Paint Calculator — How Many Gallons Do You Need?

The paint calculator above takes four inputs and returns gallons needed by surface. Start with your room dimensions, pick a paint finish, deduct doors and windows, and select coats. The widget converts perimeter and ceiling height into wall area, applies the finish-specific coverage rate, and rounds up to the nearest half gallon. Find your square footage calculator first if you only have area, not dimensions.

Step 1: Enter Your Room Dimensions

Type the room length and width in feet, plus the ceiling height. Standard residential ceilings are 8 ft; older homes and modern lofts may run 9–10 ft. The calculator computes the perimeter as 2 × (Length + Width), then multiplies by ceiling height to find total wall area.

Step 2: Choose Your Paint Finish

Pick flat, eggshell, satin, semi-gloss, or gloss. The finish sets the coverage rate the formula uses: flat covers 400 sq ft per gallon, eggshell and satin cover 350, and semi-gloss and gloss cover 300. Coverage drops as sheen rises because higher-resin finishes lay down a thicker film per pass.

Step 3: Add Doors, Windows, and Coats

Enter the number of doors and windows in the room. The calculator subtracts 20 sq ft per door and 15 sq ft per window — the industry standard from the ASPE CPE handbook. Set the number of coats: two coats is standard for any colour change; one coat works only for refresh painting on the same colour.

Step 4: Review Your Results

The widget shows total gallons, can count, and a per-surface breakdown — walls, ceiling (if enabled), trim (if enabled), and primer (if enabled). Toggle the cost tier to see the 2026 estimated price range. Round up; paint stores can match additional cans across batches, but matching is harder than buying one extra can up front.

One gallon of interior paint covers 300–400 sq ft per coat depending on finish. Flat covers 400 sq ft per gallon, eggshell and satin cover 350 sq ft per gallon, and semi-gloss and gloss cover 300 sq ft per gallon. Primer covers 200–300 sq ft per gallon. Coverage drops 20–30% on unprimed or textured walls.

Coverage Rates by Finish Type

Primer Coverage Rate

Primer covers 200–300 sq ft per gallon — lower than topcoat paint because primer is designed to seal porous surfaces and bond to the substrate. The calculator uses 250 sq ft per gallon as the working midpoint per PDCA standards. Use primer when painting bare drywall, raw wood, stained surfaces, or making a dramatic colour change. On a previously painted wall in good condition with a similar new colour, primer is optional.

Multiply room perimeter by wall height to get wall area. Subtract 20 sq ft per door and 15 sq ft per window. Multiply by the number of coats. Divide by the coverage rate (350 sq ft per gallon for eggshell). Round up to the nearest 0.5 gallon. The full formula reads:

Calculating Wall Area

Wall area equals room perimeter times ceiling height. A 12 × 15 ft room has a perimeter of 2 × (12 + 15) = 54 ft. With an 8-ft ceiling, total wall area is 54 × 8 = 432 sq ft. The four-wall geometry assumes a rectangular room. For L-shaped or irregular rooms, calculate each rectangular section and add the wall areas together.

Subtracting Doors and Windows

Each standard interior door subtracts 20 sq ft. Each standard window subtracts 15 sq ft. These are estimation industry standards from the ASPE CPE handbook — close enough for paint takeoffs without measuring each opening individually. For a sliding patio door, count it as two doors (40 sq ft). For a picture window, count it as two windows (30 sq ft).

Applying the Waste Factor

The default 10% waste factor accounts for spillage, cuts, roller transfer, and the unused paint left in the can and tray. Textured walls absorb more paint per pass — add 20–30% additional, for a total 30–40%. Unprimed surfaces take a heavier first coat; consider using primer instead of inflating waste. Dramatic colour changes may require a third coat regardless of coverage.

After drywall installation, use the drywall calculator to estimate finish coats; the same wall area then feeds back into this paint calculator.

Ceilings and trim need their own gallon count because they use different finishes than walls. The calculator above applies flat paint at 400 sq ft per gallon for ceilings (sheen would highlight every imperfection) and semi-gloss at 300 sq ft per gallon for trim (durability and wipeability). Toggle the ceiling and trim checkboxes to add each surface to your total.

Ceiling Paint

Ceiling area equals room length times width — the floor footprint of the room. A 12 × 15 ft ceiling totals 180 sq ft. At 400 sq ft per gallon and two coats, that needs about 1 gallon (rounded up to 0.5 gallon increments) including a 10% waste factor. For popcorn or knockdown texture, increase the waste factor to 30%; the texture absorbs paint faster than a smooth ceiling.

Trim and Baseboard Paint

Trim area uses linear feet of baseboard, casing, and crown moulding multiplied by an average 6-inch (0.5 ft) trim height. If you do not enter a value, the calculator estimates trim length as room perimeter × 1.1 — accounting for door and window casings on top of the baseboard run. At 300 sq ft per gallon for semi-gloss, most rooms need 0.5 to 1 gallon of trim paint per coat.

Interior paint costs $15–$80 per gallon in Q2 2026 depending on tier. Economy paint runs $15–$25 per gallon, mid-range paint runs $25–$40, and premium paint runs $60–$80. Primer adds $15–$30 per gallon as a separate line. The calculator multiplies your wall, ceiling, and trim gallons by the tier you pick and adds primer cost when included.

Interior Paint Cost by Tier

Source: Q2 2026 national average retail pricing surveyed across Home Depot, Lowe's, and Sherwin-Williams. Regional variation runs ±15%; metro markets and remote areas fall outside this range. Cost data is reviewed quarterly by Sarah Kim, CPE.

Here is a worked example using the formula above. The room is a 12 × 15 ft living room with 8-ft ceilings, one standard interior door, and two windows. The owner wants two coats of eggshell paint with the standard 10% waste factor.

At mid-range pricing ($25–$40 per gallon), the wall paint costs roughly $63–$100. Add a 1-gallon ceiling at flat finish ($25–$40) and the project totals $88–$140 for paint before tools, drop cloths, and tape. Premium paint at $60–$80 per gallon raises the total to $150–$220 — usually worth the upgrade for one-coat capability on similar colours.