What you need
A tape measure (a laser measure is handy for long rooms), a pencil and paper or a phone note, and ideally a second person to hold the other end. Make a rough sketch of the room from above and write each measurement on it as you go. It takes a few minutes and saves a second trip to the store.
Pick one unit and stay with it: feet and inches, or meters and centimeters. Measure to the nearest half inch or centimeter. There is no need to be more precise, because the waste allowance and the rounding to whole packages cover small differences.
Floor: length and width
For a rectangular room, measure the length and the width at floor level, wall to wall. Measure each in two places; if the room is not quite square, use the larger number. Floor area = length × width.
Include alcoves and the area under fitted furniture you will remove, and leave out anything that stays, such as a kitchen run or a fitted bath, if the floor will not go under it. Do not subtract doorways: the new floor usually runs into the doorway.
Walls: perimeter and height
The perimeter is the distance all the way round the room. For a rectangle it is 2 × (length + width). Measure the wall height from the floor to the ceiling, in a corner, and again on another wall. Wall area = perimeter × height.
Example: a room 3.6 m × 3.2 m with 2.4 m walls has a perimeter of 2 × (3.6 + 3.2) = 13.6 m and a gross wall area of 13.6 × 2.4 = 32.64 m². A room 13 × 11 ft with 8 ft walls has a perimeter of 48 ft and a gross wall area of 384 sq ft. The paint calculator carries an example like this through to the number of cans.
Doors, windows and other openings
For painting, measure the width and height of each door, window, archway or built-in unit you will not paint. Note how many of each size there are. The paint calculator takes them off the wall area.
- A door 30 × 80 in is 2,400 sq in, or 2,400 ÷ 144 = 16.67 sq ft.
- A window 24 × 36 in is 864 sq in, or 864 ÷ 144 = 6 sq ft.
- A door 0.762 × 1.981 m is about 1.51 m².
Measure the opening itself, not the trim around it, unless you are not painting the trim either. Small openings change little, so you can ignore a tiny window if you prefer to have some paint to spare.
For baseboard (skirting), measure the width of each doorway at floor level. There is no baseboard across a door, so the flooring calculator subtracts door widths from the perimeter. Windows do not interrupt baseboard.
L-shaped rooms
An L-shaped room is a rectangle with one corner cut out. There are two ways to measure it, and both give the same area.
Overall rectangle minus the cut-out. Measure the full length and width as if the corner were there, then the length and width of the missing corner.
Example: overall 5 m × 4 m with a 2 m × 1.5 m corner missing. Area = 20 − 3 = 17 m². The perimeter of an L-shape is the same as the perimeter of its overall rectangle: 2 × (5 + 4) = 18 m. With 2.5 m walls the gross wall area is 18 × 2.5 = 45 m².
Split into rectangles. Divide the floor into rectangles that do not overlap and add their areas. In the same room: one rectangle 5 m × 2.5 m = 12.5 m², and one 3 m × 1.5 m = 4.5 m²; 12.5 + 4.5 = 17 m². Make sure no strip is counted twice and none is left out; drawing the split on your sketch helps.
Irregular rooms
For rooms with more than one cut corner, a bay window or an angled wall, split the floor into rectangles (and, if you need to, triangles) for the area. For the walls and the baseboard, do not try to calculate the perimeter from the rectangles: walk round the room with the tape and measure the perimeter directly, wall by wall, then add the lengths up. For rooms split into several rectangles the planner needs this measured perimeter for walls and baseboard.
Worked example: a room with a chimney breast
A living room 4.0 m × 3.6 m with 2.4 m walls, one door of 0.8 × 2.0 m and one window of 1.2 × 1.2 m. A chimney breast 1.4 m wide stands 0.4 m out from the middle of one of the 4.0 m walls, so the room is neither a rectangle nor an L.
- Floor: split it into the part in front of the breast, 4.0 × 3.2 = 12.8 m², and the two alcoves beside the breast, each (4.0 − 1.4) ÷ 2 = 1.3 m wide and 0.4 m deep: 2 × 0.52 = 1.04 m². Together that is 13.84 m², the full 14.4 m² minus the 0.56 m² the breast stands on.
- Perimeter: walk round with the tape. The wall with the breast is 1.3 + 0.4 + 1.4 + 0.4 + 1.3 = 4.8 m and the others are 3.6, 4.0 and 3.6 m: 16.0 m in all. A plain 2 × (4.0 + 3.6) = 15.2 m would leave out the two sides of the breast, 1.92 m² of wall.
- Walls: 16.0 × 2.4 = 38.4 m², minus 1.6 m² for the door and 1.44 m² for the window = 35.36 m².
- Baseboard: 16.0 − 0.8 = 15.2 m.
In the planner, choose Several rectangles as the floor shape, enter the three rectangles and type 16.0 m as the Measured perimeter.
Ceilings, gables and slopes
A flat ceiling has the same area as the floor, and that is what the planner uses for the ceiling. Gables and sloping ceilings go under Painted areas in step 1 of the paint calculator or the room refresh planner, measured on their own:
- Gable wall: measure the wall up to the height of the other walls as usual. The triangle above it is a painted area of the shape Triangle (gable): its base width and its height from the base to the top. Area = ½ × base × height. Example: a gable 3.6 m wide and 1.5 m high is ½ × 3.6 × 1.5 = 2.7 m².
- Sloping ceiling: measure along the slope, not the floor below it. Each sloping part is a painted area of the shape Rectangle: its length along the slope × its width. Example: a slope 2.0 m long and 4.0 m wide is 2.0 × 4.0 = 8 m², whatever the floor under it measures. If the whole ceiling slopes, choose None for the ceiling in step 2 so it is not counted twice; a flat part can be its own rectangle.
Painted areas never have openings taken off, so leave a roof window in a slope in, or measure the slope in parts around it. Choose the paint and coats for each painted area in step 2.
Common measuring mistakes
- Writing feet and inches as decimals. 8 ft 6 in is 8.5 ft, not 8.6 ft, and 10 ft 9 in is 10.75 ft, not 10.9 ft. In the planner’s feet-and-inches fields you can type 8 6 or 8’ 6” and it is read as 8 ft 6 in; 8.6 is read as decimal feet.
- Adding up the perimeters of the rectangles. In the L-shaped room above, the two rectangles have perimeters of 2 × (5 + 2.5) = 15 m and 2 × (3 + 1.5) = 9 m. Together that is 24 m, but the walls only run 18 m: the 3 m edge the two rectangles share is counted twice and is not a wall at all. With 2.5 m walls that is 60 m² of wall instead of 45 m², a third too much paint.
Before you leave the room
Check your sketch against this list, room by room:
- The length and width of the floor, or the rectangles it is split into.
- The measured perimeter, if the room is neither a rectangle nor an L.
- The wall height.
- Each door, window and other opening: its width, its height and how many there are.
- The width of each doorway at floor level, for the baseboard.
- For wall tiles that stop below the ceiling: the tiled height and the sill height of each window, which the tile calculator asks for.
- For wallpaper on some walls only: the width of each of those walls, for the wallpaper calculator.
Then enter the numbers in the room refresh planner once and use them for both paint and flooring. Stairs are measured differently, by counting treads and risers: see how to measure stairs.