Paint Calculator

The Core Formula

Every paint estimate comes down to one relationship between area, coverage, and coats:

Paintable area is the wall (and optionally ceiling) surface minus doors and windows. Coverage rate is the square footage one gallon covers per coat, printed on the can. The waste multiplier adds a buffer for touch-ups and absorption.

Worked Example: A 12 x 14 Bedroom

Take a 12 ft by 14 ft bedroom with 8-foot walls, one door, two windows, two coats of paint, and a 350 sq ft/gal coverage rate.

• Wall area: 2 x (12 + 14) x 8 = 416 sq ft
• Openings: 1 door (3 x 6.67 = 20 sq ft) + 2 windows (2 x 3 x 4 = 24 sq ft) = 44 sq ft
• Paintable area: 416 - 44 = 372 sq ft
• Paint needed: 372 x 2 / 350 = 2.13 gallons
• With 10% waste: 2.13 x 1.10 = 2.34 gallons, so buy 3 gallons

Three gallons leaves a useful amount of matching paint for touch-ups after the furniture goes back in.

Step 1: Gross Surface Area

In room mode, the calculator treats all four walls as a perimeter band:

In single-wall mode it simply uses Length x Height. If you check Include Ceiling, the room's Length x Width is added to the total. Switching to accent-wall mode skips the ceiling entirely.

Step 2: Subtract Openings

Each door and window is removed using its actual area, defaulting to standard sizes when you leave the dimensions blank:

A standard door defaults to 3 x 6.67 ft (about 20 sq ft) and a window to 3 x 4 ft (12 sq ft). The paintable area is the gross area minus openings, never allowed to drop below zero.

Step 3: Apply Coats, Coverage, and Waste

The paintable area is multiplied by the number of coats, divided by the coverage rate, then scaled by the waste factor:

Because paint is sold in whole gallons (or whole cans), the final figure is always rounded up. The calculator also reports the un-rounded gallons so you can see how close you are to the next can.

• Buy in one batch: Paint mixed at different times can vary slightly in shade. Purchase all your gallons together so they share the same tint batch, and have the store box (combine) them if you are using several cans.
• Keep the leftover labeled: Write the room, color name, and formula on the lid. A pint of original paint is worth more than a perfect re-tint for future touch-ups.
• Round up, not down: Running out three-quarters of the way through a wall means a visible lap mark where wet meets dry. The extra gallon is cheap insurance.
• Account for the surface: Fresh drywall, patched repairs, and dark-to-light changes all increase how much paint you use. Add a coat or a primer rather than hoping one pass will hide.
• Do not forget trim and doors: This calculator estimates wall and ceiling area. Trim, baseboards, and doors usually use a separate can of a different sheen, so budget for those independently.

• Drywall texture: Knockdown or orange-peel texture changes the surface area paint must cover and lowers effective coverage by 10-20 percent. If your walls are heavily textured, nudge the coverage rate down. Use the drywall calculator to plan the surface itself before you paint.
• Tile: In wet areas like showers and backsplashes, tile outlasts paint and resists moisture. It is priced by the square foot of coverage rather than by the gallon - see the tile calculator for grout and thinset quantities.
• Wallpaper: Sold by the roll with a fixed coverage per roll and a pattern-repeat waste factor, wallpaper hides imperfections differently than paint and is harder to patch.
• Exterior stain: On decks and fences, penetrating stain replaces paint and is measured by coverage per gallon much like paint, though rough wood drinks far more. The deck calculator helps size the structure first.