How to calculate floor area
Floor area is the size of the floor surface, measured on plan. For a rectangular room it is simply length times width. Most real rooms are not perfect rectangles, so the reliable method is to split the plan into simple shapes, calculate each one and add them up. That is what this calculator does: add a row for every room or part of a room, pick its shape and type its dimensions. The total updates as you type.
Measure between the finished inside faces of the walls for the net (usable) area. If you need the gross area — for example to compare with the footprint of a building — the wall thickness also counts. The calculator estimates it by adding half of the wall thickness around every room, because each wall is shared by two rooms or by a room and the outside.
triangle = base × height ÷ 2 · circle = π × d² ÷ 4
trapezoid = (a + b) ÷ 2 × h
1 m² = 10.7639 ft² · flooring = net area × (1 + waste %)
Worked example
An apartment has a 4 × 5 m living room (20 m²), an L-shaped kitchen 6 × 5 m with a 2 × 2 m notch (30 − 4 = 26 m²), a triangular balcony corner with a 3 m base and 4 m height (6 m²) and a round entry hall 2 m in diameter (π × 2² ÷ 4 = 3.14 m²). Together that is 55.14 m², or 593.5 ft². With 10% waste you would order about 60.7 m² of flooring.
In feet: a 12 × 15 ft bedroom is 180 ft², which is 16.72 m².
Area formulas at a glance
| Shape | Inputs | Formula | Example |
|---|---|---|---|
| Rectangle | length a, width b | a × b | 4 × 5 = 20 m² |
| L-shape | outline a × b, notch c × d | a·b − c·d | 30 − 4 = 26 m² |
| Triangle | base, height | b × h ÷ 2 | 3 × 4 ÷ 2 = 6 m² |
| Circle | diameter d | π d² ÷ 4 | d 2 m = 3.14 m² |
| Trapezoid | parallel sides a, b, height h | (a + b) ÷ 2 × h | (4 + 6) ÷ 2 × 3 = 15 m² |
Net, gross and usable area in practice
Different countries measure floor area by different rules. Many European standards count balconies, terraces and loggias at a reduced percentage, and some exclude areas with low ceiling height under roofs. Real-estate listings often quote gross area, while interior work and flooring orders use net area. When you compare two numbers, make sure they were measured the same way. For apartments in Serbia, our apartment area calculator applies the local coefficients for terraces and balconies. If you produce plans in Revit, the ArchPlug Rooms plugin creates and tags room areas automatically.
Frequently asked questions
How do I calculate the square footage of a room?
Measure the length and width of the room in feet and multiply them. A 12 ft by 15 ft room is 180 square feet. For irregular rooms, split the plan into rectangles and triangles, calculate each and add them up.
How do I convert m² to square feet?
Multiply square metres by 10.7639 to get square feet, and divide square feet by 10.7639 to get square metres. 20 m² is about 215.3 ft².
How do I calculate the area of an L-shaped room?
Take the area of the full rectangle around the L and subtract the missing corner rectangle. A 6 by 5 m outline with a 2 by 2 m notch is 30 minus 4, which is 26 m².
What is the difference between net and gross floor area?
Net (usable) area is measured between the inside faces of walls. Gross area also includes the walls themselves, measured to the outside face. The calculator estimates gross area by adding half of the wall thickness around each room.
How much extra flooring should I buy?
Add about 5 to 10 percent for straight laying of tiles, laminate or parquet, and 10 to 15 percent for diagonal or herringbone patterns and rooms with many corners.
Does this area calculator save my data?
No. Everything is calculated in your browser and nothing is sent to a server. Use print to keep a copy of your list of rooms.