How to Measure a Room for Paint

Measurement Checklist

• Measure room length in metres along one wall.
• Measure room width in metres along the adjacent wall.
• Measure wall height from finished floor to ceiling in metres.
• Count the number of coats you want the calculator to apply.
• Read the paint coverage from the product label in m2 per litre.
• Measure each door or window opening as width x height and add the opening areas together in m2.
• Choose a waste percent and enter the container size in litres.

Worked Example

1. Room length 4 m, room width 5 m, wall height 2.5 m.
2. Gross wall area = 2 x (4 + 5) x 2.5 = 45 m2.
3. Door and window openings total 3.8 m2, so net wall area = 45 - 3.8 = 41.2 m2.
4. With 2 coats and 10 m2/L coverage, litres before waste = 41.2 x 2 / 10 = 8.24 L.
5. With 10% waste, litres after waste = 8.24 x 1.1 = 9.064 L.
6. With 5 L containers, containers required = ceil(9.064 / 5) = 2.

Common Mistakes

• Entering the floor area instead of wall area. The calculator builds wall area from room perimeter and height.
• Forgetting to multiply every opening width by its height before adding openings together.
• Entering coverage for one coat but setting coats to a value that does not match the project plan.
• Using zero for waste when touch-ups, surface texture, colour change, or roller loss should be allowed for separately.
• Entering a container size smaller or larger than the actual container you plan to use.

Assumptions and Limitations

• The room is treated as a rectangle with four full-height walls.
• Opening area is subtracted once from gross wall area.
• Waste is applied after the base litre calculation.
• Containers are rounded up to whole containers.
• The calculator does not account for ceilings, trim, separate feature walls, surface preparation, primer, paint type, or supplier stock.