Building Square Footage Calculator: How to Measure

This calculator works out the total square footage of a building from its length and width (or from multiple rectangular sections). You enter the dimensions in feet (or meters, which it converts), and it returns the area in square feet and square meters. The result is the gross floor area of a simple rectangular footprint, which is the standard starting point for real-estate listings, material estimates, and zoning checks.

Formula

The formula for a rectangular building is simply:

Area = Length × Width

• Length – the longer side of the building footprint, in feet (ft) or meters (m).
• Width – the shorter side, in the same unit.
• Area – the floor area, in square feet (sq ft) or square meters (m²).

If the building is not a single rectangle, you break it into rectangular sections, calculate each one, and add them together. That is the same method used by appraisers and assessors for most simple residential shapes.

Worked Example

Using the default values from the calculator – length = 40 ft, width = 30 ft:

1. Multiply length by width: 40 × 30 = 1200
2. The area is 1200 square feet.
3. To get square meters, convert each dimension to meters first: 40 ft × 0.3048 = 12.192 m; 30 ft × 0.3048 = 9.144 m.
4. Multiply: 12.192 × 9.144 = 111.48 m² (rounded to two decimals).

The calculator shows 1200.00 sq ft and 111.48 m² – exactly the same result.

What “Square Footage� Really Means

Square footage is the two-dimensional floor area of a building, measured from the outside of the exterior walls (gross living area, or GLA). It does not include:

• Unfinished basements or attics (unless finished and meet local codes)
• Garages, porches, or decks (unless they are enclosed and heated)
• Common areas in condos or apartments (those are listed separately)

For a single-family home, the standard practice is to measure the exterior perimeter at each floor level and add the floors together. A two-story house with a 40×30 footprint therefore has 2400 sq ft of living space (1200 per floor), not 1200.

How to Measure a Building Yourself

If you need the square footage for a real estate listing, refinance, or permit, follow these steps:

1. Draw a rough sketch of the building footprint, breaking it into rectangles.
2. Measure each rectangle with a tape measure or laser distance measurer. Measure from the outside of the walls, not the interior.
3. For each rectangle, multiply length × width to get its area.
4. Add all the areas together to get the total square footage.
5. If the building has multiple floors, repeat for each floor and add those totals.

For a simple rectangular house, this calculator does steps 3–4 automatically. For complex shapes, you can use it repeatedly for each section and sum the results.

Common Mistakes and How to Avoid Them

• Mixing units – If you measure length in feet and width in meters, the result is meaningless. Use the same unit for both, or let the calculator convert (it does, but only if you enter both in the same unit).
• Measuring interior instead of exterior – Interior dimensions exclude wall thickness, which can add or subtract several percent. For official listings, measure from the outside.
• Forgetting to add all floors – A two-story house has double the footprint area. The calculator gives one floor; multiply by the number of floors if needed.
• Rounding too early – If you convert feet to meters and round each dimension to two decimals, you lose accuracy. Do the conversion in one step, as the calculator does.
• Including non-living spaces – Garages, unfinished basements, and screened porches are usually not counted in GLA. Check your local rules.

Limits of This Calculator

This calculator assumes a perfect rectangle. Real buildings have alcoves, bay windows, irregular corners, and multiple levels. For those, you must break the shape into rectangles and add the results. It also does not account for:

• Sloped ceilings (finished attics – only areas with at least 7 ft ceiling height count)
• Stairwells (counted once, usually on the lower floor)
• Wall thickness (the difference between exterior and interior measurements)

For official appraisals or permits, a professional measurement is recommended. The calculator is a good estimate for planning, material ordering, or comparing properties.

Square Footage vs. Square Meters

Most of the world uses square meters; the U.S., Canada, and the U.K. commonly use square feet. To convert:

• 1 square foot = 0.092903 square meters
• 1 square meter = 10.7639 square feet

The calculator gives both, so you don’t need to remember the factors. If you have a measurement in square meters and want square feet, multiply by 10.7639. For example, a 50 m² apartment is about 538 sq ft.

Frequently Asked Questions

How do I calculate square footage of an irregular shaped room?

Break the room into rectangles (and triangles if needed). For each rectangle, multiply length × width. For a triangle, multiply base × height and divide by 2. Add all the areas together. This calculator handles the rectangular parts; do the triangles separately.

Does square footage include walls?

For gross living area, yes – the measurement is taken from the outside of the exterior walls, so wall thickness is included. If you measure interior walls, you get “net� area, which is smaller. Most real estate listings use gross living area.

What is the difference between square footage and floor area?

They are often used interchangeably, but “floor area� can mean the area of a single floor, while “square footage� usually refers to the total living area of the whole building. A two-story house has a floor area of 1200 sq ft per floor, but a total square footage of 2400 sq ft.

Can I use this calculator for a multi-story building?

Yes, for each floor that has the same dimensions, multiply the result by the number of floors. If floors differ, calculate each floor separately and add them. Do not forget to exclude unheated or unfinished areas if you are calculating living space.

Why does my appraisal show a different square footage than my builder’s plan?

Appraisers and assessors follow specific rules (e.g., ANSI standards in the U.S.) that may exclude certain areas like unfinished basements or count finished attics differently. Builders may include all constructed area. Differences are common and usually due to definitions, not errors.

How many square feet is a 20×20 room?

400 square feet. Multiply 20 by 20. This calculator gives the same result for length=20, width=20. A 20×20 room is a common size for a two-car garage or a large bedroom suite.

Related

• Bolt Pitch Diameter Calculator and Thread Formula
• Board on Board Fence Calculator: Estimate Materials
• Board on Board Fence Picket Calculator and Formula

in length | Length | ft | 40 | 0
in width | Width | ft | 30 | 0
out sqft | Square footage | sq ft | length * width | 2
out sqm | Square meters | m² | (length * 0.3048) * (width * 0.3048) | 2