THE NUMORIX GUIDE
How to use the Paint Calculator
Last reviewed September 14, 2026
What this calculator does
The engine estimates the four-wall area as 2 x (length + width) x height, subtracts the entered door-and-window area, multiplies the nonnegative result by the number of coats, and divides by coverage per gallon.
Formula and method
The engine estimates the four-wall area as 2 x (length + width) x height, subtracts the entered door-and-window area, multiplies the nonnegative result by the number of coats, and divides by coverage per gallon. It returns gallons rounded to two decimals and converts gallons to liters with 3.785 liters per gallon.
Variables and inputs
Enter room length, width, and wall height in feet; number of coats; paint coverage in square feet per gallon; and the combined area of doors and windows in square feet. The result's coverage field is the total painted area after coats, not the product's coverage rate.
Worked example
For a 12 ft x 10 ft room with 8 ft walls, two coats, 350 sq ft per gallon, and 20 sq ft of openings, wall area is 2 x (12 + 10) x 8 - 20 = 332 sq ft. Two coats require 664 sq ft, so paint is 664 / 350 = 1.90 gallons, about 7.18 liters.
How to interpret the result
The result is the theoretical liquid volume needed for the entered four walls and coats. It does not round the answer to a purchasable can size, so use the decimal gallons to compare with the package sizes available for the product.
Common mistakes to avoid
Enter the total square footage of openings rather than the number of doors and windows, and enter the can's coverage as square feet per gallon. Include every coat in the coats field instead of multiplying the final answer again.
Assumptions and limitations
The model assumes a rectangular room and four paintable walls. It does not estimate ceilings, trim, primer, texture, porous surfaces, color changes, touch-up waste, or product-specific spread rate. If openings exceed the calculated wall area, the painted area is clamped to zero, although the intermediate wall-area step can be negative.