How the area is worked out
For each room, the wall area is the perimeter, two times length plus width, times the height. Doors and windows are taken out at the sizes you set, and the ceiling is added if you paint it. The total is the area your painter should quote per square foot.
Paint needed is that area times the number of coats, divided by the coverage per litre for one coat, which is printed on the tin or the product data sheet. The calculator also shows the figure with 10% extra.
On the quotation
Quote painting as an area with a rate, and name the product, number of coats and whether putty and primer are included. Two painting quotations with different areas for the same flat cannot be compared; showing the area settles it. For ceilings with cove lights, see the false ceiling calculator.
Questions
How is interior painting cost per sq ft calculated?
Work out the paintable area: the four walls (perimeter times height) less doors and windows, plus the ceiling if it is painted. Multiply that area by the rate per sq ft your painter quotes for the paint and finish.
How much paint do I need for a room?
Multiply the paintable area by the number of coats and divide by the coverage per litre printed on the tin. Buy about 10% more for touch-ups and absorption on new walls.
Should doors and windows be deducted?
Yes, for a fair quotation. The calculator deducts a standard door of 21 sq ft (7 × 3 ft) and window of 16 sq ft (4 × 4 ft); change these to match your openings.
Why is there no default coverage or rate?
Coverage differs between primers, emulsions and enamels, and painting rates differ by city and finish. The tin and your painter give the right figures for your job; a default would be wrong for most.