How to Estimate How Much Paint You Need

Measure, subtract, multiply by coats, divide by coverage. Here is the method, with the numbers paint makers publish.

By the Mirovi Home editorial team · Updated September 2026

Step 1: measure the walls

Measure each wall’s width and the ceiling height. For a rectangular room the total wall area is:

A 14 × 12 ft room with 8 ft ceilings has 2 × 26 × 8 = 416 sq ft of wall.

Step 2: subtract doors and windows

Measure openings if you can. As a planning shortcut we use a standard door of 3 × 7 ft (21 sq ft) and a window of 3 × 5 ft (15 sq ft). One door and two windows remove 51 sq ft, leaving 365 sq ft. Skip this step for small openings if you prefer a margin of safety.

Step 3: multiply by coats

Two coats is typical for a new color and gives even coverage; one coat may be enough when refreshing the same color. Two coats on 365 sq ft means painting 730 sq ft.

Step 4: divide by coverage

Sherwin-Williams states that a gallon of paint typically covers about 350–400 square feet and a quart about 100 square feet. Coverage falls on textured, porous or unprimed surfaces, and when covering a dark color with a light one. Using 350 sq ft per gallon:

Step 5: decide what to buy

2.09 gallons is two gallons and a little more. Rather than a third gallon, a quart covers the remainder (a quart is a quarter gallon, about 100 sq ft). So: 2 gallons + 1 quart. If the remainder needs four quarts, buy another gallon — it’s usually cheaper.

Ceilings, trim and primer

• Ceiling: length × width. Our room’s ceiling is 168 sq ft — about half a gallon per coat.
• Trim and doors: usually a different sheen, often bought by the quart.
• Primer: Sherwin-Williams gives about 200–300 sq ft per gallon for primer. Prime new drywall, patched areas, stains and big color changes.

Tips that save a second trip

• Buy all the paint for a room from the same batch, or box (mix) gallons together to avoid slight color differences.
• Keep a labeled leftover for touch-ups.
• Check the coverage on your product’s label — premium and one-coat products may state different figures.

The paint calculator does all of this for you, including quarts and primer.

Sources

• Sherwin-Williams, “Paint Calculator” and FAQs (coverage per gallon, quart and primer)