Measuring & waste
Measuring L-shaped, angled and odd rooms for flooring
Any room can be measured as rectangles that do not overlap. Here is how to split the awkward ones, what to include and leave out, and why an L-shape has the same perimeter as a plain rectangle.
The rule: rectangles that do not overlap
Every flooring measurement comes down to rectangles. Split the room into pieces with straight sides and right-angled corners, measure each piece’s length and width, multiply, and add them. The only rule is that the pieces must not overlap: where two pieces share a line, measure one of them up to the line and the other from it.
Measure at floor level, wall to wall, at the longest and widest points. Measure each dimension at both ends of the wall and use the larger figure, because walls are rarely parallel and the floor has to reach the wider end. Round to the nearest inch, not up to the nearest foot; rounding six measurements up a foot each adds a lot of floor.
L-shaped rooms
Split the L at its inside corner, either way. A room that is 12 ft wide for 10 ft and then narrows to a 6 × 4 ft leg is 12 × 10 = 120 plus 6 × 4 = 24, which is 144 sq ft. Splitting it the other way gives the same total. What does not work is measuring the bounding rectangle, 12 × 14 = 168 sq ft, which orders a fifth too much, or the main rectangle alone, which orders a box short.
An L-shaped bedroom
A 12 × 10 ft main area and a 6 × 4 ft leg by the window.
- Part 1: 12 × 10 = 120 sq ft; part 2: 6 × 4 = 24 sq ft
- Floor area: 144 sq ft, 16 sq yd, 13.38 m²
The bounding rectangle would have said 168 sq ft; the main area alone, 120. The real figure is between them, and it is the only one worth taking to a calculator.
Bays, alcoves and closets
A bay window with straight sides is a rectangle and two triangles, or close enough to a rectangle at its average depth. A closet or alcove is its own rectangle, measured from the line where it meets the main room. Closets are the most commonly forgotten part of a floor, and a 6 × 3 ft closet is 18 sq ft, most of a box.
Angled walls and triangles
A triangle with a right angle is half its two short sides multiplied: a corner cut at 45° that takes 4 ft off each wall removes 4 × 4 ÷ 2 = 8 sq ft. To enter a triangle in a rectangle-based calculator, enter it as a rectangle with half the width. Rooms with a single angled wall are a rectangle minus a triangle.
Curves
A curved wall or a round bay is approximated well enough by its average depth: measure the depth at the deepest point and at the ends and use the average for a rectangle. For a semicircle, the area is 1.57 times the radius squared; a 4 ft radius bay is about 25 sq ft. The error is a few percent of a small area, which the waste allowance covers.
What to subtract and what not to
| Subtract | Do not subtract |
|---|---|
| A raised hearth | A sofa, bed or table |
| A kitchen island or cabinets set on the subfloor | Appliances that stand on the floor |
| A built-in cabinet or bench | A bathtub alcove the floor runs under (check) |
| A stair opening | The floor under a door swing |
The test is whether the new floor will stop at the thing or go under it. The floor area calculator lets you mark any part as subtracted.
A room with a closet and a hearth
A living room
A 16 × 12 ft main area, an 8 × 6 ft dining leg, a 6 × 3 ft closet, and a 4 × 2 ft raised hearth.
- Added: 192 + 48 + 18 = 258 sq ft
- Subtracted: 4 × 2 = 8 sq ft
- Floor area: 250 sq ft
That 250 is what goes to the material calculator, which adds its own waste. The complete method carries this room through to boxes and trim.
The perimeter for trim
Baseboard, shoe moulding and tack strip follow the walls. For a rectangle the perimeter is twice the length plus twice the width. For an L-shape, surprisingly, it is the same as the enclosing rectangle’s: cutting a notch out of a corner moves two walls inward without changing their total length. A 14 × 12 ft L-shape has a 52 ft perimeter whatever the size of the notch.
An 18 × 14 ft bedroom with a closet
- Perimeter: 2 × (18 + 14) = 64 ft; plus the closet’s three inside walls, 9 ft = 73 ft
- Less two 3 ft doors: trim run 67 ft
The perimeter calculator gives the trim run and the tack strip run, which differ where cabinets stand against a wall. The baseboard calculator turns the run into pieces, and the tack strip calculator into tack strip for carpet.
Sketch first
A quick sketch with each rectangle numbered and its dimensions written on it is the best check there is. It shows overlaps, forgotten closets and the doorways, and it is what an installer or a store will ask for. The units guide covers converting the result to square yards or square metres, and the rug size guide uses the same measurements for choosing a rug.