Free paint calculator. Litres for a room's walls from its size, doors, windows and coats, in 4L and 10L tins with 10% wastage. Check your product's spread rate.
This calculator estimates the litres of paint a room's walls take, and turns that into 4 litre and 10 litre tins. Give it the room dimensions, the wall height, the area of any doors and windows to leave out, and how many coats you are putting on.
Wall area is the room perimeter (twice the length plus the width) multiplied by the wall height. The doors and windows area you enter is taken off: measure each opening, width times height, and add them up; the page does not assume a size for them. The result is floored at zero so an over-deduction cannot produce a negative area.
That net area is then multiplied by the number of coats, divided by the spread rate, and CostMate's allowance of 10% is added for wastage.
The wall height defaults to 2.4m, which is the standard stud height here, but it is an input: set it to what you measured.
The page uses Resene's own data-sheet figures: Zylone Sheen at 14 square metres per litre (the low end of Resene's 14 to 16) for interior walls, Lumbersider at 12 for exterior weatherboard, and Lustacryl at 12 for fascia and trim. With no paint type picked it uses 12, the lowest of the three.
Real spread rate moves a long way with the product and what it is going onto. A sealer on bare plasterboard, a topcoat on a previously painted wall, and an exterior coat on rough-sawn weatherboard do not cover the same area per litre, and the tin states the manufacturer's figure.
Treat the litres here as a starting point for an order, then check it against the spread rate printed on the product you are actually buying.
Spread rate, the number of coats and the preparation a surface needs are product and site decisions. Check the tin's stated spread rate and confirm the system with your paint supplier before you order.