Paint quantity formula
Paint is a coverage calculation, not simply room square footage. The calculator finds paintable wall area, subtracts measured openings, optionally adds the ceiling, multiplies by coats, then divides by the coverage rate you enter.
wall area = 2 × (length + width) × height − openings
coating area = (walls + optional ceiling) × coats
gallons = coating area ÷ product coverage
Coverage changes with product, substrate, texture, color change, spray/roll method, and application. Use the number on the actual product data sheet or can label when possible.