How to use the paint calculator
- Measure the room: length, width and ceiling height. For several rooms the same size, set Rooms this size.
- Count doors and windows. Standard sizes are subtracted; change them under Adjust assumptions.
- Choose coats, and tick ceiling or primer if needed.
- Read what to buy. The calculator rounds to 5-gallon buckets, gallons and quarts so you are not left short.
The paint formula
Wall area = 2 × (length + width) × height − doors − windows
Gallons = wall area × coats ÷ coverage per gallon
Ceiling = length × width × coats ÷ coverage
Worked example
A 12 × 14 ft bedroom, 8 ft ceiling, 1 door, 2 windows, 2 coats
- Walls: 2 × (12 + 14) × 8 = 416 sq ft; less 21 + 30 = 365 sq ft.
- 365 × 2 ÷ 350 = 2.09 gallons: buy 2 gallons + 1 quart.
- Ceiling: 168 × 2 ÷ 350 = 0.96 gallons: buy 1 gallon of ceiling paint.
Paint needed by room size (walls, 2 coats, 8 ft ceiling)
| Room | Wall area* | Gallons |
|---|---|---|
| 10 × 10 ft | 269 sq ft | 1.5 |
| 12 × 12 ft | 333 sq ft | 1.9 |
| 12 × 14 ft | 365 sq ft | 2.1 |
| 14 × 16 ft | 429 sq ft | 2.5 |
| 16 × 20 ft | 525 sq ft | 3.0 |
* After one door and two windows. 350 sq ft per gallon.
Common mistakes to avoid
- Forgetting the second coat. Most jobs need two.
- Trusting the can on rough walls. Texture and fresh drywall soak up paint.
- Buying gallon by gallon. Batches can differ slightly; buy enough at once.
- Skipping primer on new drywall. Bare paper and mud absorb paint unevenly.
Frequently asked questions
How much paint do I need for a 12 × 14 room?
With 8 ft ceilings, the walls are 2 × (12 + 14) × 8 = 416 sq ft. Less one door (21 sq ft) and two windows (30 sq ft) leaves 365 sq ft. Two coats at 350 sq ft per gallon need 2.09 gallons, so buy 2 gallons and 1 quart.
How many square feet does a gallon of paint cover?
Most interior paint covers 350 to 400 sq ft per gallon per coat on smooth, primed walls. Textured, porous or dark surfaces can drop that to 250 to 300.
Do I need two coats of paint?
Usually. Two coats give even color and sheen, especially when changing color. One coat can work when repainting the same color with a high-quality paint.
Do I need primer?
Use primer on new drywall, patched areas, stains, glossy surfaces and big color changes (dark to light). Many paints are "paint and primer", which helps but does not replace primer on bare drywall.
How much paint for a ceiling?
Length × width of the room. A 12 × 14 ceiling is 168 sq ft: about 1 gallon for two coats.
Should I buy a 5-gallon bucket?
Once you need about 4 gallons or more of one color, a 5-gallon bucket is usually cheaper and keeps the color consistent. The calculator switches to buckets automatically.