Building Square Footage Calculator
Building square footage calculator for houses and offices. Add wings, floors and walls for gross and net area in sq ft, sq m, sq yd and acres.
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How the Building Square Footage Calculator Formula Works
Break the building outline into rectangles. The main block is length x width, each wing is its length along the wall times how far it sticks out, and a corner cut-out is subtracted. A 40 × 30 ft block with a 20 × 20 ft wing is 1,600 sq ft on one floor. The wall perimeter is the main perimeter plus two times each wing's depth, which is 180 ft here.
For several floors the gross area is the footprint times 1 plus the number of extra floors times the upper floor size. With two floors and a 75% upper floor, 1,600 × 1.75 = 2,800 sq ft. To get the area inside the walls, the calculator takes off the wall thickness t along the perimeter and adds back 4 t squared for the corners. For a house it then subtracts the garage to give living area and reports any basement separately. For a commercial building it applies a common and service share to estimate rentable area.
footprint = main + wings − corner cut-out; inside = footprint − t × perimeter + 4 t2; gross = footprint × (1 + (floors − 1) × upper size %)- L, Wft or mLength and width of the main block, measured outside or inside the walls.
- wing length, depthft or mLength along the wall and how far the wing sticks out.
- tin or cmExterior wall thickness, used to convert between outside and inside sizes.
- floorscount2Number of floors above ground with this footprint.
- upper size%Size of each floor above the first, as a share of the ground floor.
- + 1 more input below
- L, W
- ft or m
- Length and width of the main block, measured outside or inside the walls.
- wing length, depth
- ft or m
- Length along the wall and how far the wing sticks out.
- t
- in or cm
- Exterior wall thickness, used to convert between outside and inside sizes.
- floors
- count
- Number of floors above ground with this footprint.
- upper size
- %
- Size of each floor above the first, as a share of the ground floor.
- common share
- %
- Commercial only: share of gross area used for common and service space.
Using the Building Square Footage Calculator
Enter units
Choose from Imperial (feet, inches), Metric (metres, centimetres).
Enter building type
Choose from House (garage and basement options), Commercial (rentable estimate).
Enter my sizes are measured
Choose from Outside the walls (exterior), Inside the walls (interior).
Enter exterior wall thickness (inches, or cm in metric)
Type the figure you already have, then press Calculate.
Enter main block length (ft, or m)
Type the figure you already have, then press Calculate.
Enter main block width (ft, or m)
Type the figure you already have, then press Calculate.
Enter wing a length along the wall (0 for none)
Type the figure you already have, then press Calculate.
Enter wing a depth, how far it sticks out
Type the figure you already have, then press Calculate.
Enter wing b length along the wall (0 for none)
Type the figure you already have, then press Calculate.
Enter wing b depth, how far it sticks out
Type the figure you already have, then press Calculate.
Enter corner cut-out length (0 for none)
Type the figure you already have, then press Calculate.
Enter corner cut-out width
Type the figure you already have, then press Calculate.
Enter floors above ground
Any value between 1 and 100.
Enter size of each upper floor (% of the ground floor)
Any value between 0 and 200.
Enter garage floor area (sq ft, or sq m)
Type the figure you already have, then press Calculate.
Enter basement size (% of the footprint, 0 for none)
Any value between 0 and 200.
Enter common and service share of gross area (%)
Any value between 0 and 99.
Read the result
Press Calculate to update the result. Until you do, the last result is dimmed and marked as out of date, so you always know which numbers belong to the values in the boxes.
A Real Worked Example
A two-storey house is a 40 × 30 ft main block with a 20 ft wide garage wing that sticks out 20 ft. The ground floor is 1,200 + 400 = 1,600 sq ft. The second floor sits over the main block only, so it is 75% of the ground floor, and there is a basement under the main block at 75%.
Gross floor area above ground is 1,600 × 1.75 = 2,800 sq ft, which is 260.13 sq m, 311.11 sq yd or 0.0643 acres. Taking off the 400 sq ft garage leaves 2,400 sq ft of living area. The basement adds 1,200 sq ft on its own line, so all levels total 4,000 sq ft. With 6 inch walls the floor area inside the exterior walls is about 2,644.25 sq ft.
A three-floor office building, 100 x 60 ft
Choose commercial, enter 100 × 60 ft, three equal floors, 12 inch walls and a 15% common and service share. The footprint is 6,000 sq ft, so the gross floor area is 18,000 sq ft, which is 2,000 sq yd.
The floor area inside the walls is 17,052 sq ft. Taking 15% off the gross for common and service space gives an estimated 15,300 sq ft rentable. Your lease's measurement standard decides the real figure.
An L-shaped ranch house by subtracting a corner
Enter a 50 × 30 ft main block with a 20 × 12 ft corner cut-out, one floor, no garage or basement. The area is 1,500 − 240 = 1,260 sq ft, which is 140 sq yd.
The perimeter stays 160 ft, so with 6 inch walls the inside area is 1,181 sq ft.
A cabin measured inside, 24 x 18 ft with 8 inch walls
Choose 'inside the walls' and enter 24 × 18 ft with 8 inch walls. The inside area is 432 sq ft. Adding the wall thickness gives an outside size of 25.33 × 19.33 ft, which the calculator reports as 489.78 sq ft gross.
The outside wall perimeter is 89.33 ft, and in square metres the gross area is 45.50.
A metric house, 12 x 9 m plus a 4 x 3 m wing
Choose metric, enter 12 × 9 m with a wing 4 m along the wall and 3 m out, two equal floors, 20 cm walls and a 36 sq m garage. Each floor is 120 sq m, so the gross area is 240 sq m, or 2,583 sq ft.
Taking off the garage leaves 204 sq m of living area, and the area inside the walls is 221.1 sq m.
A 20 x 20 ft building
Enter 20 × 20 ft, one floor, 6 inch walls, no garage or basement. The gross area is 400 sq ft, 37.16 sq m or 44.44 sq yd.
The inside area is 361 sq ft, so the walls take 39 sq ft, about 9.8% of the outside figure.
What Else to Know
How to Easily Calculate the Square Footage of a Building
The easiest method is to break the building into rectangles. Measure the length and width of each part in feet, multiply, and add the results. A 40 × 30 ft building is 1,200 sq ft. A 20 × 20 ft building is 400 sq ft, and a 2,000 sq ft building is about 44.7 × 44.7 ft or 40 × 50 ft on one floor.
The calculator does the adding. Enter the main block, then up to two wings for the parts that stick out, then an optional corner cut-out for the part that is missing. Add the number of floors and it returns the footprint, gross floor area and the same area in square metres, square yards and acres.
A wing is described by its length along the wall and its depth beyond it. A cut-out is a rectangle removed from one corner of the main block. These two ideas cover most house and small commercial outlines, including L-shapes, T-shapes and U-shapes made from several blocks. Height is not needed for floor area; it only matters for wall area and volume.
Gross, Net and Rentable Area Explained
Gross floor area is measured to the outside face of the exterior walls. Gross internal area is measured to the inside face. Net internal area then removes lobbies, stairs, lifts, plant rooms and similar space. These definitions follow Wikipedia's article on floor area, and different countries use their own versions.
For leasing, gross leasable area is the space a tenant can rent. Wikipedia notes it usually excludes corridors and public areas, and that the Building Owners and Managers Association worked with ANSI on a standard for measuring floor area and loss factors. Your lease will name the edition it uses.
The calculator gives gross area, the area inside the exterior walls, and for commercial buildings a rentable estimate. The rentable figure applies a common and service share you enter. In the example, 18,000 sq ft gross at 15% common space leaves 15,300 sq ft. That 15% is a placeholder, not a market value.
Outside Versus Inside Measurements and Wall Thickness
Measuring the outside of a building gives a bigger number than measuring inside, because the walls take space. For a rectangle the inside area is (L - 2t) × (W - 2t), where t is the wall thickness. A 40 × 30 ft footprint with 6 inch walls has 1,131 sq ft inside, and with 12 inch walls 1,064 sq ft.
The calculator uses a form that works for any outline made of right angles: inside area = outside area - t x perimeter + 4 t squared. If your sizes are inside measurements, it goes the other way and adds the walls. Either way you see both numbers.
The default 6 inch wall is a placeholder. Measure a window or door opening, read the plans, or ask your builder. Interior partitions are not deducted, so the inside area is the space within the exterior shell, not the sum of room sizes.
Houses: Floors, Garage and Basement Conventions
Listings often follow a convention for houses. The National Association of Realtors describes the ANSI Z765 approach as using exterior dimensions of each floor for detached homes, and leaving out garages, porches and patios. It also says any area partly or fully below grade is excluded from the main figure, though it can still add value.
The calculator mirrors that logic in a simple way. Gross floor area above ground is the footprint times the floors. Living area subtracts the garage you enter. The basement appears on its own line, and a separate total adds it back.
Do not read this as a legal or appraisal result. ANSI Z765, BOMA standards, local codes and tax records differ, and they change. A professional measurer or appraiser decides what applies to a sale, a loan or a permit. Use this page to plan, compare and check a listing against your own measurements.
Multiple Floors, Wings and L-Shaped Buildings
For several floors, the calculator multiplies the footprint by 1 plus the extra floors times the upper floor size. A 1,200 sq ft footprint with three full floors is 3,600 sq ft. With upper floors at 80% it is 3,120 sq ft.
The floor size percentage handles an upper floor that is smaller than the ground floor, such as one over a house but not over a garage. All floors are assumed to share the same shape scaled down, so the area inside the walls is scaled the same way. It is an estimate; for an odd upper floor, run the calculator separately.
For L-shapes, use a wing or a cut-out. A 50 × 30 ft block with a 20 × 12 ft corner removed is 1,260 sq ft with a 160 ft perimeter. A wing adds two times its depth to the perimeter.
Square Feet to Square Metres, Square Yards and Acres
One square foot is 0.09290304 square metres and one ninth of a square yard. An acre is 43,560 sq ft, which is 4,840 square yards. A 2,000 sq ft building is 185.8 sq m, 222.2 sq yd and 0.046 acre of floor area.
The acre figure compares floor area with land measured in acres. It is not the size of the lot. A building with a 1,200 sq ft footprint can sit on a quarter-acre lot of 10,890 sq ft with plenty of room around it.
Enter lengths in metres and the calculator converts everything to feet before it works out areas, so the metric and imperial results agree.
Measuring Tips, Limits and Related Calculators
Measure each outside wall with a tape or laser, record the corners, and sketch the outline as blocks. Check that opposite sides add up. Round to the nearest inch or centimetre; rounding to the foot can move a 2,000 sq ft total by tens of square feet.
The calculator assumes right-angle corners, flat walls and the wing and cut-out layout described above. It does not handle curved walls, bay windows, sloped ceilings, staircases or shared walls between attached units. It is not an appraisal or a legal measurement.
For related work, see the rafter length calculator for framing the roof, the stair calculator for the stairs between floors and the percentage calculator for upper floor or common space shares.
Reference Tables
| Floors | Footprint (sq ft) | Square footprint (ft) | A 2:1 footprint (ft) | Total (sq m) |
|---|---|---|---|---|
| 1 | 2,000 | 44.7 × 44.7 | 31.6 × 63.2 | 185.8 |
| 2 | 1,000 | 31.6 × 31.6 | 22.4 × 44.7 | 185.8 |
| 3 | 667 | 25.8 × 25.8 | 18.3 × 36.5 | 185.8 |
| Footprint (ft) | Sq ft | Sq m | Sq yd | Acres |
|---|---|---|---|---|
| 20 × 20 | 400 | 37.2 | 44.4 | 0.0092 |
| 24 × 24 | 576 | 53.5 | 64.0 | 0.0132 |
| 30 × 40 | 1,200 | 111.5 | 133.3 | 0.0275 |
| 40 × 50 | 2,000 | 185.8 | 222.2 | 0.0459 |
| 50 × 60 | 3,000 | 278.7 | 333.3 | 0.0689 |
| 60 × 100 | 6,000 | 557.4 | 666.7 | 0.1377 |
| 100 × 200 | 20,000 | 1,858.1 | 2,222.2 | 0.4591 |
| Wall thickness (in) | Inside size (ft) | Interior area (sq ft) | Walls take (sq ft) | Share lost |
|---|---|---|---|---|
| 4 | 39.33 × 29.33 | 1,153.8 | 46.2 | 3.9% |
| 6 | 39.00 × 29.00 | 1,131.0 | 69.0 | 5.8% |
| 8 | 38.67 × 28.67 | 1,108.4 | 91.6 | 7.6% |
| 10 | 38.33 × 28.33 | 1,086.1 | 113.9 | 9.5% |
| 12 | 38.00 × 28.00 | 1,064.0 | 136.0 | 11.3% |
| 16 | 37.33 × 27.33 | 1,020.4 | 179.6 | 15.0% |
| Floors | Full-size upper floors (sq ft) | Upper floors at 80% (sq ft) | Sq m at full size |
|---|---|---|---|
| 1 | 1,200 | 1,200 | 111.5 |
| 2 | 2,400 | 2,160 | 223.0 |
| 3 | 3,600 | 3,120 | 334.5 |
| 4 | 4,800 | 4,080 | 445.9 |
| Footprint | Blocks | Area (sq ft) | Perimeter (ft) |
|---|---|---|---|
| Rectangle | 40 × 30 | 1,200 | 140 |
| L-shape by cut-out | 50 × 30 minus 20 × 12 corner | 1,260 | 160 |
| L-shape by wing | 40 × 30 plus 20 × 12 wing | 1,440 | 164 |
| House with garage wing | 40 × 30 plus 20 × 20 wing | 1,600 | 180 |
| Two wings | 40 × 30 plus 20 × 12 and 10 × 5 | 1,490 | 174 |
| Wing and corner notch | 40 × 30 plus 20 × 12 minus 10 × 8 | 1,360 | 164 |
| Space | Usually in the main figure? | Why |
|---|---|---|
| Finished living space above ground | Yes | This is the gross living area, measured on the outside |
| Attached or detached garage | No | Built for vehicles, not living |
| Porch or patio | No | Not enclosed living space |
| Finished basement or any space partly below grade | No, reported separately | Below grade, even when finished |
| Unfinished space | No | Not finished living area |
| Upper floor over a garage | Yes, if finished | The floor is above ground and finished |
| Term | Measured to | Typical use |
|---|---|---|
| Gross floor area (GFA) | Outside face of the exterior walls | Planning, permits, house listings |
| Gross internal area (GIA) | Inside face of the exterior walls | Floor space inside the shell |
| Net internal area (NIA) | GIA minus lobbies, stairs, lifts, plant and similar | Usable space for an occupier |
| Gross leasable area (GLA) | Space available to rent, usually without corridors and public areas | Commercial leases |
| Loss factor | Share of space not directly rentable | Rent calculations, set by the standard in the lease |
| Sq ft | Sq m | Sq yd | Acres |
|---|---|---|---|
| 500 | 46.45 | 55.6 | 0.0115 |
| 1,000 | 92.90 | 111.1 | 0.0230 |
| 1,500 | 139.35 | 166.7 | 0.0344 |
| 2,000 | 185.81 | 222.2 | 0.0459 |
| 2,500 | 232.26 | 277.8 | 0.0574 |
| 3,000 | 278.71 | 333.3 | 0.0689 |
| 5,000 | 464.52 | 555.6 | 0.1148 |
| 10,000 | 929.03 | 1,111.1 | 0.2296 |
| 43,560 | 4,046.86 | 4,840.0 | 1.0000 |
| Common and service share | Common area (sq ft) | Rentable (sq ft) | Rentable (sq m) |
|---|---|---|---|
| 5% | 500 | 9,500 | 882.6 |
| 10% | 1,000 | 9,000 | 836.1 |
| 15% | 1,500 | 8,500 | 789.7 |
| 20% | 2,000 | 8,000 | 743.2 |
| 25% | 2,500 | 7,500 | 696.8 |
| Building | Sq ft | Sq m | Sq yd | Acres | Perimeter (ft) |
|---|---|---|---|---|---|
| 20 × 20 ft | 400 | 37.16 | 44.44 | 0.0092 | 80 |
| 40 × 50 ft | 2,000 | 185.81 | 222.22 | 0.0459 | 180 |
| 44.7 × 44.7 ft | 2,000 | 185.81 | 222.22 | 0.0459 | 178.9 |
| Mistake | Effect | Fix |
|---|---|---|
| Mixing inside and outside measurements | Differences of several percent | Pick one basis and say which |
| Counting the garage as living area | Living area overstated | Subtract the garage, as the calculator does for houses |
| Adding a finished basement to the main figure | Number does not match listings | Report it as a separate line |
| Measuring the same wall twice at a wing | Area too high or too low | Enter the wing as depth beyond the main wall |
| Entering inches as feet | Area 144 times off | Check the unit and the wall thickness unit |
| Assuming upper floors match the footprint | Gross area overstated | Set the upper floor size percentage |
Frequently Asked Questions About the Building Square Footage Calculator
How do you calculate the square footage of a building?
Measure the outside length and width in feet and multiply them. A 40 ft by 30 ft building is 1,200 sq ft on one floor. For several floors, add each floor's area. For an L-shape, add the rectangles or subtract the missing corner. A 50 × 30 ft footprint with a 20 × 12 corner missing is 1,260 sq ft. The calculator handles wings, cut-outs and floors.
What is the easiest way to calculate square footage?
Split the building into rectangles, multiply length by width for each, and add them. Use outside dimensions for a gross figure. A 40 × 30 ft block plus a 20 × 20 ft wing is 1,600 sq ft per floor. Enter the main block and wings in the calculator and it adds floors, subtracts the garage and converts to square metres, yards and acres.
How big is a 2000 sq ft building?
On one floor, 2,000 sq ft is about 44.7 ft by 44.7 ft if square, or 40 ft by 50 ft as a plain rectangle. On two equal floors each floor is 1,000 sq ft, about 31.6 ft by 31.6 ft. It equals 185.8 square metres, 222.2 square yards or 0.046 acre of floor area.
What is the square footage of a 20x20 building?
A 20 ft by 20 ft building is 400 sq ft, or 37.16 square metres. With 6 inch walls the area inside the walls is 361 sq ft. Two such floors give 800 sq ft of gross floor area. If the 20 × 20 is in metres, it is 400 square metres, which is 4,306 sq ft.
How do I calculate square feet of a house?
Use the outside size of each floor, add the floors that are finished and above ground, and leave out the garage. Our example house has a 1,600 sq ft footprint, a 75% upper floor and a 400 sq ft garage, so the living area is 2,400 sq ft. Appraisers and local rules may count details differently, so ask a measurer if the number matters.
How do I calculate square feet from length, width and height?
Square feet need only length and width. Height changes the wall area and the volume, not the floor area. A room 20 ft by 15 ft is 300 sq ft of floor. At 8 ft high the four walls are 560 sq ft before openings, and the volume is 2,400 cubic feet.
How do I calculate square feet of a wall?
Multiply the wall's length by its height. A wall 40 ft long and 9 ft high is 360 sq ft. Subtract doors and windows if you need the net surface. For all four outside walls of a building, use the perimeter times the height: a 40 × 30 ft building at 9 ft is 1,260 sq ft.
How do I calculate square feet of a room with height?
Floor area is length times width; height does not change it. With height you can find wall area and volume. For a 12 by 14 ft room with a 9 ft ceiling the floor is 168 sq ft, the walls are 468 sq ft before openings, and the volume is 1,512 cubic feet.
How do I convert feet to square feet?
You cannot convert one to the other directly, because feet measure length and square feet measure area. Multiply two lengths in feet. A strip 50 ft long and 3 ft wide is 150 sq ft. A single length of 50 ft has no area until you give it a width.
How do I find a house's square footage online for free?
Public property records, a listing, or the plan set often show a size, but these can differ from what you measure and from each other. Tax records and listings may follow different rules. Measure the outside yourself with this calculator, then compare. If a sale or loan depends on the figure, use a professional measurer or appraiser.
What is the difference between gross and net square footage?
Gross area is measured to the outside face of the walls. Net or internal area is inside the walls and often leaves out stairs, lobbies and plant space. For a 40 × 30 ft footprint with 6 inch walls the gross area is 1,200 sq ft and the area inside the walls is 1,131 sq ft.
Does square footage include the garage?
For a house, the usual listing convention leaves the garage out of living area. The National Association of Realtors says garages are excluded under the ANSI Z765 approach. The calculator reports gross area, then subtracts the garage you enter to give living area. Commercial leases follow their own measurement standard.
Does square footage include the basement?
Usually not in the main figure. Under the ANSI Z765 approach space that is partly or fully below grade is reported separately, even if it is finished. In our example a 75% basement adds 1,200 sq ft on its own line, and the total of all levels is 4,000 sq ft.
Should I measure inside or outside walls?
It depends on the purpose. House listings under ANSI Z765 use outside dimensions, according to the National Association of Realtors. Floor coverings and furniture planning need inside sizes. The calculator takes either and converts with your wall thickness: a 24 × 18 ft interior with 8 inch walls is 489.8 sq ft on the outside.
How do you measure an L-shaped building?
Use one main rectangle plus a wing, or one rectangle minus a corner. For 50 × 30 ft with a 20 × 12 ft corner missing, the area is 1,500 minus 240, which is 1,260 sq ft. The perimeter stays 160 ft, the same as the full rectangle, because a corner notch does not change it.
How do you calculate square footage for multiple floors?
Add the area of each floor. If the upper floors are the same size, multiply by the number of floors. Our example has a 1,600 sq ft ground floor and a second floor at 75%, so 1,600 + 1,200 = 2,800 sq ft above ground. Use the upper floor size box for smaller floors.
How do you calculate commercial square footage?
Measure the gross floor area on the outside, then apply the measurement standard in your lease. Many leases use a BOMA standard to split gross into rentable and common space. In our three-floor example, 18,000 sq ft gross with 15% common space is 15,300 sq ft rentable. That share is an example; use your lease.
How many square feet are in an acre?
An acre is 43,560 sq ft, or 4,840 square yards, about 4,047 square metres. A building with 18,000 sq ft of gross floor area is 0.413 acres of floor space, which is not the same as land area. The calculator gives floor area in acres only as a size comparison.
What are ANSI Z765 and BOMA?
ANSI Z765 is the American National Standard for measuring single-family houses. It is used in appraisal. BOMA is the Building Owners and Managers Association, whose standards measure commercial space and loss factors. They differ in method and purpose, and editions change, so a measurer or the lease decides which applies.
How thick are walls when switching inside and outside sizes?
Use your actual wall thickness, from the plans or by measuring a door opening. This calculator defaults to 6 inches as a placeholder. At 6 inches an outside 40 × 30 ft footprint is 1,131 sq ft inside, and at 12 inches it is 1,064 sq ft.
How many square yards is a 2000 sq ft building?
Divide by 9. A 2,000 sq ft building is 222.2 square yards. In square metres it is 185.8, using 1 sq ft = 0.09290304 sq m. The calculator shows square feet, square metres, square yards and acres together, so you can read whichever unit a plan, a quote or a local rule uses without a second conversion step.
Why does listed square footage differ from my measurement?
Reasons include inside versus outside measuring, different treatment of garages, basements and stairs, wall thickness, rounding and older records. The National Association of Realtors notes that public record standards need not match lending standards. Compare the basis first, then the numbers.
References
- [1] National Association of Realtors. “Why Do Square Footage Disputes Arise?.” NAR. Accessed 2026-10-07.
- [2] Wikipedia contributors. “Floor area.” Wikipedia. Accessed 2026-10-07.
- [3] Wikipedia contributors. “Gross leasable area.” Wikipedia. Accessed 2026-10-07.
- [4] Wikipedia contributors. “Square foot.” Wikipedia. Accessed 2026-10-07.
- [5] Wikipedia contributors. “Acre.” Wikipedia. Accessed 2026-10-07.
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