Paint Calculator
Estimate how many gallons of paint a room needs — walls, optional ceiling, coats, and a waste allowance all included.
Calculator
Free, runs entirely in your browser, and your numbers never leave your device. Results are estimates for planning only.
What this calculator does
This paint calculator estimates how much paint it takes to cover the walls — and, if you choose, the ceiling — of a single room. It measures the paintable surface, subtracts the area taken up by doors and windows, multiplies by the number of coats, adds a waste allowance for trimming and touch-ups, and divides by your paint's coverage rate to return an amount in both gallons and liters.
How to use it
- Measure the room's length and width and the wall height, then enter all three.
- Enter how many doors and windows the room has so their area can be subtracted.
- Switch the ceiling option on if you plan to paint it, and set the number of coats (two is typical).
- Enter the coverage rate from your paint can — around 350 sq ft per gallon is a common figure.
- Raise the waste allowance if you expect lots of cutting-in or touch-ups, then read off the gallons needed.
The formula
Paintable area = 2 × (length + width) × height + ceiling − openings, where each door counts as 21 sq ft and each window as 15 sq ft. Paint = paintable area × coats × (1 + waste %) ÷ coverage per gallon.
Example calculation
A 12 ft × 12 ft bedroom with 8 ft walls, one door, two windows, two coats, no ceiling, and paint rated at 350 sq ft per gallon:
- Wall area: 2 × (12 + 12) × 8 = 384 sq ft
- Openings: 1 door (21) + 2 windows (30) = 51 sq ft
- Paintable area: 384 − 51 = 333 sq ft
- Two coats: 333 × 2 = 666 sq ft
- With 10% waste: about 733 sq ft to cover
- Paint: 733 ÷ 350 ≈ 2.1 gallons
Result: About 2.1 gallons — so two gallons plus a quart for touch-ups is a sensible buy.
Buying and planning tips
- Buy every can from the same batch and note the batch number, so the color matches across the room.
- Bare drywall, patched repairs, and dramatic color changes soak up more paint — budget an extra coat in those areas.
- Keep a labeled quart of leftover paint for touch-ups; it is far cheaper than re-matching the color months later.
Common mistakes to avoid
- Estimating for one coat when the job really needs two for an even finish.
- Subtracting every closet door and tiny window, which can leave you a gallon short.
- Trusting the can's coverage figure on textured or unprimed walls, where real coverage is noticeably lower.
Assumptions and limits
- Each interior door is treated as 21 sq ft and each window as 15 sq ft.
- Coverage is whatever you enter; 350 sq ft per gallon is a typical starting point, not a guarantee.
- The estimate covers finish paint only — bare or stained surfaces usually need a separate primer coat.
Frequently asked questions
How many gallons of paint do I need for a 12x12 room?
A 12 ft by 12 ft room with 8 ft walls has about 384 sq ft of wall. After subtracting a door and two windows and applying two coats, that works out to roughly 2 gallons — pick up an extra quart for touch-ups.
Should I subtract doors and windows from the estimate?
Yes. This calculator treats each door as about 21 sq ft and each window as about 15 sq ft. Subtracting real openings keeps you from buying paint you will not use, but don't bother with very small ones.
How many square feet does a gallon of paint cover?
Most interior wall paints cover about 350 to 400 square feet per gallon on smooth, primed walls. Textured, porous, or unprimed surfaces cover less, so use the lower end of the range — around 350 sq ft per gallon — if you are unsure.
Do I need primer as well as paint?
Bare drywall, patched repairs, stains, and big color changes usually need a primer coat first. This tool estimates finish paint only, so plan for primer separately when the surface calls for it.
How many coats of paint should I plan for?
Two coats is standard for an even, durable finish. One coat may be enough when repainting a similar color, while deep or drastic color changes can need three.
Does the calculator include the ceiling?
Only when you switch the ceiling option on. When it is on, the tool adds the ceiling area (length × width) to the paintable surface before working out the gallons.
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