Buying too little means a second trip to the shop and a colour mismatch. Buying too much wastes money. Here's the exact formula for any room, plus adjustments for doors, windows and textured walls.
Most people either guess and run out, or buy far too much. Neither is ideal. Here's how to calculate paint quantities accurately for any room, in under five minutes.
Step 1: Measure the perimeter of the room. Add together the width of all four walls.
Step 2: Multiply by ceiling height. This gives you total wall area in m².
Step 3: Subtract doors and windows. A standard interior door is about 1.85m². A medium window is roughly 1.2m². Subtract these from your total.
Step 4: Divide by coverage rate. Then multiply by the number of coats.
For the ceiling, simply multiply the room length by width.
Textured or porous surfaces absorb more paint. Bare plaster, brick, and textured finishes typically reduce effective coverage by 20–30%. Use 10 m²/litre instead of 12–14 for these surfaces.
Dark colours over light usually need an extra coat. Budget for 3 coats when going dark over white, or use a tinted primer as your first coat to reduce coverage needed for the top coat.
Light colours over dark often need 3–4 coats. This is where people consistently underestimate. A white over charcoal grey can need 4 full coats for solid coverage.
Always buy a little extra. Add 10% to your calculated amount for touch-ups, mistakes, and the inevitable bit you missed. A tin kept sealed in a cool place will last 2+ years for future touch-ups — but only if it's the same batch and same colour mix.
Trade-size tins (10L) are significantly cheaper per litre than consumer 2.5L tins. If you're painting a large space or multiple rooms in the same colour, buying trade size saves noticeably.