Paint Calculator

Calculate how many gallons of paint you need for a room, including ceiling, multiple coats, and optional primer.

Length (ft)

Width (ft)

Ceiling Height (ft)

Doors (qty)

Windows (qty)

Number of Coats

Coverage (sqft/gal)

Typical coverage: 350–400 sqft/gal on smooth painted surfaces; 250–300 on new drywall or rough texture.

Include ceilingAdd primer coat(new drywall, dark→light, stains)

Paint Needed

2.8 gallons

480 sqft paintable · 2 coats · ~$135 est. cost

Wall Area	396 sqft
Deductions (1D + 1W)	−36 sqft
Ceiling Area	120 sqft
Paintable Area	480 sqft
Paint (2 coats @ 350 sqft/gal)	2.8 gal
Est. Paint Cost	~$135
Total Est. Cost	~$135

Paintable area = perimeter × height + ceiling (if selected), minus door and window deductions. Gallons = area × coats ÷ coverage. Coverage defaults to 350 sqft/gal — reduce to 300 for new drywall or rough surfaces. Primer calculated at 300 sqft/gal. Cost estimates use $45/gal for paint and $30/gal for primer — use actual product pricing for final quotes.

Frequently Asked Questions

How many gallons of paint do I need for a 12×10 room?

For a 12×10 room with 9-ft ceilings, 1 door, and 1 window: walls = 2 × (12+10) × 9 = 396 sqft, minus 21 (door) and 15 (window) = 360 sqft wall area, plus 120 sqft ceiling = 480 sqft total. At 350 sqft/gal with 2 coats = 480 × 2 ÷ 350 ≈ 2.75 gallons. Round up to 3 gallons to be safe.

What does paint coverage mean?

Coverage is the square footage one gallon of paint covers in a single coat — typically 350–400 sqft/gal for latex on smooth, previously painted surfaces. Porous or rough surfaces (new drywall, bare wood, rough texture) absorb more and yield 250–300 sqft/gal. Always use the lower end for bidding to avoid running short.

Do I need primer before painting?

Yes, in these situations: new drywall (drywall paper is very porous), major color changes from dark to light, surfaces with stains or water damage, or bare wood. One coat of quality primer effectively replaces one coat of paint and improves adhesion, color uniformity, and final coverage. Skipping primer on new drywall leads to “flashing” — uneven sheen that shows through the paint.

How much does interior paint cost?

Quality interior latex ranges from $35–65 per gallon at retail. Budget $45/gal as a rule of thumb for estimating — use the actual product spec for final quotes. Premium paints in the $55–65 range often deliver better coverage and hide, sometimes reducing the total gallons needed. Contractor accounts at paint stores typically save 20–30% off retail.

How the paint calculator works

The Buildermuse paint calculator estimates gallons for an interior room from its dimensions, openings, coats, and coverage rate. Wall area is 2 x (length plus width) x ceiling height. It deducts 21 square feet per door and 15 square feet per window to get the paintable wall area, then adds the ceiling (length times width) when the ceiling option is on.

Gallons are the paintable area times the number of coats divided by the coverage rate, rounded up to the nearest tenth of a gallon. Coverage defaults to 350 square feet per gallon — typical for latex on a smooth, previously painted surface. Drop it toward 250 to 300 for new drywall or rough texture, which drink more paint. If you enable the primer option, primer is figured separately at 300 square feet per gallon over the full paintable area. Cost estimates use 45 dollars per gallon for paint and 30 dollars per gallon for primer, billed on whole gallons since that is how paint is sold.

Worked example: a 12-by-10 room with 9-foot ceilings, one door, one window, two coats, 350 square feet per gallon coverage, and the ceiling included. Wall area is 2 x (12 plus 10) x 9 = 396 square feet. Subtract 21 for the door and 15 for the window for 360 square feet of wall, add 120 square feet of ceiling, and paintable area is 480 square feet. Walls need 360 x 2 / 350 = 2.06 gallons and the ceiling 120 x 2 / 350 = 0.69 gallons, totaling 2.8 gallons after rounding up. At 45 dollars per gallon on three whole gallons, the paint estimate is 135 dollars.

Frequently Asked Questions

How does the paint calculator estimate gallons?

Paintable area times the number of coats divided by the coverage rate, rounded up to the nearest tenth of a gallon. A 480-square-foot room at two coats and 350 square feet per gallon needs about 2.8 gallons.

Does it deduct doors and windows?

Yes. It removes 21 square feet per door and 15 square feet per window from the wall area before calculating how much paint you need.

How does it handle primer?

Enable the primer option and it estimates a separate primer coat at 300 square feet per gallon over the full paintable area, priced at 30 dollars per gallon.

Why is the cost based on whole gallons?

Paint is sold by the gallon, so the cost estimate rounds the gallon count up to the next whole gallon at 45 dollars per gallon, even though the quantity display shows tenths of a gallon.