Not sure how much paint to buy? Enter your room dimensions below and this free calculator works out the paintable area, the exact gallons you need, and how much to actually buy — accounting for doors, windows, coats, ceiling and primer.
How much paint do you need?
Enter your room size and we will work out exactly how much paint to buy — walls, ceiling, doors and windows included.
Your estimate
Based on standard door (21 sq ft) and window (15 sq ft) sizes. One gallon covers about 350–400 sq ft on smooth walls; rough or unpainted surfaces cover less — lower the coverage number if your walls are textured or bare drywall.
How to use this paint calculator
- Measure your room's length, width and wall height.
- Enter how many doors and windows are in the room — we subtract standard sizes automatically.
- Choose your number of coats (2 is right for most rooms).
- Optionally add the ceiling and primer.
- Read the 'Buy this much paint' figure — it already includes a waste buffer for touch-ups.
How we calculate paint
Wall area = 2 × (length + width) × wall height. We subtract 21 sq ft per standard door and 15 sq ft per standard window, add the ceiling area if selected, then multiply by the number of coats and divide by the paint's coverage rate (about 350 sq ft per gallon for standard interior paint). Primer is calculated separately at 300 sq ft per gallon.
Typical paint coverage
| Smooth, previously painted drywall | 350–400 sq ft per gallon |
| Textured or rough walls | 250–300 sq ft per gallon |
| Bare drywall or primer | 200–300 sq ft per gallon |
| Brick, masonry or stucco | 150–250 sq ft per gallon |
Paint calculator FAQs
How much paint do I need for a 12×12 room?
For a 12×12 room with 8 ft walls, one door and two windows, you need roughly 2 gallons for two coats — about 333 sq ft of paintable wall area. Use the calculator above with your own door and window count for an exact figure.
How many coats of paint do I need?
Two coats is the standard for an even finish and a true color. One coat can work when repainting the same color over a surface in good condition. Three coats may be needed for dramatic color changes, like covering a dark wall with a light color.
How much area does one gallon of paint cover?
About 350–400 sq ft on smooth walls. Rough, textured or unpainted surfaces soak up more paint and cover less — lower the coverage figure in the calculator if your walls are textured or bare.
Does this calculator include primer?
Only if you tick the primer option. Primer is calculated as one coat at 300 sq ft per gallon. Primer is recommended on bare drywall, bare wood, and when making big color changes.
Should I buy more paint than calculated?
Yes — the buy figure already adds a 10% buffer for roller loss, spills and future touch-ups. Keep leftover paint sealed and labeled with the room and color; it is invaluable for scuff repairs.