Calculation details
Planning estimate only. Verify dimensions, waste, coverage, product specifications, and local requirements before purchasing materials.
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What does this calculator estimate?
A paint calculator estimates how much paint a room needs from wall perimeter, height, doors, windows, and coats. Enter your room details to get the paintable area and gallons required.
- Openings: 20 sq ft per door, 15 per window
- Coated area = paintable area × coats
- Gallons = ceil(coated ÷ coverage)
How paint quantity is estimated
Paint coverage starts with the gross wall area (perimeter × height), subtracts doors and windows that won't be painted, then multiplies by the number of coats. A gallon typically covers about 350 sq ft per coat.
Limitations to watch for
Actual coverage varies by paint type, surface texture, and color change — dark-over-light or rough surfaces use more. Ceilings, trim, and doors themselves need paint too and aren't included. The 20/15 sq ft per opening values are averages.
How to use it in practice
Measure the wall perimeter and ceiling height. Count doors and windows to subtract. Multiply by coats (2 is standard) and round up to whole gallons. Buy slightly more than needed for touch-ups.
['Enter the wall perimeter and ceiling height.', 'Enter the number of doors and windows.', 'Enter the coats and coverage per gallon; the tool computes gallons.']
How paint coverage is estimated
Most paints cover about 350 square feet per gallon. Coverage = wall area ÷ 350 × coats. A 10×12-foot room with 8-foot walls has about 352 sq ft of wall — one gallon per coat, two gallons for two coats. The calculator runs the arithmetic.
Why two coats
A second coat evens color, covers primer show-through, and extends durability. Coverage estimates assume two coats for a finished job; one coat is for touch-ups. The calculator's coat input makes the difference explicit.
Subtracting openings
Doors and windows reduce paintable area: a standard door is about 20 sq ft, a window 12–15 sq ft. The calculator subtracts what you enter, but many estimators count full walls and let the openings absorb waste.
A worked example
A 12×14-foot room, 8-foot ceilings: perimeter 52 ft × 8 = 416 sq ft, minus 40 sq ft of openings = 376 sq ft. At 350 sq ft/gallon with two coats: 376 × 2 ÷ 350 ≈ 2.15 gallons — buy 3. The calculator returns the gallon count.
Beyond the walls
Ceilings, trim, and primer are separate lines: ceiling adds the room's footprint, trim adds a coat at a different sheen, and primer covers new drywall. Budget them separately with the same coverage math.
How this calculator works
Formula
Gross wall area = perimeter × wall height. Openings = doors × 20 + windows × 15 sq ft. Paintable area = max(0, gross − openings). Coated area = paintable area × coats. Gallons = ceil(coated area ÷ coverage per gallon).
Worked example
A room with 60 ft of wall perimeter and 10 ft walls: gross = 600 sq ft. Subtract 2 doors (40) and 3 windows (45) → 515 sq ft paintable. Two coats = 1,030 sq ft; at 350 sq ft/gallon → ceil(1030 ÷ 350) = 3 gallons.
Assumptions to verify
- Openings average 20 sq ft per door and 15 sq ft per window.
- A gallon covers the entered coverage (350 sq ft is typical).
- Walls are smooth and already primed.
Frequently asked questions
How much paint do I need for a room?
Compute wall area (perimeter × height), subtract doors and windows, multiply by coats, and divide by the coverage per gallon. A typical 12×15 ft room with two coats needs about 2–3 gallons.
How many square feet does a gallon cover?
About 350 sq ft per coat for smooth, primed walls. Textured or very porous surfaces cover less.
Why subtract doors and windows?
They aren't painted with wall color, so subtracting avoids over-buying. This tool uses 20 sq ft per door and 15 sq ft per window.
How many coats do I need?
Two is standard. Dark-to-light color changes or bare drywall may need three; a same-color repaint can need one.
Does this include the ceiling?
No — ceilings, trim, and doors are separate painting jobs with their own area calculations.
Should I buy extra?
Yes — keep leftover paint for touch-ups. Rounding up to whole gallons usually provides enough margin.
Why does my painter's estimate differ?
Painters account for texture, trim, spray loss, and brand-specific coverage. Use this as a planning number and confirm with your painter.
How much paint do I need?
Wall area ÷ 350 sq ft per gallon × number of coats.
Do I need two coats?
Usually yes for even color and durability; one coat is for touch-ups.
Do windows and doors reduce the count?
Yes — subtract their area or let the waste allowance absorb it.
Cite this tool
BoringToolsKit. “Paint Coverage Calculator.” boringtoolskit.com/paint-coverage-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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