Worked example — 16 × 12 × 8 room, 2 coats, 375 ft²/gal
Walls 16×8 twice plus 12×8 twice = 448 ft², minus a 20 ft² door and 15 ft² window = 413 ft² paintable walls. Two coats at 375 ft²/gal is 2.20 gallons of film, which purchases as 3 gallons. Ceiling is a second takeoff.
Walls, ceilings, openings
Use room length, width and ceiling height. Deduct doors and windows you will not paint. Ceilings, trim and doors are different products — keep them on different lines. Metric litres for emulsion, with the room table and tin rounding, are in how many litres of paint per m².
A common planning coverage is 350–400 ft² per gallon on smooth walls. That is a starting input, not a universal law. Deep colours, primer and texture change it. Two coats double the film; you still round purchase up.
Gallons are not labour hours
Once gallons exist, cost is a rate you type. Do not hide cut-in, masking or furniture moves inside the can price if you need a transparent quote.
Do it in order
- Measure walls and ceilings separately and deduct openings.
- Enter coats and the coverage from the selected can.
- Round up to whole gallons, then add primer and trim as extras if they use a different product.
Planning boundary
These tools estimate gallons. They do not specify prep, moisture, lead-safe work or colour matching.
Sources
This guide is original worked arithmetic for the calculator linked above. It does not cite a laboratory test or a supplier price. The method, the internal check and the planning limit are on How we calculate.
One coat if the colour is close?
Film build and sheen still usually want two. Enter the coats you will actually roll.
Do sample pots use the same coverage?
No. Do not scale a sample into a gallon count.
Is this a production rate?
No. It estimates gallons, not hours.