# How to calculate the area of an L-shaped room

By FloorDraft · Updated 2026-09-13

Original: https://floordraft.online/en/guides/l-shaped-room-area

Split the L into two rectangles that do not overlap and add their areas. Or calculate the enclosing rectangle and subtract the missing corner. Both methods should give the same result when the dimensions are consistent.

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Illustrative diagram and caption: [view the original guide](https://floordraft.online/en/guides/l-shaped-room-area). Dimensions in examples are assumptions, not building standards.

## Measure four lengths and label the missing corner

For a right-angled L, record the outer width W and outer depth D, then the width w and depth d of the missing corner. Sketch which corner is missing so you do not swap the short dimensions.

Use clear internal dimensions if you want the floor area within the room. Keep built-in exclusions and separate cupboards explicit. This simple geometric calculation is not a property valuation or a regulated floor-area measurement method.

## Method 1: subtract the missing rectangle

Assume W = 6 m, D = 4 m, w = 2 m and d = 1.5 m. The enclosing rectangle is 6 × 4 = 24 m². The missing corner is 2 × 1.5 = 3 m². The room area is 24 − 3 = 21 m².

Formula: area = W × D − w × d. Keep all input lengths in one unit before multiplying. Metres produce square metres; feet produce square feet.

## Method 2: add two non-overlapping rectangles

The two parts meet along an edge and do not overlap. A common mistake is adding the full width times one leg depth to the full depth times the other leg width; that counts the shared corner twice.

| Part of the same example | Dimensions | Area |
| --- | --- | --- |
| Full-width lower strip | 6 m × (4 − 1.5) m | 15 m² |
| Upper remaining leg | (6 − 2) m × 1.5 m | 6 m² |
| Total | 15 + 6 | 21 m² |

## Check the geometry in FloorDraft

Do not expect an area from wall centreline dimensions to equal an area measured to internal faces. Choose a consistent boundary. A traced source image may also contain proportions that need correcting against the real measurements.

1. Draw the six-sided outline from your measurements, closing the corners.
2. Set a trustworthy scale before reading any area.
3. Create or select the floor region and inspect its area in the properties panel.
4. Compare that value with the rectangle calculation.
5. If they differ, check wall faces, gaps, the missing-corner dimensions and the selected floor boundary.

## Keep material ordering separate from geometric area

The 21 m² result describes the example shape. It does not include cutting allowance, pack rounding, pattern matching or material-specific fitting needs. Use the supplier’s instructions and the installation plan before ordering flooring.

For curved or angled edges, the two-rectangle shortcut is insufficient. Split the shape into appropriate geometric parts or measure a correctly scaled polygon, and check the assumptions.

## Questions

### Can I use centimetres?

Yes. The product is then in square centimetres; divide by 10,000 for square metres. Convert consistently before comparing results.

### Why do the two methods disagree?

Check for overlapping rectangles, swapped dimensions or mixed units. With the same right-angled outline, they must agree.

## References

Use the example dimensions to follow the calculations, then repeat them with your own measurements. The examples are not building standards.

- [Had2Know: L-shape area and perimeter formulas](https://www.had2know.org/househome/l-shaped-room-area-perimeter-calculator.html)

## Continue your plan

- [How to measure a room for a floor plan](https://floordraft.online/en/guides/measure-a-room-for-a-floor-plan)
- [How to draw a floor plan to scale](https://floordraft.online/en/guides/draw-a-floor-plan-to-scale)
- [Long narrow living room layout: keep the route clear](https://floordraft.online/en/guides/narrow-living-room-layout)

[Draw and check your room outline](https://floordraft.online/en/editor)