A paint estimate starts with surface area, not the floor size alone. For walls, measure the room perimeter and multiply it by wall height. Subtract large doors and windows, then include the number of coats you expect to apply.
1. Measure each wall
For a rectangular room, add the length and width, multiply by two, then multiply by ceiling height. A 4 m by 3 m room with a 2.4 m ceiling has a wall area of 33.6 m²: (4 + 3) × 2 × 2.4.
2. Deduct the largest openings
A standard interior door and large windows reduce the paintable area. You do not need to deduct every socket or tiny opening; that small margin usually helps cover cutting-in, touch-ups and awkward corners.
3. Add the ceiling only if it will be painted
A ceiling is simply length × width. Treat it as a separate paint product if you are using a dedicated ceiling paint, because its coverage and price may differ from wall paint.
4. Multiply by coats
Most colour changes need two full coats. A dramatic colour change, porous new plaster or very uneven repair work may need a primer plus two topcoats. The paint label has priority over any general rule.
Worked example: 33.6 m² of walls minus 4 m² of openings leaves 29.6 m². Two coats require 59.2 m² of coverage. At 10 m² per litre, that is 5.92 L before a small planning buffer.
Common mistakes
- Using floor area instead of wall area.
- Assuming every paint covers exactly the same area.
- Forgetting a second coat or primer.
- Buying exact quantities with no allowance for waste or later touch-ups.
Use the Paintwise calculator when you have several rooms, mixed paint types or a supplier shopping list to prepare.
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