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Interior Paint Calculator — Walls, Doors & Windows

Interior paint jobs require careful measurement to avoid either wasted leftover paint or a frustrating mid-project shortfall. This calculator handles rooms with multiple doors and windows and lets you specify the number of coats for an accurate result.

Enter your measurements

Standard emulsion covers 10–14 m²/L. Matte finishes are at the higher end.

How we calculate

Wall area = 2 × (length + width) × height. Deduct 1.9 m² per door, 1.2 m² per window. Paint = (paintable area ÷ coverage) × coats × (1 + waste %). Cans = ⌈litres ÷ can size⌉.

Worked example

Bedroom: 4 m × 3.5 m, 2.4 m walls, 1 door, 2 windows, 2 coats, 12 m²/L, 10% waste. Paintable area ≈ 33.2 m². Paint ≈ 6.1 L → 3 × 2.5 L cans.

Common mistakes

  • •Measuring total perimeter without deducting openings — overestimates by 10–20%.
  • •Applying only one coat for a significant colour change.
  • •Ignoring primer when painting over bare plaster or a heavily patched surface.
  • •Buying exactly enough without a waste buffer — coverage varies with surface texture.

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