Interior Paint Calculator

For a rectangular room:

Room Perimeter = 2 × (Length + Width)

Wall Area = Perimeter × Wall Height

Net Wall Area = Wall Area − Openings

Paint Required = (Net Wall Area × Coats) ÷ Coverage

Example: a 12 × 10 ft room with 8 ft walls has 352 ft² of wall. Subtracting one 3 × 7 ft door and two 3 × 4 ft windows (45 ft²) leaves 307 ft². Two coats need 614 ft² of coverage; at 350 ft² per gallon that's about 1.75 gallons, or 1.84 gallons with a 5% touch-up allowance — two 1-gallon cans.

1. Measure room length and width.
2. Calculate the room perimeter.
3. Multiply by wall height.
4. Subtract large doors and windows that won't be painted.
5. Multiply by the number of coats.
6. Divide by the paint's coverage.

Manufacturer guidance is often around 350–400 ft² per gallon, and some paints list broader ranges such as 250–400 ft² per gallon. Actual coverage depends on:

• The product
• Wall texture
• Porosity
• Color change
• Application method

Use the coverage on your product label whenever it's available. The calculator's 350 ft²/gal (about 8.59 m²/L) is an example planning value only.

Large openings that won't receive wall paint can be subtracted from the wall area — paint manufacturers such as Sherwin-Williams advise subtracting large windows and doors when estimating interior wall paint. Enter your own measurements; no standard door or window size is assumed.

BuildCalcWorks doesn't choose the coat count. It can depend on:

• The paint product
• Existing color
• New color
• Wall condition
• Primer
• Finish

You enter the planned number of coats.

For a rectangular room:

Ceiling Area = Length × Width

Ceiling paint can use different coverage, coats and product from wall paint, so it has its own settings. For vaulted or sloped ceilings, enter the measured ceiling surface area instead.

Calculate an accent wall separately when it uses a different color or product. The calculator subtracts its area from the main wall paint so it isn't counted twice.

Trim Area = Linear Trim Length × Average Trim Width

Keep trim paint separate when it uses a different product. Example: 100 ft of 4-inch trim is about 33.33 ft²; two coats at 350 ft²/gal is about 0.19 gallons.

Door paint quantity depends on door dimensions, the number of doors, whether you paint one or both sides, coats and product coverage. Two 3 × 7 ft doors painted on both sides are 84 ft² per coat.

Primer follows the same area and coverage logic. Don't assume primer is always necessary — and some paint-and-primer products may still need a separate primer depending on surface conditions. That choice stays yours.

BuildCalcWorks calculates each room on its own, then combines paint quantities where the same product is used. Containers are only rounded once for the whole project unless you choose to buy for each room separately — which can require more paint because each room is rounded up.

Material cost can include wall paint, ceiling paint, primer, trim paint, door paint and optional supplies. Labor is not included.