How much paint do I need for a room?
Four numbers decide it: wall area, coats, coverage per gallon and waste. Here's the math, then one room worked through. The calculator does every room at once.
1. Wall area
2 × (length + width) × height, minus the openings you won't paint. Sheenbid uses 21 sq ft per door (about 3 × 7 ft) and 15 sq ft per window. Got big windows? Measure them and subtract the real size.
2. Coats ÷ coverage
gallons = area × coats ÷ coverage. Coverage, in square feet per gallon, is on the can or data sheet. The calculator starts from the Sheenbid workbook's sample rates: 375 for wall paint, 350 for ceiling paint, 400 for trim enamel. Use your product's figure when you have it.
Texture drinks more paint. The calculator cuts coverage 15% for light texture and 30% for heavy. Both factors are Sheenbid's assumption, not a manufacturer figure.
3. Ceiling, trim, primer
- Ceiling: length × width, then the same coats ÷ coverage.
- Trim: Sheenbid counts baseboard as perimeter × 0.5 sq ft, each painted door as 80 sq ft (both sides plus jamb), each window casing as 15 sq ft.
- Primer: new, unprimed drywall gets one coat of PVA primer on walls and ceiling; stains, patches or glossy old paint get one stain-blocking coat.
4. Waste, then cans
Add 10% for rolling, cutting in and what's left in the tray. Then round up to cans: a quart for 0.25 gal or less; otherwise gallons. Five singles usually cost more than one 5-gallon pail, so at five gallons, grab the pail.
Example: 12 × 10 ft room, 8 ft ceiling
One door, two windows, two coats of everything, smooth painted walls.
| Step | Math | Result |
|---|---|---|
| Wall area | 2 × (12 + 10) × 8 − 21 − 2 × 15 | 301 sq ft |
| Wall paint | 301 × 2 ÷ 375 = 1.61, × 1.10 | 1.77 gal → 2 gal |
| Ceiling paint | 12 × 10 × 2 ÷ 350 = 0.69, × 1.10 | 0.75 gal → 1 gal |
| Trim area | 44 × 0.5 + 80 + 2 × 15 | 132 sq ft |
| Trim paint | 132 × 2 ÷ 400 = 0.66, × 1.10 | 0.73 gal → 1 gal |
On new drywall, add primer: (301 + 120) ÷ 300 = 1.40 gal, × 1.10 = 1.54 gal, so 2 gal of PVA primer.
Where it goes wrong
- Big color changes, especially dark to light, can need a third coat or tinted primer.
- The can's coverage assumes a sealed, smooth surface. Porous patches guzzle paint.
- Sloped ceilings and stairwells need their real wall shapes measured.
Measure, then run every room. If you paint for a living, Sheenbid turns these quantities into labor hours, a price at your margin and a one-page quote. Related: production rates, what to charge for a room, free estimate template, how to bid a paint job, margin vs markup. Hiring it out? See the cost per square foot to paint a room.