Paint Calculator

What It Does

This calculator works out what a room's walls need. It measures the wall area from the room's length, width and height, or takes an area you already know, and subtracts the doors and windows you list. In paint mode it divides that area, times the number of coats, by the coverage of one litre and rounds up to whole tins, giving a second colour its own tins. In wallpaper mode it counts the drops the perimeter needs, cuts each drop to the wall height rounded up to the pattern repeat, and reports the rolls to buy.

Real-World Usage

Use it before buying paint for a bedroom, hallway or whole flat, and before ordering wallpaper for a wall with a large pattern. It answers the two questions a paint shop asks — how many tins and how many rolls — from one room measurement, and it shows what a second colour or a bold pattern adds to the bill.

Local Context

Coverage is the figure on the tin and it varies by paint: a typical emulsion covers about 10 to 12 square metres per litre, while a primer or a strongly coloured paint covers less. Metric tins are sold in 1, 2.5, 5 and 10 litres, and US paint in gallons of 3.79 litres. Wallpaper rolls are commonly 0.53 m wide and about 10 m long, and the pattern repeat is printed on the roll label.

How much paint do I need for a 12 × 10 ft room?

With 8 ft ceilings the four walls are 352 sq ft; one door and two windows take 51 sq ft out, leaving 301 sq ft. At two coats and 100 sq ft per litre that is 6.02 litres, so two 5-litre tins.

Does a second colour need its own tins?

Yes. Paint is bought by colour, and each colour is rounded up to whole tins on its own share of the wall. Half a wall in a second colour at a 5-litre tin means a tin of each colour, even though the two shares together need only a litre.

Why does the pattern repeat change the number of rolls?

Every drop is cut to the wall height rounded up to the repeat, so the pattern starts at the same point on each strip. A 2.5 m wall with a 0.64 m repeat is cut at 2.56 m, which can cost a drop per roll: 9 rolls where 7 would do without the pattern.

Should I deduct doors and windows?

Yes — the area behind them is not painted or papered. Enter how many of each there are and the area of one; the calculator takes the whole opening out. Leave them at zero if you would rather keep the spare paint.