at 350 ft² per gallon: 301 × 2 ÷ 350 = 1.72 gal → round up to 1.8 gallons (7.2 quarts). Primer for bare drywall: about 1.4 gallons.
1,000 ft² of wall, 1 coat at 400 ft²/gal
Using the total-area method with 1,000 ft² of wall, one coat, and 400 ft² per gallon coverage (smooth previously-painted walls): 1,000 × 1 ÷ 400 = 2.5 gallons exactly. That is 10 quarts. Primer is not needed over sound existing paint, but spot-prime any patched areas.
How to measure
- Measure each wall’s length at the baseboard and the height floor-to-ceiling; rooms are rarely perfectly square.
- Count every door and window opening — closets with doors count too.
- Do not subtract small fixtures like outlets or thermostats; the paint you would "save" is your cutting-in margin.
- For open-plan spaces, measure each wall run separately and use the total-area method.
- Note the surface: fresh drywall drinks paint, so budget the low end of coverage (300 ft²/gal) there.
Assumptions & limitations
Planning estimate
- Coverage of 350 ft² per gallon, a door allowance of 21 ft², and a window allowance of 15 ft² are rules of thumb for standard interior work — textured, porous, or dark-to-light jobs need more paint.
- Two coats are assumed for color changes; one coat may suffice for same-color refreshes on sound paint.
- Ceilings, trim, and doors are not included — estimate those separately.
- This is a material estimate, not a substitute for professional advice on specialty coatings.
Frequently asked questions
A 12 × 12 ft room with 8 ft ceilings has about 384 ft² of gross wall area; after one door and two windows it is roughly 333 ft². Two coats at 350 ft² per gallon need 333 × 2 ÷ 350 = 1.9 gallons — buy 2 gallons. Enter your exact dimensions above for a precise figure rather than relying on the rule of thumb.
The industry rule of thumb is about 350 ft² per gallon on smooth, primed interior walls — roughly a 12 × 12 ft room’s walls for one coat after openings. Porous surfaces like bare drywall or brick can drop that to 250–300 ft², while a second coat over sealed paint stretches further. Adjust the coverage field above for your surface.
Bare drywall, patched spots, and dramatic color changes need primer; painting over sound existing paint usually does not. Budget primer at about 80% of your finish-paint gallons — it is thinner and covers more. The calculator shows a primer row using that ratio so you can decide based on your walls’ condition.
One coat works for same-color refreshes on walls in good shape, and some premium "paint + primer" products are formulated for it. Color changes, especially light over dark, almost always need two coats for uniform hiding. When in doubt, plan for two — a thin single coat that does not hide costs more in rework than the second gallon would have.
Paint is sold in fixed sizes and a shortfall mid-wall leaves visible lap marks where wet meets dry. Rounding up to the nearest tenth (and then to whole gallons at checkout) guarantees you finish each coat from a single batch. Leftover paint is also your touch-up supply — store it sealed, labeled with the room and color.