Measure and MendStart measuring

How much paint does this room need?

Measure the walls and actual openings. The calculator shows the cited coverage range, then uses your can label when you enter one.

A 15 × 12 × 8 ft room with 58 sq ft of openings and two coats covers 748 sq ft. At 350 sq ft per gallon, buy 3 one-gallon cans.

Paint calculator

Measure the wall perimeter and height, then subtract the actual total area of doors and windows.

3 one-gallon cans

374 sq ft of paintable wall area × 2 coats = 748 coated sq ft.

The cited 350–400 sq ft/gal range is 2–3 whole cans. The main result uses the entered 350 sq ft/gal label value.

See the formula and assumptions

(2 × (length + width) × height − measured openings) × coats ÷ coverage. Purchase quantities are rounded up only at the final can step. Surface condition and application method can change real coverage.

Unbranded paint, a roller, brush, tape, drop cloth, and tape measure arranged beside a blank wall
Measure the actual project and read the selected product label before purchase.

Why the answer is a range

Manufacturer planning guidance is a spread-rate range, not a promise for every wall. Porous or textured surfaces, a strong colour change, application method, and the selected coating can change coverage. The range remains visible even when the can-label field supplies the main purchase quantity.

Measure openings instead of assuming them

Doors and windows vary widely. Add their measured width × height areas and enter the total. The calculator does not assume a “standard” opening size or silently add waste.

What to verify before purchase

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. Sherwin-Williams: Exterior Painting How-TosNorth America · manufacturer guide

    Use the selected coating label when it provides a more specific spread rate.

  3. 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.