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.
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.
Frequently asked questions
What is the standard coverage rate for interior paint?+
Most standard interior emulsions cover 10–14 m² per litre. Matte finishes tend to cover slightly more than silk or satin finishes. Always check the coverage rate printed on your specific tin, as premium paints can vary.
How many coats of interior paint do I need?+
Two coats is the standard for most interior walls. You may need three coats when painting a very light colour over a dark base, or when using cheap paint with low opacity. One coat is only sufficient for a refresh coat over the same colour.
How do I account for awkward room shapes?+
For L-shaped or irregular rooms, measure each wall separately (height × width), sum the areas, and deduct door and window openings. Use the total paintable area as your starting point rather than room perimeter.
Should I add a waste allowance for interior painting?+
Yes — 10% is appropriate for smooth walls. Increase to 15% if you have textured walls or are rolling into tight corners and mouldings, where paint usage is noticeably higher than on flat surfaces.