How much paint for a room

Measure the walls first

Paint volume starts with wall area. For a rectangular room, add the four wall lengths (or use 2 × (length + width)), multiply by ceiling height, then subtract doors and windows. A common planning shortcut is one door at about 20 sq ft and one window at about 15 sq ft, but measuring the openings is better when they are large.

The Paint Calculator asks for room length, width, height, door and window counts, coats, coverage from the can, and a waste percent. It then estimates gallons.

Coverage on the can

Coverage is not a universal constant. Many interior latex paints claim around 350 sq ft per gallon per coat on a smooth, primed surface. Rough plaster, new drywall, or a dark colour under a light topcoat can cut that number. Always prefer the label on the product you will buy, and enter that coverage in the calculator.

Coats and colour changes

One coat rarely hides a strong colour change. Two coats are the usual plan for a solid finish. If you are going from dark red to pale grey, a primer coat plus two finish coats is safer than hoping a single thick coat will hide everything. Each coat multiplies the paint you need.

Waste and odd walls

A 10% waste margin covers roller loss, touch-ups, and slightly uneven walls. Rooms with niches, chair rails, or heavy texture often need more. Round up to the next gallon or quart size the store sells. Leftover paint in a sealed can is useful for repairs; running out mid-wall is worse.

Ceilings and trim

Ceilings are a separate rectangle: length × width. Trim and doors use smaller volumes and often a different sheen. Do not fold trim into the wall gallon count if you are buying a different product.

Worked example

A 12 × 10 ft room with 8 ft ceilings has a raw wall area of 2 × (12 + 10) × 8 = 352 sq ft. One door (20) and two windows (30) leave about 302 sq ft. Two coats at 350 sq ft per gallon need about 1.73 gallons before waste. With 10% waste, plan on roughly 1.9 gallons, which means buying 2 gallons.

Confirm the final number with the Paint Calculator, and compare floor area separately with the Square Footage Calculator if you are also ordering flooring.

Checklist before you buy or rely on the number

Write down the raw inputs you used, including units. Re-run the calculator once with those exact figures. If a store pack size or a syllabus rule forces a round-up, round up rather than chasing a perfect match. Keep a note of the date you calculated so you can compare later when prices, labels, or grades change.

A note on units

Keep units consistent inside one calculation. Mixing feet with metres, or cups with grams without a density, is the usual source of bad results. When a label uses a different unit, convert once with the Unit Converter and then return to the specialty calculator.

Limits

These figures are planning aids. Confirm product labels, syllabus rules, employer policy, or veterinary advice when a decision depends on the number.