Measure the shape you actually have
Measure interior length, width and height separately. For an irregular room, divide the floor or wall into rectangles, record each piece and add the results rather than forcing one misleading measurement.
Separate openings from surfaces
Record doors and windows separately from the wall dimensions. Behr’s paint calculator also asks for opening information, which is a useful prompt to decide whether a material estimate should exclude them.
Match the product label
Use coverage for the exact paint or coating. Record the number of coats and any allowance for texture, transfer loss or touch-ups. Keep the resulting estimate separate from a purchase decision until container sizes and project conditions are checked.
Sources
- Interior & Exterior Paint Coverage Calculator; Behr; accessed Jul 27, 2026
This guide is educational and does not provide financial, accounting, tax or legal advice.