Paint calculator

How to use the paint coverage calculator

Estimate paint for a room’s walls, ceiling and trim before you shop. Enter the room dimensions, openings, number of coats and the coverage printed for your paint to see the exact amount and purchasable can sizes.

1. Choose Feet or Metres under Units.
2. Enter the room length, width and wall height.
3. Set the number of coats and enter the number of doors and windows.
4. Turn on the ceiling, trim or primer options when needed.
5. Open the advanced options to adjust coverage, opening areas or trim dimensions, then review the exact amount and amount to buy.

What to enter

Units

Choose Feet for imperial measurements or Metres for metric measurements. Imperial results use sq ft and gal; metric results use m² and L.

Room length

Enter the room length in ft or m, based on Units.

Room width

Enter the room width in ft or m, based on Units.

Wall height

Enter the floor-to-ceiling height in ft or m. The field help gives 8 or 9 ft as common examples.

Coats

Enter 1 to 5 coats. The field help suggests two coats for a color change and one for a same-color refresh.

Doors

Enter the number of doors. If Area per door is blank, the calculator subtracts 21 sq ft per door in imperial mode or 1.95 m² per door in metric mode.

Windows

Enter the number of windows. If Area per window is blank, the calculator subtracts 15 sq ft per window in imperial mode or 1.4 m² per window in metric mode.

Paint the ceiling too

Turn this on to add the room length × width to the paintable area.

Paint baseboard and trim

Turn this on to add trim. The calculator uses the room perimeter unless Trim length is entered in the advanced options.

Add one coat of primer

Turn this on to calculate one primer coat separately. The tool describes this option for bare drywall, repairs, stains or a dramatic color change.

Paint coverage

Enter the paint's coverage in sq ft/gal or m²/L. The interface help shows 350–400 sq ft per gallon as an example range; use the coverage shown for your paint.

Primer coverage

When primer is selected, enter primer coverage in sq ft/gal or m²/L.

Area per door

Override the opening default with the area of one door in sq ft or m².

Area per window

Override the opening default with the area of one window in sq ft or m².

Trim length

When trim is selected, enter the trim length in ft or m. If left unset, the calculator uses the room perimeter.

Trim face width

Enter the visible trim face width in inches. This field appears when Paint baseboard and trim is selected.

Example: two coats on four walls

Use imperial Units with a 10 ft room length, 12 ft room width and 8 ft wall height. Set Coats to 2, Doors to 1, Windows to 2, and leave the ceiling, trim and primer options off. Leave each opening at its default of 21 sq ft per door and 15 sq ft per window. Perimeter: 2 × (10 + 12) = 44 ft. Gross wall area: 44 × 8 = 352 sq ft. Openings: (1 × 21) + (2 × 15) = 51 sq ft. Net wall area: 352 − 51 = 301 sq ft. Two-coat area: 301 × 2 = 602 sq ft. At 350 sq ft/gal, exact paint: 602 ÷ 350 = 1.72 gal. The fractional remainder is 0.72 gal, so the purchase amount rounds up to 2 gal. The can mix is 2 × 1-gal cans.

How the paint coverage calculation works

The calculator converts metric measurements to internal feet and square feet, then converts the displayed results back to m, m² and L when metric is selected. Wall perimeter is 2 × (room length + room width). Gross wall area is perimeter × wall height. Door area is the door count × area per door, and window area is the window count × area per window. Net wall area is gross wall area − opening area. If selected, the ceiling adds room length × room width. Trim uses the room perimeter unless a trim length is entered, then uses trim length × trim face width converted from inches to feet. The paint requirement is the applicable paintable area × coats ÷ paint coverage. Primer, when selected, is calculated as one additional coat using Primer coverage. Areas and volumes are rounded to two decimals for display. Exact gallons are then rounded up for purchase: a fractional remainder below 0.5 gallon is rounded up to the next quarter gallon, while a remainder of 0.5 gallon or more is rounded up to the next whole gallon. Whole-gallon amounts stay unchanged. Can suggestions split the purchase into 5-gallon buckets, 1-gallon cans and quarts. Metric volumes use 3.78541 L per US gallon, but the can suggestion still uses US sizes.

Reading the results

Use the exact quantity to understand the calculated requirement and To buy as the shopping amount after can-size rounding. Review the wall, ceiling, trim and opening areas in the breakdown to check the geometry. If primer is selected, review its quantity separately from finish paint. In metric mode, volume results are shown in L; the suggested can mix remains in US bucket, gallon and quart sizes.

Limitations and common mistakes

• The form has no separate waste-percentage field. Purchase rounding provides an allowance for available quart, gallon and 5-gallon sizes, while the tool disclaimer advises buying an extra quart for textured or porous walls and for dark-to-light changes.
• The room model uses a rectangular footprint, one wall height and equal area for each door or each window unless you override those areas.
• Coverage is an input. A different coverage value changes the result, so use the coverage stated for the paint or primer you plan to use.
• The calculator rejects inputs when the total door and window area is greater than the gross wall area. Metric can suggestions still refer to US can sizes even though the volume result is shown in liters.

Frequently asked questions