Start with a sketch
Draw the room from above and mark every wall you plan to paint. Use the same unit throughout and keep your decimal measurements. Rounding individual walls before adding them can change the total unnecessarily.
For a rectangular room, use the perimeter. A 4 × 3 m room has four sides totalling 4 + 3 + 4 + 3 = 14 m. Multiplying by a height of 2.5 m gives 35 m² of wall area.
Subtract unpainted openings
Measure doors and windows separately and add their areas. If they occupy 3 m², the net wall area is 35 − 3 = 32 m². Subtract only surfaces that will not receive the paint in this calculation.
Do not subtract an opening you already excluded by measuring separate wall sections. Keep woodwork separate when it needs a different product. Each group should have its own coverage figure and number of coats.
Keep different surfaces separate
A 4 × 3 m ceiling adds 12 m². Include it in the same operation only when the paint and application are the same as the walls. Otherwise keep separate calculations and separate entries on your shopping list.
For walls of different heights, calculate each rectangle separately. Break complicated shapes into manageable parts and record what you included. The tool organises quantities, but the actual surface area and its condition still depend on careful measuring.
- Use a consistent method for walls and openings.
- Keep your sketch with the saved calculation.
- Check height at several points if the room appears uneven.
Frequently asked questions
Can I measure just the floor?
Not for wall paint: you also need perimeter and height. Two rooms with the same floor area can have different perimeters.
Must I subtract every small opening?
Measure them if you need greater precision. The important point is consistency: do not remove areas and then compensate with an unexplained extra allowance.