Define the room before choosing a can
This example uses a square room measuring 12 feet by 12 feet with an 8-foot ceiling. The painted surface is the four walls, not the floor area. One 21 ft² door and two 12 ft² windows are deducted using the calculator’s standard-opening defaults.
| Input | Value | Why it matters |
|---|---|---|
| Room | 12 ft × 12 ft × 8 ft | Sets wall perimeter and height |
| Openings | 1 door + 2 windows | Removes 45 ft² from wall area |
| Coats | 2 | Doubles the prepared surface |
| Coverage | 350 ft²/gal | Conservative end of a common range |
| Allowance | 5% | Allows for application loss and touch-up |
Calculate walls, coatings, then gallons
- Perimeter: 2 × (12 + 12) = 48 feet.
- Gross wall area: 48 × 8 = 384 ft².
- Openings: 21 + 12 + 12 = 45 ft².
- Prepared wall area: 384 − 45 = 339 ft².
- Two coats: 339 × 2 = 678 coated ft².
- With 5% allowance: 678 × 1.05 = 711.9 coated ft².
- Paint: 711.9 ÷ 350 = 2.034 gallons.
The calculator rounds upward to quarter-gallon increments. Confirm that the exact paint is sold in compatible can sizes.
Sherwin-Williams describes typical gallon coverage as about 350–400 ft² and notes that application method and surface condition change requirements. The product label remains the controlling input.
What changes if the ceiling is included?
The ceiling adds 12 × 12 = 144 ft² of surface. At two coats and the same 5% allowance, total coated area becomes (339 + 144) × 2 × 1.05 = 1,014.3 ft². At 350 ft² per gallon, that is 2.898 gallons and rounds to 3 gallons.
Run or price the quantities separately when the finish, color, coverage, or can size differs.
When 2.25 gallons is not the right answer
- Replace the standard openings when the room has a large patio door, picture window, built-ins, or open wall.
- Use the exact coverage printed for the chosen paint and surface.
- Increase coats when the manufacturer’s system, primer, major color change, or finish requires it.
- Keep primer separate; primer coverage and coat count may differ from finish paint.
- Round to the can sizes the store actually stocks and confirm whether color-matched quarts are available.
Source and scope
Coverage guidance comes from the Sherwin-Williams paint calculator FAQ. This is an original worked example using Project Matter’s published method. It estimates quantity, not surface preparation, coating compatibility, lead-safe work, or application safety.