Start with wall area, not floor area
For a rectangular room, wall area is 2 × (length + width) × height. A 3.2 × 2.4 m room with 2.4 m walls has 26.88 m² of wall area before deductions. Subtract the area of doors and windows you will not paint with that product.
The wall and ceiling paint calculator keeps wall openings separate from ceiling exclusions. Removing a window from the wall area must not reduce the ceiling area.
Apply coverage and coats once
With no deductions, two coats over 26.88 m² at a hypothetical 10 m² per litre require 26.88 × 2 ÷ 10 = 5.376 litres. If you choose 4-litre tins, buy 2 tins, providing 8 litres.
Use the coverage stated for your product and surface. The example rate is not a promise of actual coverage. The calculator does not add a separate loss allowance, so check preparation and application guidance before buying.
Keep different paints on separate lines
Ceiling paint and primer each need their own coverage rate, coats and tin size. Do not assume a primer covers the same area per litre as the finish paint. See how primer quantities are calculated.
When entering gallons, choose US or Imperial gallons explicitly. They are different volumes. The calculator converts the coverage rate and selected tin size together when you switch units.