FixerCalc

Paint Calculator

Paint math is paintable area ÷ coverage × coats. This calculator builds that area from a rectangular room (or your measured walls), subtracts openings, and rounds gallons up so you are not one quart short.

Last reviewed 2026-08-25. How we calculate

Results update as you type. Units stay imperial inside the math.

Room
Openings
Paint

Results

  • Gallons to buy

    4

  • Exact gallons (before rounding)

    3.08

  • Wall area (minus openings)

    346 sq ft

  • Ceiling area

    168 sq ft

  • Paint cost (your $/gal)

    $140.00

  • Texture, raw drywall, masonry, and drastic color changes use more paint than the can’s best-case coverage.
  • This does not include caulk, tape, or the value of your time. Primer can be a separate product.

How this calculation works

  1. For a box room, wall area is 2 × (length + width) × height.
  2. Doors and windows subtract. Defaults (20 and 15 sq ft) are typical; measure odd sizes.
  3. Gallons = (area × coats × waste) ÷ coverage. You buy whole gallons, so we round up. Ceiling is length × width when enabled.

walls = 2(L+W)H − doors − windows; total = walls + optional ceiling; gallons = ceil(total × coats × (1+waste%) / coverage)

What to do next

  • Use the coverage number on your can, not the default, if they disagree.
  • Buy all wall color from the same batch. Keep a labeled quart for scuffs.
  • If you are painting over dark colors or new drywall, add primer as extra coats or a second run of the math.

FAQ

Do I subtract trim I will paint a different color?
If walls and trim are different SKUs, subtract trim area from the wall color and calculate trim separately (often a quart).
Why not use 400 sq ft per gallon?
You can—change the coverage field. 350 is a slightly conservative planning default for two-coat interior work on ordinary drywall.

Sources

  • Wall area ≈ perimeter × height, or 2×(L+W)×H for a rectangular room
  • Many US interior latex labels cite about 350–400 sq ft per gallon per coat on sealed smooth drywall

Cost defaults are typical US planning bands, last reviewed 2026-08-25. Edit the price fields on this page rather than treating them as a local market index.