Paint Calculator
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Enter your details and hit “Calculate” to see your results.
About the Paint Calculator
Paint runs out in the worst possible place — mid-wall, with a wet edge drying while you drive to the store, and the new can's tint fractionally different. Slightly over-buying is cheap insurance by comparison, but a whole extra gallon is wasted money. The estimate that lands you in the right spot is plain area arithmetic with honest allowances.
Enter the room's dimensions and wall height, count the doors and windows (they subtract standard opening areas), choose one or two coats and your paint's coverage rating, and optionally add the ceiling. The result comes as exact gallons plus a practical buy list in gallons and quarts — including the liters equivalent.
Painting fresh board? New drywall needs primer first — count the sheets with the Drywall Calculator
The Paint Math
The estimator's standard method, allowances and all:
Wall area = 2 × (length + width) × height − 21 ft² per door − 15 ft² per window Gallons = area × coats ÷ coverage (350–400 ft²/gal)
Worked example: a 12 × 12 ft room with 8′ walls, one door and two windows: 384 − 21 − 30 = 333 sq ft. Two coats at 400 sq ft/gal = 1.67 gallons — buy one gallon plus three quarts, or round to two gallons if you want touch-up left over.
Gallons for Common Rooms
Two coats at 400 sq ft/gal, 8′ walls, one door, two windows — computed by this calculator:
| Room | Paintable area | Exact need | Practical buy |
|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 1.34 gal | 1 gal + 2 qt |
| 12 × 12 ft | 333 sq ft | 1.67 gal | 1 gal + 3 qt |
| 12 × 16 ft | 397 sq ft | 1.99 gal | 2 gal |
| 15 × 20 ft | 509 sq ft | 2.54 gal | 2 gal + 3 qt |
Ceilings add length × width ÷ coverage per coat — a 12 × 12 ceiling is another 0.72 gallons for two coats, usually in a different (flat) paint.
Buying & Coverage Tips
Where real-world coverage departs from the label:
- New drywall and skim coats drink the first coat — prime first (primer is cheaper than paint) and rate the first finish coat at the 350 end.
- Dramatic color changes (especially covering dark or red walls) often need a tinted primer or a third coat; budget accordingly.
- Textured walls can cut coverage 10–20% — that's what the 350 sq ft/gal setting is for.
- One gallon ≈ 128 fl oz: a 2 fl oz sample pot covers only ~6 sq ft, fine for swatches, useless for touch-up of a whole wall.
- Have the store shake all cans of one color together (boxing) or box them yourself in a bucket — tint drifts slightly can-to-can.
Keep the leftover quart labeled with the room and date — future-you doing touch-ups will be grateful.
What the Paint Will Cost
Paint cost = gallons needed × price per gallon, and quality is most of the price spread: budget interior lines, mid-range, and premium one-coat-promise paints occupy very different shelves. Example math at illustrative prices: the 12 × 12 room needing 1.67 gallons costs about $50 in a $30 budget line and $117 at a $70 premium — before the roughly $25–50 of one-time supplies (roller frames and covers, tape, tray, drop cloth) a first project adds. Enter your store's actual price against this calculator's gallon figure; better paint often needs fewer coats, which the coats setting lets you price both ways.
Frequently Asked Questions
How much paint do I need for a 12x12 room?
With 8′ walls, one door, and two windows: 333 sq ft of paintable wall → 1.67 gallons for two coats at 400 sq ft/gal. Buy one gallon plus three quarts, or two gallons for leftover touch-up. Add ~0.75 gallons if you're doing the ceiling.
How much does 1 gallon of paint cover?
Most interior paints are rated 350–400 sq ft per gallon for one coat on a sealed, smooth surface. Textured, porous, or unprimed surfaces land below the label figure — which is why this calculator lets you pick the honest end of the range.
How much paint per square foot?
Inverted, the coverage rating says 1 sq ft takes about 1/400 to 1/350 of a gallon per coat — roughly a third of a fluid ounce. A 2 fl oz sample covers only about 6 sq ft, which is exactly one swatch, not a touch-up reserve.
Is one coat of paint ever enough?
Only when repainting a sound surface in the same (or very close) color and sheen. Color changes, new drywall, patched walls, and sheen changes all need two coats for even color and washability — and dark-to-light sometimes needs a tinted primer besides.
Do I subtract windows and doors from wall area?
Yes — unlike drywall (where offcuts are waste), paint genuinely isn't applied over openings. The standard estimating allowances are 21 sq ft per door and 15 sq ft per average window, which is what this calculator subtracts.
How many gallons for a whole house interior?
Sum room by room — an average 3-bedroom interior (walls only, two coats) typically lands between 8 and 12 gallons, but hallways, ceilings, and trim change it fast. Run each room above and add; trim and doors usually take 1–2 gallons of their own in a different sheen.
Methodology. This calculator uses standard construction and material-estimation formulas. It is reviewed and maintained by the Vast Calculators editorial team.
Last updated · July 2026
Results are estimates for general use; verify critical figures independently.