Measuring the area to paint, wall by wall

What you are trying to measure

The calculator needs two things about the geometry of your job: the gross area, meaning all the surfaces you are going to paint as if they were solid, and the list of openings, the parts that will not receive the wall paint. It subtracts the second from the first to get the actual area, which the rest of the calculation is built on.

Before you pick up the tape, decide what goes into the calculation. The calculator handles one paint at a time: if the walls get a velvet-finish paint and the ceiling a ceiling paint, run two separate calculations. In the same way, woodwork and radiators painted with another product are not part of the wall area.

Tools and the right level of precision

A tape measure is enough for an ordinary room; a laser distance meter saves time on long walls and ceiling heights. Write each measurement down in metres to two decimal places, which means to the nearest centimetre: finer precision adds nothing to a paint estimate, and anything coarser lets errors build up from one wall to the next.

Keep one simple rule: measure, write it down, then calculate, without rounding along the way. In the example room below, writing the height down as 2.60 m instead of 2.50 m is enough to add 14.60 × 0.10 = 1.46 m².

Straight walls: length × height

A rectangular wall is its length multiplied by its height. In a rectangular room where every wall has the same height, it is quicker to work from the perimeter: add the room's length and width, double the result, then multiply by the ceiling height.

Example. A room measuring 4.20 m by 3.10 m, with a 2.50 m ceiling: the perimeter is 2 × (4.20 + 3.10) = 14.60 m, and the gross wall area 14.60 × 2.50 = 36.50 m².

Measure the height in several places if the ceiling or floor is not perfectly level, and keep the largest value: running short of paint costs more than a little left in the tin.

Doors, windows and unpainted areas

Each opening is measured the same way: width × height of the part that will not get the wall paint. For a door, that is usually the leaf and its frame; for a window, the frame and glazing. The calculator adds these areas up and deducts them from the gross area.

Continuing the example. The room has a 0.90 × 2.10 m door and a 1.20 × 1.35 m window. The openings come to 1.89 + 1.62 = 3.51 m², and the actual wall area to 36.50 − 3.51 = 32.99 m².

The same logic applies to anything that permanently covers the wall: tiling, a kitchen splashback, built-in cupboards, fixed wall units. Enter them as openings, with their real dimensions. Conversely, leaving a small opening undeducted simply keeps a little paint in reserve: that is a choice, not a mistake, as long as you make it knowingly.

Window reveals: the area people forget

When a wall is thick, the opening is lined with returns, the reveals, which are usually painted along with the walls. Their area is added to the gross area: multiply the depth of the reveal by the length of the edges concerned. For a window, that usually means both sides and the top; the sill, if it is stone or timber, is dealt with separately.

Example. A 1.20 × 1.35 m window set in a 20 cm deep wall: 0.20 × (1.35 + 1.35 + 1.20) = 0.78 m² to add.

Sloping walls and gables

In a loft room, walls are no longer rectangles. Two shapes cover almost every case.

• The trapezoid, for a wall whose height changes steadily from one end to the other (an end wall under a slope, seen face on): average the two heights, then multiply by the length. A wall 3.80 m long, 0.90 m high at one end and 2.40 m at the other, measures (0.90 + 2.40) ÷ 2 × 3.80 = 6.27 m².
• The triangle, for the top part of a gable: base × height ÷ 2. A gable 6.00 m wide whose peak rises 2.20 m above the straight part of the wall adds 6.00 × 2.20 ÷ 2 = 6.60 m².

A full gable is therefore worked out in two pieces: the rectangle at the bottom, then the triangle on top. The sloping ceiling itself, if you paint it, is measured like a tilted ceiling: its true length along the slope multiplied by its width.

The ceiling

A flat ceiling is worked out like the floor: length × width. In the example room, 4.20 × 3.10 = 13.02 m². There is usually nothing to deduct, apart from a stairwell opening or a large roof light. As it usually gets a different paint from the walls, give it a calculation of its own.

Irregular rooms: break them into simple shapes

An L-shaped room, an alcove or a wall interrupted by a chimney breast are all handled the same way: split the surface into rectangles, trapezoids and triangles, work out each piece, then add them up. Note the pieces on a freehand sketch with their dimensions; it is the surest way not to count anything twice.

A free-standing pillar or a boxed-in pipe is measured around its perimeter: add up the widths of its visible faces, then multiply by its height.

The most common mistakes

• Mixing units: a measurement in centimetres slipped into a calculation in metres multiplies the error by a hundred. Convert everything before you calculate.
• Rounding too early: round the final result, never the intermediate measurements.
• Deducting the same area twice: a splashback already counted under the wall units comes off only once.
• Forgetting reveals and returns: in a room with a lot of glazing, they quickly add up to several square metres.
• Adding walls and ceiling together when they get different paints: each paint has its own coverage, so its own calculation.

Entering your measurements in the calculator

Enter the total for the walls (or for the ceiling) in square metres as the gross area, then add each opening with its width and height in centimetres: the calculator shows the total opening area, then the actual area to paint in the result details. If the openings ever exceeded the gross area, the calculation would be blocked with a message rather than producing a meaningless quantity. If you measure in imperial units, the unit selector accepts the area in square feet and the openings in inches, and converts the result to gallons.

To see the method applied from start to finish, with quantities and suggested tins, have a look at our worked examples, room by room.