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ProjectCalcMeasure. Calculate. Build.

MEASURE THE WALLS. EXCLUDE THE OPENINGS.

Paint Calculator

Work out how much paint to buy for one room or a whole house — walls and ceilings, doors and windows subtracted, coats and spread rate you can edit, and the answer in tins or gallons rather than a decimal.

v1.0.0 · reviewed 2026-09-18

✓ Visible formulas↔ Imperial + metric＋ Live results
PAINTABLE WALL AREA WITH ROLLER AND OPENINGS STUDY
Paintable wall area with roller and openings studyConstruction material artwork with ProjectCalc measurement overlays.roll on each coatsubtract openings10 ftC
TECHNICAL STUDY / NOT TO SCALEMEASURE → ALLOW → RESULT

01Choose your project

LIVE ESTIMATE / NO SUBMIT NEEDED

02Measure your surfaces

Include units in any measurement, such as 10 ft, 4 in, or 2.4 m. Bare numbers use the selected unit system.

PAINTABLE WALL AREA WITH ROLLER AND OPENINGS STUDY
Paintable wall area with roller and openings studyConstruction material artwork with ProjectCalc measurement overlays.roll on each coatsubtract openings12 ftC
TECHNICAL STUDY / NOT TO SCALEMEASURE → ALLOW → RESULT
352 ft²

Doors & windows

Add the openings you are not covering. Sizes are common defaults — edit any of them to match what you measured.

03Set coats & coverage

352 ft²3 × 1 gallons

The math behind your estimate

The inputs, allowance, and rounding stay visible.

Wall area
Walls = 2 × (Length + Width) × Height
Ceiling
Ceiling = Length × Width
Paintable
Paintable = Gross − (each opening's Width × Height × quantity)
Paint
Volume = Paintable area × Coats ÷ Spread rate
Buy
Containers = Volume ÷ container size, rounded up

Assumptions

  • The default 350 ft²/gal (≈8.6 m²/L) applies to interior latex rolled onto primed or previously painted drywall. Bare drywall, plaster, masonry and textured surfaces can drop that by a third or more — the tin's own figure is the one to trust.Source: Typical manufacturer spread rates for interior latex
  • The element value shown beside each room is its wall area; the Surfaces setting decides whether ceilings are added into the totals.
  • No waste percentage is added. Rounding up to whole containers already leaves slack, and adding a waste factor on top routinely over-orders paint by a full tin.
  • Primer is not modelled separately. If you are priming, run the calculation a second time with the primer's own spread rate — primer coverage differs from topcoat.

Frequently asked questions

How much paint do I need for a 12 × 10 ft room?
With 8 ft walls that is 352 ft² of wall before openings. Subtract a door and a window (about 32 ft²) and you have 320 ft² of paintable wall. Two coats at 350 ft²/gal needs about 1.8 gallons, so buy 2 gallons.
Should I subtract doors and windows?
Yes, once they add up. A single door and window in a small room is roughly 10% of the wall area, which is close to the slack you get from rounding up to a whole tin. In a room with sliding doors or a wall of glass, skipping the subtraction can cost you an extra gallon.
How many coats of paint do I need?
Two is the normal answer for a repaint. Use one if you are refreshing the same colour over a sound surface, and plan for three when going dramatically lighter, covering a strong colour, or painting bare drywall — where a primer coat first is usually cheaper than a third topcoat.
Why does the coverage on the tin vary so much?
Spread rate depends on the surface as much as the paint. Smooth primed drywall might give the full 350–400 ft²/gal; porous bare drywall, rough plaster, brick or heavy texture can take twice the paint for the same area. Edit the spread rate here to match your tin and your surface.

Sources

  • Manufacturer spread-rate ranges for interior latex paints
  • Standard door and window rough-opening dimensions