L-Shaped Room Square Footage Calculator

When to use the L-shape calculator

L-shaped rooms are everywhere: living-and-dining spaces that wrap a staircase, kitchens with a pantry cut into one corner, decks that turn a corner of the house, and lots that fold around a neighbour’s plot. The calculator treats the shape as a full rectangle with a smaller rectangle removed — an outer 20 × 16 ft with a 6 × 8 ft notch is 320 − 48 = 272 square feet.

Measuring the outer rectangle and the notch

Measure the overall width and length as though the missing corner were filled in, running the tape along the two longest walls. Then measure the notch itself: its width along the same direction as the outer width, and its length along the same direction as the outer length. Getting those two axes crossed is the one mistake this shape invites, and it produces a plausible-looking but wrong answer, so check that the notch is smaller than the outer rectangle in both directions before you accept the result.

Splitting into two rectangles instead

You can equally split the L into two rectangles and add them — the answer is identical. Splitting is easier when you can only measure inside the room, because both rectangles are fully enclosed by walls. Subtracting a notch is easier when the outer dimensions are the ones you can reach along a wall.

Whichever you use, make sure the two rectangles meet along the dividing line without overlapping, or that strip gets counted twice.

More than one notch

For a shape with two or three missing corners, keep subtracting — or sketch the outline on the drawing canvas in the calculator above, drag the corners to your measurements, and let it compute the whole polygon at once. To fold the result into a house total, add it as a row in the multi-room calculator.