The Paint Calculator estimates the paintable wall area of a rectangular room and the litres of paint required. It accounts for room length, width, height, doors, windows, coats, manufacturer coverage, surface condition, and a waste allowance.
Calculation method
Room dimensions entered in feet are converted to metres before area is calculated. The tool estimates the four-wall area, subtracts fixed allowances of 1.8 square metres per door and 1.4 square metres per window, then applies coats, coverage, surface factor, and waste.
Worked example
For a 5 m by 4 m room with 2.4 m walls, one door, two windows, two coats, 14 m2/L coverage, and 10% waste:
- Wall area:
2 x (5 + 4) x 2.4 = 43.2 m2. - Opening allowance:
1.8 + (2 x 1.4) = 4.6 m2. - Net area:
43.2 - 4.6 = 38.6 m2. - Base paint:
38.6 x 2 / 14 = 5.51 L. - With 10% waste: about
6.06 L, displayed as 6.1 purchase litres.
Reading the estimate
The result includes raw litres, litres with waste, and simple counts for 1 L, 2.5 L, or 5 L tins if buying only one tin size. Actual pack combinations may use less leftover paint than any single-size count.
Frequently Asked Questions
Does the calculator include the ceiling?
No. It calculates four walls only. Calculate the ceiling separately as length multiplied by width and add that paint requirement if needed.
What coverage figure should I use?
Use the coverage printed on the selected paint's technical sheet or tin. Porous surfaces, textured walls, and major colour changes can reduce practical coverage.
Why add a waste allowance?
Waste covers paint left in trays and rollers, touch-ups, measurement variation, and small differences in surface absorption.
Limitations
Door and window deductions use fixed average areas rather than their exact dimensions. The tool does not account for ceilings, trim, cabinets, unusual room shapes, primer, or product-specific application losses. Measure complex surfaces separately before purchasing.