Paint Calculator

Paint is the one material where people usually over-buy rather than under-buy, and the reason is that they add up wall area and forget to take anything off.

The calculation is straightforward: perimeter times ceiling height gives wall area, then subtract the doors and windows, then multiply by the number of coats and divide by the coverage rate. The manufacturer's 350 square feet per gallon is genuinely achievable on smooth, previously painted drywall.

The part worth getting right is buying in the correct tin sizes. Needing 2.3 gallons does not mean buying three — it means two gallons and two quarts, which costs less and leaves a sensible touch-up amount instead of most of a full tin going hard in the garage.

1. Step 1 Wall area walls = 2 × (length + width) × ceiling_height
2. Step 2 Subtract the openings openings = doors × 21 + windows × 15 sq ft Standard door and window sizes. Count a large picture window as two.
3. Step 3 Add the ceiling if you are painting it ceiling = length × width
4. Step 4 Multiply by coats and divide by coverage gallons = paintable_area × coats ÷ coverage_per_gallon 350 sq ft/gal on smooth drywall; 300 on texture or bare board.
5. Step 5 Split into tins buy whole gallons plus quarts for the remainder

Example: a 12 × 12 ft room with 8 ft ceilings has 384 sq ft of wall. Take off one door and two windows — 51 sq ft — and you have 333 sq ft. Two coats at 350 sq ft per gallon is 1.9 gallons, so buy 1 gallon plus 4 quarts, or simply 2 gallons.

• Coverage defaults to 350 square feet per gallon, which is realistic for a quality paint on smooth, previously painted drywall. Textured walls, bare drywall, and deep or bright colours all reduce it — 300 is a safer figure for those.
• Doors are taken as 21 square feet and windows as 15. These are standard US sizes; measure your own if they are unusual, and count a large picture window as two.
• Trim, doors and window frames are not included. Those are normally a different paint in a different sheen, and a quart usually covers the trim in an average room.
• The first coat on bare or heavily patched drywall soaks in and covers less than the stated rate. Priming solves this and is cheaper than an extra finish coat.
• Closet interiors, alcoves and stairwells are not included in the room dimensions. Add them as a separate calculation.
• Primer is estimated at one coat over the same paintable area at the same coverage rate. Stain-blocking primers often cover less.
• Roller nap affects consumption noticeably — a 1/2 inch nap holds and deposits more paint than a 3/8 inch nap on the same wall.