Most floor plans in textbooks are rectangles, but real homes are full of exceptions: an L-shaped living room, a bay window, a chamfered corner, a stairwell cut-out. The good news is that every odd shape can be reduced to simple pieces, and the pieces can be added or subtracted. This checklist shows how to do it without losing track.
Start with a sketch
Draw the room roughly to scale on paper. You do not need an architect's precision, just enough to label each wall. Write the measurement next to each wall as you take it. A sketch reveals missing measurements before you leave the room.
Choose a dividing strategy
There are two ways to handle an irregular shape:
- Addition: split the shape into rectangles, triangles and circles, find each area and add them.
- Subtraction: imagine a larger rectangle that encloses the shape, find its area and subtract the missing corners.
Choose whichever needs fewer measurements. An L-shape works well with either: two rectangles added, or one big rectangle minus the missing corner.
Worked example: an L-shaped room
The long arm of the L is 18 feet by 10 feet, which is 180 square feet. The short arm adds a 8 by 6 foot rectangle, which is 48 square feet. The total is 228 square feet. Using subtraction instead: the bounding rectangle is 18 by 16, which is 288; the missing corner is 10 by 6, which is 60; 288 minus 60 gives the same 228.
Angled corners and triangles
If a corner is cut at an angle, treat the cut-off piece as a right triangle. Measure the two short sides, multiply them and halve the result. A 4 foot by 3 foot corner cut removes 6 square feet from the bounding rectangle.
Bay windows and bump-outs
Measure the depth and the width at the widest point and treat a simple bump-out as a rectangle. A curved bay can be approximated as a half circle: use pi times the radius squared, halved. Measure the width across the opening and divide by two to get the radius.
Curved walls
For gentle curves, break the curve into a rectangle plus a segment and approximate. For precise results on expensive materials, draw the curve on graph paper and count squares, or ask the supplier for a template.
Cut-outs and fixtures
Stairwells, fireplaces and built-in cabinets may need to be subtracted. Decide whether the material goes underneath built-ins. For flooring it often does not, but for paint and plaster the answer is different.
Common errors
- Measuring wall centre to wall centre instead of the usable interior.
- Forgetting to convert inches to feet before multiplying.
- Adding the overlap between two pieces twice.
- Skipping a closet or a hallway.
Adding a margin
Odd shapes usually waste more material than rectangles. Add 10 to 15 percent rather than the 5 to 10 percent you might use for a plain room, and round the final figure up to whole boxes or rolls.
Checking your total
When your pieces are measured, a tool such as an irregular shape area calculator can add the parts and confirm the total, which is useful for spotting an arithmetic slip. If your total differs, compare piece by piece rather than starting again.
Keep the sketch
File the labelled sketch with your project notes. Next time you need to paint, recarpet or heat the same room, you will have reliable measurements, and you can edit the sketch when you renovate.
Summary
Sketch, divide or subtract, treat corners as triangles, approximate curves, subtract only what you will not cover and add a larger margin. Every odd shape is a few simple shapes in disguise.