How to measure a room for painting

The five numbers

Everything a quote needs from a room comes out of five measurements. Take them once, write them down, and the pricing is arithmetic from there.

• Length and width of the floor — gives you the ceiling area directly
• Perimeter — length plus width, doubled; gives you skirting and, with the height, the walls
• Ceiling height — floor to cornice
• Count of doors, and whether frames are included
• Count of window frames

Walls = perimeter × height. Ceiling = length × width. Skirting = perimeter, less the doorways.

When to subtract openings

For an ordinary room, do not bother. A door and a window come to about four square metres, which sits inside the waste allowance you are carrying anyway, and the time saved not painting them is spent cutting in around them.

Subtract when an opening is genuinely large — a wall that is mostly glass, stacking doors, a void over a stairwell. At that size it stops being a rounding error.

Take the height once, carefully

Height is the measurement that multiplies. An error of 100 mm across a 16 metre perimeter is 1.6 square metres of wall per room, which across a house is most of a tin of paint and several hours.

It is also the number most often estimated rather than measured, because it needs a ladder or a laser and the floor plan does not.

Write down what you cannot see

The measurements are the easy half. The half that costs money is the condition — the crack above the window, the patch where a shelf came off, the bathroom ceiling that has been quietly wet for a year.

Note them against the room while you are standing in it. Every one of those is either a preparation line on the quote or an exclusion, and it is far cheaper to decide which at the time than at handover.