Square footage of a room that is not a rectangle
Every generic calculator asks for a width and a depth, which is another way of saying it has already decided your room is a box. Most rooms worth measuring are not.
Why width × depth is wrong on a real room
Take an L: 20' across the front, and a 8' square bitten out of the back corner. Ask for a width and a depth and the only answer available is 400.0 sq ft. The walk encloses 336.0 sq ft.
64 square feet out, on one room, in the expensive direction. And the perimeter is identical either way — 80.00 lf both times — so nothing about the wrong answer looks wrong.
What a room actually is, to anybody measuring it, is a walk: a run, a turn, a run, a turn, all the way back to where you started. The area is whatever polygon that walk closes, and there is a formula for it that has been right since surveyors were the people who needed it.
How to walk a room so the numbers come out
Start in a corner and pick a direction
Any corner. Any direction. What matters is that you keep going the same way round — all the way clockwise or all the way anticlockwise, never both.
Write down every straight run
Corner to corner. A wall that changes direction is two runs, and a wall with a chamfer on the end is a run and a diagonal.
Say which way each run goes
North, east, south, west, as you walk them. It does not matter whether north is really north — it only has to be consistent for the whole room.
Finish where you started
The last run has to bring you back to the first corner. If it does not, one of the runs is wrong, and that is worth knowing before it is worth pricing.
The example above, written out: 20' east, 12' south, 8' west, 8' south, 12' west, 20' north.
The check nothing else does: does the room close?
A room that does not close is a room where at least one measurement is wrong. Walk the runs, and if you do not finish where you started, the gap is the error — and it is a real number, in inches, that you can go and look for.
Every other calculator will happily give an area for a set of measurements that cannot be a room. This one says how far out the walk finished, on which axis, and refuses to print an area until it closes. An area worked out from a walk that does not close is the area of a shape that is not the room you are standing in.
It is also the fastest way to find a transposed figure. A run written down as 12 ft when the tape said 21 leaves nine feet of gap, on one axis, and nothing else in a takeoff would have caught it.
What the walk gives you after the area
The same walk produces every quantity the room can produce. On the L, at a 9' ceiling: 336.0 sq ft of floor, 80.00 lf of perimeter, 720.0 sq ft of wall face, 80.00 lf of base — before any opening is taken out, which the form above will do if you tell it about them.
That is the same order the app works in, because it is the same code. Trueline walks the room off the scan, lets you correct any run by hand, and then every quantity — floor, ceiling, wall face, base — falls out of the corrected walk rather than being measured again.
Questions people ask
- How do you calculate the square footage of an L-shaped room?
- Walk it. Write down every straight run in order with the direction you walked it, finish where you started, and the area is the polygon that walk closes. Splitting the L into two rectangles gets the same answer when it is genuinely two rectangles, and stops working the moment there is an angle in the room.
- What if my room has an angled wall?
- A diagonal run is still a run. The walk handles it — the area of the polygon does not care whether an edge is square — which is exactly the case that defeats splitting a room into rectangles.
- My measurements do not add up. What does that mean?
- It means one of them is wrong, and the size of the gap tells you how wrong. The calculator says how far the walk finished from where it started, on each axis, instead of printing an area for a shape that cannot exist.
- Does the direction I walk the room matter?
- Only that it is consistent. Clockwise or anticlockwise both work; changing your mind halfway round does not. Which way is "north" does not matter either — it only has to mean the same thing for the whole room.
This is the app’s arithmetic, not a copy of it
The figures above are not worked out by anything written for this page. The form hands what you typed to Trueline’s own measuring engine — the same modules the iPhone app runs — and prints what comes back. Lengths are held as whole nanometres and money as whole cents, so there is no floating-point number anywhere in a measurement, here or in the app.
The one thing a web form cannot do is know whether anybody put a tape on the wall. That is the whole of the difference between this page and the app it comes out of: Trueline carries where every length came from — scanned, drawn or measured — onto the drawing, the takeoff and the proposal a client reads. You can also take the blank forms and use them with whatever you already have.