Last updated: October 2026
How much paint do I need?
Short answer: Multiply your paintable wall area by the number of coats, then divide by the paint's coverage per gallon — 350 sq ft for standard interior paint, 400 for primer, 300 for semi-gloss or exterior. Round up to whole gallons; you can't buy part of a can.
How to measure a room for paint
Measure the length × height of each wall in feet and add them — or enter each wall separately above. A standard door removes 21 sq ft and a standard window removes 15 sq ft, which the calculator subtracts automatically. Two coats is the standard for a clean, even finish; one coat only works for same-color touch-ups.
Paint coverage by type
| Paint type | Coverage per gallon | Use it for |
|---|---|---|
| Standard interior | 350 sq ft | Flat, eggshell and satin wall paint |
| Primer | 400 sq ft | Bare drywall, patches, dark-to-light color changes |
| Semi-gloss / exterior | 300 sq ft | Trim, doors, cabinets, outdoor siding |
Check your paint can's label — premium paints can stretch a little further. When in doubt, buy the extra gallon: a second run can be a slightly different shade, and leftover paint is perfect for future touch-ups.
Frequently asked questions
How much paint do I need for a 12x12 room?
A 12x12 room with 8-foot ceilings has 384 sq ft of wall. Subtract one door (21 sq ft) and one window (15 sq ft) and you get about 348 sq ft of paintable area. At 350 sq ft per gallon, one coat needs roughly 1 gallon — buy 2 gallons for two full coats.
How do you calculate paint coverage per gallon?
Multiply your paintable area by the number of coats, then divide by the paint's coverage per gallon: gallons = (area × coats) ÷ coverage. Standard interior paint covers about 350 sq ft per gallon; primer covers about 400; semi-gloss or exterior paint about 300. Read more about paint types and how coverage works.
Do I need to paint the ceiling too?
Not always, but a fresh ceiling makes the whole room look new. A 12x12 ceiling adds 144 sq ft — less than half a gallon at 350 sq ft per gallon. Use the ceiling toggle in the calculator to include it in your gallon count.
How much area do doors and windows take off?
The calculator subtracts 21 sq ft per standard door and 15 sq ft per standard window. You don't paint those surfaces, so subtracting them keeps you from overbuying paint.
Is my project data private?
Completely. Every calculation runs locally in your browser with JavaScript — your measurements are never sent to a server, stored, or tracked.