Measure and MendStart measuring

Paint

Interior room paint

This page treats coverage as a range because Sherwin-Williams describes a typical gallon as covering 350–400 square feet. Texture, porosity, color change, application, and the selected product can move actual use, so the can label takes priority.

First answer

Add the wall areas, subtract doors and windows you will not paint, multiply by the number of coats, then divide by 400 and 350 square feet per gallon to show a planning range.

Run the calculator

Formula or decision rule

(total wall area − excluded openings) × coats × allowance factor ÷ 400 to 350 ft²/gal
  • The lower liquid estimate uses 400 ft²/gal; the higher estimate uses 350 ft²/gal.
  • Round up only when converting the liquid range to purchasable whole-gallon cans.
  • An allowance is optional and user-selected, not a claimed universal waste rate.

Interior paint planning inputs

Interior paint planning inputs
InputHow to measureUnit
Wall areawall width × wall height, summedft² or m²
Excluded openingswidth × height for each unpainted openingft² or m²
Coveragemanufacturer typical range350–400 ft²/U.S. gal
Coatswhole coats selected by the usercount

Work through the project

  1. Sketch each wall

    Record width and height separately so sloped ceilings, alcoves, and partial walls are not hidden inside one rough perimeter.

  2. Separate painted surfaces

    Subtract only doors, windows, cabinets, or panels that will remain unpainted; keep trim and ceiling paint in their own calculation.

  3. Check the actual can

    Replace the planning range with the chosen coating label when it gives a product-specific spread rate, then buy whole containers.

Safety and scope

  • Test suspect coatings before sanding or scraping in older homes.
  • Provide ventilation and follow the coating safety data sheet and label.

Sources and scope

Source links reviewed July 16, 2026. A review date is not the document's publication date.

  1. Sherwin-Williams: Paint CalculatorNorth America · manufacturer calculator

    Actual coverage varies with surface condition, color change, application, and product label.

  2. National Institute of Standards and Technology: NIST Guide to the SI, Appendix B — Conversion FactorsUnited States · government standard

    Code retains exact defining constants where NIST identifies an exact relationship.

  3. U.S. Environmental Protection Agency: Steps to Lead Safe Renovation, Repair and PaintingUnited States · government guide

    Lead rules and certified-contractor requirements may apply; this site does not replace regulatory guidance.

  4. Health Canada: Lead-based paintCanada · government guide

    Paint history is a screening clue, not a laboratory identification.