How to calculate square metres
Square metres are length times width. The hard part is the rooms that are not rectangles, and knowing which area you actually need.
I worked out the area of my living room three times when I was laying a floor, and got three different answers. The mistake was the same each time: I mixed units. Here is the method that holds.
1. Measure in metres
Measure the length and the width of the room. If you measured in centimetres, divide by 100 before you calculate. 420 centimetres is 4.2 metres.
2. Rectangle: length times width
A room that is 4.2 metres long and 3.5 metres wide is 4.2 × 3.5 = 14.7 square metres.
3. Rooms with a corner: split them
Split an L-shaped room into two rectangles. Work out each one, and add them up.
Example: a living room of 5 × 4 metres with a dining nook of 2 × 3 metres. That is 20 + 6 = 26 square metres. Make sure you do not count the area where the parts meet twice.
4. Slanted wall: use a triangle
If a wall runs at an angle, split the room into a rectangle and a triangle. The area of a right-angled triangle is the two short sides multiplied together, divided by two.
Example: a triangle with sides of 2 and 3 metres is 2 × 3 ÷ 2 = 3 square metres.
5. Subtract and add
Add alcoves and extensions as rectangles of their own. Subtract chimneys and fixed shafts, if they stand on the floor.
6. Walls: perimeter times height
If you are painting or wallpapering, you need the wall area. Add up the length of all the walls, and multiply by the ceiling height. Then subtract doors and windows.
Example: a room of 4.2 × 3.5 metres has a perimeter of 15.4 metres. With a ceiling height of 2.4 metres, that is 15.4 × 2.4 = 36.96, so about 37 square metres of wall, before you subtract the openings.
7. Allow for waste
Flooring, tiles and wallpaper have to be cut, and some is lost. The manufacturer usually states how much extra to buy. Work out the area first, and add the waste afterwards.
In Plantekn: The area is calculated as you draw. Each room shows its own area, and the total is always in the toolbar. With Pro, the renovation calculator estimates costs per room for flooring, paint, electrical work and plumbing, from the areas you have drawn.
Common mistakes
- Multiplying centimetres by metres. Convert to metres first.
- Treating an L-shaped room as one big rectangle. Split it.
- Forgetting the waste. The exact area is too little material.
- Using your own number in a property listing. What counts as floor area in a sale follows separate rules that vary by country, and that measurement is a surveyor’s job.