About this paint calculator
TrueCoat Paint Calculator is a free tool that estimates how much paint and primer to buy for a single room. Unlike simpler calculators, it deducts doors and windows, separates walls, ceiling and trim, calculates primer as its own product, and turns the result into an exact purchase combination of gallons and quarts — or litre tins in metric mode.
Basis for the default assumptions (checked 30 Oct 2026)
- Coverage — default 350 sq ft per gallon per coat (editable). Sherwin-Williams' official paint selection FAQ explains the standard method: calculate the square feet to be coated and divide by the coverage of the product, using 400 sq ft per gallon as its worked example, and notes that figure is theoretical because surface texture and paint left on applicators are not counted (sherwin-williams.com, "Paint Selection Frequently Asked Questions", checked 30 Oct 2026). Manufacturer and retailer guidance for smooth, primed interior walls commonly falls in a roughly 250–400 sq ft per gallon range, with rough or porous surfaces at the low end. We default to 350, the middle of that range, and label it on the tool as a working assumption you can edit to the figure printed on your can.
- Door size — 21 sq ft (editable assumption). A standard US interior door slab is about 3 ft × 7 ft = 21 sq ft. We label this as an assumption on the tool; larger or glazed doors should be edited.
- Window size — 15 sq ft (editable assumption). A typical window of about 3 ft × 5 ft = 15 sq ft. Actual windows vary widely, so edit this if yours differ.
- Trim allowance — 30 sq ft (editable estimate). A planning allowance for baseboards, door and window frames in one average room. It is an estimate, not a standard — measure your trim if you need precision.
- Metric equivalents. 350 sq ft per gallon converts to about 8.6 m² per litre; 21 sq ft ≈ 1.95 m²; 15 sq ft ≈ 1.39 m². Metric mode calculates natively in metres, m² and litres.
Honesty note
Every default on this site is presented as an editable assumption, never as a guaranteed fact. Real paint usage depends on the product, surface, application method and colour change. This tool gives you a well-reasoned estimate and an exact buy combination — confirm the coverage on your chosen product before a large purchase.