Paint Calculator

Estimate paint by area, coats, and coverage

This paint calculator turns room dimensions into a practical gallon estimate. It starts with wall area, optionally adds the ceiling, subtracts standard door and window areas, multiplies by the number of coats, and divides by the coverage rate printed on the paint can.

The goal is to avoid two common mistakes: buying too little because doors, windows, and second coats were handled loosely, or buying far too much because the room was guessed instead of measured.

Formula

Wall area = 2 x (length + width) x height.

Ceiling area = length x width, when the ceiling is included.

Paintable area = wall area + ceiling area - door area - window area.

Total coverage needed = paintable area x coats.

Gallons needed = total coverage needed / coverage per gallon.

The calculator rounds up to whole gallons because most paint is bought by the gallon and a small reserve is useful for touch-ups.

Example

A 12 ft by 10 ft room with 8 ft walls has 352 square feet of wall area. If you include the ceiling, add 120 square feet. With one standard door and two standard windows, subtract about 45 square feet. Two coats at 350 square feet per gallon require about 2.44 gallons, so buying 3 gallons is the practical estimate.

Assumptions and limitations

• Doors are assumed to be about 21 square feet each.
• Windows are assumed to be about 12 square feet each.
• Walls are treated as simple rectangles.
• Actual coverage depends on paint quality, surface texture, primer, color change, and application method.

Use the result well

Check the paint label and update the coverage field if the product lists a different number. Add a little extra margin for textured walls, dramatic color changes, fresh drywall, or rooms with many cut-in areas.