Coverage is a starting pointTexture, porosity, color change, application method, and primer can move real coverage away from the number on the can.
How wall area is calculated
For a rectangular room, wall area equals the room perimeter multiplied by wall height. The calculator subtracts 21 sq ft per door and 15 sq ft per window, then adds the ceiling if selected. It multiplies by coats and divides by coverage per gallon.
- Measure rooms separately when heights change.
- Treat closets, accent walls, trim, and doors as separate paint scopes.
- Do not subtract every small opening when touchups and edge work will use material.
Primer and paint are separate products
Primer can improve adhesion, control uneven absorption, and help with major color changes. Do not count a primer coat as one of the finish coats unless the manufacturer describes a combined system and the surface qualifies.
Buy by product line and sheen
Keep the same color, sheen, and batch together when possible. If you round to multiple cans, boxing the paint—mixing cans together before application—can reduce small color differences.
BuildMath Lab provides planning estimates, not engineering, code approval, or a supplier quote. Confirm the final order with the product manufacturer, supplier, plans, and qualified professional.