Free Paint Calculator – Gallons, Liters & Cost

Paint Calculator Guide

The Paint Calculator estimates how many gallons or liters you need to cover a room's walls (and optionally the ceiling) for a chosen number of coats. Measure the height of each wall floor-to-ceiling and the length of each wall, then add the wall lengths together to get the room perimeter. You will also need a count of doors and windows to deduct, the number of coats you plan to apply, and the paint's coverage rate, which is printed on the can and defaults here to 350 square feet per gallon per coat. For ceilings, measure length and width separately and add that area. Note the existing wall color and condition: bare drywall, patched repairs, and dark-to-light color changes all soak up more paint and usually justify an extra coat. Decide up front whether trim, doors, and the ceiling are part of this estimate, because each surface type often uses a different product and coverage rate and should be calculated as its own line item.

The formula is wall area minus openings, multiplied by coats, divided by coverage. Wall area = perimeter x height. A 12 ft x 10 ft room with 8 ft ceilings has a perimeter of 2 x (12 + 10) = 44 ft, so wall area = 44 x 8 = 352 sq ft. Deduct one door at 21 sq ft and two windows at 15 sq ft each (30 sq ft): 352 - 21 - 30 = 301 sq ft of paintable wall. For 2 coats: 301 x 2 = 602 sq ft of coverage required. At 350 sq ft per gallon, 602 / 350 = 1.72 gallons, which rounds up to 2 gallons. Metric check: 301 sq ft is about 28 sq m; at roughly 12 sq m per liter per coat, two coats need about 4.7 liters, so a 5 liter can suffices. Buying in whole gallons or standard cans means you almost always round up to the next container size.

Add about 10% for touch-ups and keep a sealed remainder for future repairs, since later batches can vary slightly in tint. The single largest accuracy error is coverage rate: deep, saturated colors, sprayed application, and porous or textured surfaces can drop real-world coverage well below the can's stated figure, while a smooth previously painted wall may exceed it. Primer is a separate calculation and is strongly advised over bare gypsum board, stains, or strong color changes; do not count primer toward your finish-coat total. Other common mistakes include forgetting the ceiling area, ignoring closets and returns, and over-deducting for small openings. Standard rough openings are about 21 sq ft (2.0 m²) per door and 15 sq ft (1.4 m²) per window, but measure oversized units directly. ASTM D5324 and related volume-solids standards describe how coverage is derived from a coating's solids content; higher volume solids generally means better hide and truer coverage, which is why premium finishes can reach the upper end of the stated range.