# Room area and square footage calculator

> Floor area in square metres and square feet, wall area, perimeter and volume for one or more rectangular rooms or an L-shaped room.

Interactive version: https://www.calcopenly.com/construction/room-area-calculator
Subject: Construction and home calculators

Floor area is length × width for each rectangle, summed; an L-shaped room is its enclosing rectangle minus the missing corner. Wall area is the perimeter times the ceiling height, before doors and windows are taken off, and volume is floor area times ceiling height. Every result is given in metric and imperial units, using the exact foot of 0.3048 m, so 1 m² = 10.764 ft².

People use it to order flooring, paint or plasterboard and to size heating or ventilation by room volume. The default, a 4 × 3.5 m room plus a 2.5 × 2 m alcove under a 2.7 m ceiling, gives 19 m² (204.51 ft²) of floor, 64.8 m² of wall and 51.3 m³ of air.

Each rectangle is counted as a separate room with four walls. When the rectangles are parts of one open space, the wall area comes out too high; use the L-shape option for that case.

## How to calculate the square footage of a room

Measure the length and width at floor level, from finished wall to finished wall, and write the result in feet and inches. Convert to a single unit before multiplying: either all inches, then divide the product by 144, or decimal feet, where 4 in is 0.3333 ft, not 0.4 ft.

A bedroom measuring 13 ft 4 in by 11 ft 9 in, with a closet 5 ft 6 in by 2 ft 2 in:

1. Bedroom in inches: 160 × 141 = 22,560 in², and 22,560 ÷ 144 = 156.67 ft².
2. Closet in inches: 66 × 26 = 1,716 in², and 1,716 ÷ 144 = 11.92 ft².
3. Total: 168.58 ft², or 15.66 m².

In the calculator, choose inches and enter one rectangle per line, `160 x 141` and `66 x 26`. Reading 13 ft 4 in as 13.4 ft and 11 ft 9 in as 11.9 ft gives 159.46 ft² for the bedroom alone, 2.79 ft² too much, because inches are twelfths of a foot, not tenths.

## Irregular rooms: split the floor into rectangles

Any floor made of straight walls at right angles can be divided into rectangles. Draw the outline, cut it into rectangles that do not overlap, and enter each one on its own line. Alcoves, closets and hallway strips each become a line.

Angled walls and bay windows need a hand calculation first. A triangle is ½ × base × height; a trapezoid is the average of the two parallel sides times the depth. A bay 6 ft wide at the wall, 4 ft across the front and 2 ft deep covers (6 + 4) ÷ 2 × 2 = 10 ft². Enter it as a rectangle with the same area, such as `5 x 2`. The floor area comes out right, but that line's perimeter and wall area no longer describe the real walls. For curved or many-sided outlines, the [area and perimeter calculator](/geometry/shape-area-perimeter-calculator) covers 15 shapes including circles and ellipses, and the [polygon area calculator](/geometry/polygon-area-from-coordinates) takes the corner points of any straight-sided outline.

The calculator treats every rectangle as a room with four walls. For the bedroom above under an 8 ft ceiling, it reports 524 ft² of wall, which includes all four sides of the closet. Use its floor area as it stands, and work out wall area for paint or plasterboard from the real walls, or with the [paint calculator](/construction/paint-calculator).

## L-shaped rooms

An L-shaped room can be measured two ways, and both give the same floor area. Take a living room whose outline is 20 × 16 ft, with an 8 × 6 ft corner missing:

- **Subtract the cut-out:** 20 × 16 − 8 × 6 = 320 − 48 = 272 ft², or 25.27 m².
- **Add two rectangles:** 12 × 16 + 8 × 10 = 192 + 80 = 272 ft².

The difference shows up in the walls. With the L-shape option and an 8 ft ceiling, the calculator reports a 72 ft perimeter and 576 ft² of wall. Entered as the two rectangles, it reports 736 ft², because the 10 ft edge the two rectangles share is counted as a wall on both sides: 2 × 10 × 8 = 160 ft² of wall that does not exist. Use the L-shape option whenever the parts form one open room.

## Converting square feet to square metres

The international foot is exactly 0.3048 m, and [NIST](https://www.nist.gov/pml/us-surveyfoot/revised-unit-conversion-factors) has advised against the older US survey foot since 31 December 2022. Squaring the foot gives the exact factors: 1 ft² = 0.09290304 m², and 1 m² = 10.7639 ft². A square yard is 9 ft², or 0.83612736 m².

| Room (ft) | Square feet | Square metres | Square yards |
|---|---|---|---|
| 10 × 10 | 100 | 9.29 | 11.11 |
| 10 × 12 | 120 | 11.15 | 13.33 |
| 12 × 12 | 144 | 13.38 | 16.00 |
| 12 × 14 | 168 | 15.61 | 18.67 |
| 12 × 16 | 192 | 17.84 | 21.33 |
| 14 × 16 | 224 | 20.81 | 24.89 |
| 15 × 20 | 300 | 27.87 | 33.33 |
| 20 × 20 | 400 | 37.16 | 44.44 |

The slip to avoid is using the length factor for an area. A metre is 3.28 ft, but a square metre is 3.28 × 3.28 = 10.76 ft². Dividing the 168.58 ft² bedroom by 3.28 gives 51.40, more than three times its real 15.66 m². For single conversions without a room, use the [square feet to square metres converter](/conversion/square-feet-to-square-meters).

## How much flooring to buy

Flooring is sold in whole cartons, and cutting to fit walls, doorways and the ends of rows wastes some of it. Manufacturers state the allowance in their installation instructions. [Bruce's solid hardwood instructions](https://www.bruce.com/assets/content/sites/2/Bruce-Installation-Solid-3-4.pdf), for example, require 5 % added to the measured area for cutting and grading, 10 % for diagonal installations and 10–15 % for glue-down installations. Use the figure for the product you are buying.

For the 168.58 ft² bedroom and a carton that covers 20 ft²:

1. Straight lay at 5 %: 168.58 × 1.05 = 177.01 ft², and 177.01 ÷ 20 = 8.85 cartons, so buy 9 (180 ft²).
2. Diagonal lay at 10 %: 168.58 × 1.10 = 185.44 ft², or 9.27 cartons, so buy 10 (200 ft²).

Round up once, at the end. Rounding each dimension up first already adds area: taking the bedroom as 13 ft 6 in × 12 ft gives 162 ft² against the real 156.67 ft², 3.4 % extra before any allowance. Measure closets and door openings the flooring will run into, and leave out areas it will not cover, such as the floor under fixed kitchen cabinets. The [tile calculator](/construction/tile-calculator) does the same carton arithmetic for tiles, including grout joints.

## Room area versus a home's square footage

A house's official square footage is not the sum of its rooms. [ANSI Z765-2021](https://www.homeinnovation.com/services/national_standards/square_footage_method_standard_faq), the US measuring standard for single-family houses, covers above-grade and below-grade finished area and does not cover room dimensions. It measures each level at floor level to the outside finished surface of the exterior walls, leaves out garages, counts finished space only where the ceiling is at least 7 ft high (with exceptions under beams and for sloped ceilings), reports below-grade area separately and rounds to the nearest whole square foot. Because the measurement runs to the outside of the exterior walls and takes in hallways and stairs, a house's figure is larger than its rooms added together.

## Inputs

- **Room shape** (options: Rectangles, L-shape)
- **Rectangles (length x width, one per line)**: Each line is one rectangle, e.g. 4 x 3.5. Add a line per room or per part of a room.
- **Rectangles measured in** (options: m, cm, ft, in)
- **Overall length**
- **Overall width**
- **Cut-out length**: The missing corner, measured along the overall length.
- **Cut-out width**
- **Ceiling height**

## Results

- Floor area (m²) — main result
- Floor area (ft²)
- Wall area (m²)
- Wall area (ft²)
- Perimeter (m)
- Perimeter (ft)
- Volume (m³)
- Volume (ft³)

## Formula

$$
A = \sum \ell_i w_i,\quad A_{\text{walls}} = h \sum 2(\ell_i + w_i),\quad V = A\,h
$$

## Worked examples

### Two rectangles in metres

- Room shape: Rectangles
- Rectangles (length x width, one per line): 4 x 3.5 / 2.5 x 2
- Rectangles measured in: m
- Ceiling height: 2.7 m
- **Floor area: 19 m²**
- **Floor area: 204.51 ft²**
- **Wall area: 64.8 m²**
- **Perimeter: 24 m**
- **Volume: 51.3 m³**
- Checked against: Python decimal: 4×3.5 + 2.5×2 = 19 m²; perimeters 15 + 9 = 24 m; ÷ 0.3048² for ft²

### 12 × 10 ft room, 8 ft ceiling

- Room shape: Rectangles
- Rectangles (length x width, one per line): 12 x 10
- Rectangles measured in: ft
- Ceiling height: 8 ft
- **Floor area: 120 ft²**
- **Wall area: 352 ft²**
- **Volume: 960 ft³**
- **Floor area: 11.15 m²**
- Checked against: Hand calculation: 12 × 10 = 120 ft²; 2(12 + 10) × 8 = 352 ft²; 120 × 8 = 960 ft³

### L-shape 6 × 5 m with a 2 × 3 m cut-out

- Room shape: L-shape
- Overall length: 6 m
- Overall width: 5 m
- Cut-out length: 2 m
- Cut-out width: 3 m
- Ceiling height: 2.4 m
- **Floor area: 24 m²**
- **Perimeter: 22 m**
- **Wall area: 52.8 m²**
- **Volume: 57.6 m³**
- Checked against: Hand calculation: 30 − 6 = 24 m²; an L-shape's perimeter equals its bounding rectangle's, 2(6 + 5) = 22 m

### Edge: one square written with × and a blank line, in cm

- Room shape: Rectangles
- Rectangles (length x width, one per line): 300 × 300 / 
- Rectangles measured in: cm
- Ceiling height: 250 cm
- **Floor area: 9 m²**
- **Perimeter: 12 m**
- **Wall area: 30 m²**
- **Volume: 22.5 m³**
- Checked against: Python decimal: 3 m × 3 m square, 2.5 m ceiling

## Questions

### How do you calculate the square footage of a room?

Multiply the length by the width, both in feet. A 12 × 10 ft room is 120 ft². For an irregular room, split the floor into rectangles, work out each one and add them: a 12 × 10 ft room with a 4 × 3 ft closet totals 132 ft². Measurements in inches divide by 144 to give square feet.

### How do you find the area of an L-shaped room?

Take the rectangle that encloses the room and subtract the missing corner. A 6 × 5 m room with a 2 × 3 m cut-out is 30 − 6 = 24 m². The perimeter equals the enclosing rectangle's, 2 × (6 + 5) = 22 m, so under a 2.4 m ceiling the walls total 52.8 m².

### How many square feet are in a square metre?

10.7639. The foot is exactly 0.3048 m, the only definition NIST uses since the US survey foot was retired on 1 January 2023 (NIST Handbook 44, Appendix C), so 1 m² = 1 ÷ 0.3048² ≈ 10.764 ft² and 1 ft² ≈ 0.0929 m². A 19 m² room is 204.51 ft², and a 120 ft² room is 11.15 m².

### What is the minimum ceiling height for a room?

In US homes, 7 ft for habitable rooms and 6 ft 8 in for bathrooms, toilet rooms and laundry rooms (2021 IRC R305.1; 2024 IRC R313.1). A 12 × 10 ft room at the 7 ft minimum holds 840 ft³ of air, against 960 ft³ at 8 ft. The section's exceptions and local amendments can change the figure.

### How do you calculate the wall area of a room?

Multiply the perimeter by the ceiling height, then subtract doors and windows. A 12 × 10 ft room with an 8 ft ceiling has 2 × (12 + 10) × 8 = 352 ft² of wall. This calculator reports the gross figure; the paint and drywall calculators take off openings.

### How accurate is the room area and square footage calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “Two rectangles in metres” is checked against Python decimal: 4×3.5 + 2.5×2 = 19 m²; perimeters 15 + 9 = 24 m; ÷ 0.3048² for ft².

### Where does the method come from?

NIST Handbook 44, Appendix C — units of area and volume; NIST — U.S. survey foot: revised unit conversion factors (foot = 0.3048 m exactly from 2023); ANSI Z765-2021 — square footage method for single-family houses: finished area of a whole home, measured to the outside walls (it does not cover single rooms).

## Sources

- [NIST Handbook 44, Appendix C — units of area and volume](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition)
- [NIST — U.S. survey foot: revised unit conversion factors (foot = 0.3048 m exactly from 2023)](https://www.nist.gov/pml/us-surveyfoot/revised-unit-conversion-factors)
- ANSI Z765-2021 — square footage method for single-family houses: finished area of a whole home, measured to the outside walls (it does not cover single rooms)