Square Footage Calculator

How to calculate square footage

Calculating the square footage of a standard rectangular room is a simple process. It requires measuring the length and the width of the space and multiplying those two dimensions. For residential real estate, square footage measurements are formally standardized and measured according to the ANSI Z765 standard, which establishes the industry-recognized methodology for calculating finished area in single-family dwellings.

1. Measure the length: Use a tape measure to find the distance along one side of the room, from wall to wall, measuring between the interior wall surfaces. Make sure the tape is straight. This calculator multiplies whatever you type, so it works in whichever convention you measure in; what it cannot do is tell you which one you used.
2. Measure the width: Measure the perpendicular wall next to the first one, from wall to wall.
3. Convert to feet (if necessary): If your measurements include inches, convert them to decimal feet by dividing the number of inches by 12. For example, 10 feet 6 inches becomes 10.5 feet (6 ÷ 12 = 0.5).
4. Multiply length by width: Multiply the two measurements together. The formula is: Length (ft) × Width (ft) = Square Footage (sq ft).

How to measure and calculate different shapes

Depending on the layout of your space, you may need to measure and calculate areas for shapes other than simple rectangles. Here is how to handle each shape supported by this calculator:

• Rectangles: Measure the length and width of the space in feet. Multiply them together.
  Formula: Length × Width = Area
• Squares: Since all four sides of a square are equal, you only need to measure one side. Multiply the side length by itself.
  Formula: Side Length² = Area
• Circles: Find the center of the circle and measure the straight line to the outer edge (the radius). Multiply the radius by itself, then multiply by pi (approximately 3.14159).
  Formula: π × Radius² = Area
• Triangles: Measure the base (flat bottom) and the height (perpendicular distance from base to opposite tip). Multiply the base by the height, then divide by 2 (or multiply by 0.5).
  Formula: 0.5 × Base × Height = Area
• L-Shaped / Irregular Rooms: Divide the L-shape into two simpler rectangles (Section A and Section B). Calculate the square footage of Section A and Section B separately using the rectangular formula, then add them together.
  Formula: (Length A × Width A) + (Length B × Width B) = Total Area

Square footage of irregular or L-shaped rooms

Not all rooms are perfect rectangles. If you are dealing with an L-shaped room, a space with alcoves, or a multi-room project, you can easily calculate the area by breaking the layout down into simpler sections:

1. Divide the space: Sketch the room on paper and draw lines to divide it into separate rectangles or squares. For an L-shaped room, draw a single line to split it into two rectangular areas.
2. Measure each section: Measure the length and width of each individual section separately.
3. Calculate the areas: Multiply the length and width of each section to find its square footage.
4. Sum the results: Add the square footages of all the sections together to find the total square footage of the entire space.

Square footage for flooring, paint, and sod

Understanding square footage is the baseline for budgeting and purchasing materials for home improvement projects:

• Flooring (hardwood, laminate, tile): Flooring is sold by the square foot, often packaged in boxes covering a specific square footage. You calculate the room's square footage, add a waste margin (5 percent is the figure a manufacturer publishes; 10 is trade convention), and divide by the square footage per box to determine how many boxes to purchase. Check out our flooring calculator and tile calculator for precise box estimations.
• Paint: One gallon of wall paint generally covers about 350 to 400 square feet with a single coat. To find the wall square footage, measure the wall heights and widths, multiply them, subtract doors and windows, and divide the total area by 350 to find the required gallons of paint. We have a dedicated paint calculator that computes this automatically.
• Landscaping (sod, soil, mulch): Sod is typically sold in rolls or pallets by the square yard or square foot. Measuring the lawn area in square feet helps you order the correct amount of turf grass. If you are ordering bulk soil or mulch, you'll need to combine the square footage with a target depth to compute the volume in cubic yards using our mulch calculator or cubic yard calculator.

Square feet vs square yards vs square meters

Depending on the material or location, you might need to convert your square footage into other units of area:

• Square Yards (sq yd): Mainly used for carpet, turf, and large-scale landscaping. One square yard is a square that measures 3 feet by 3 feet, which equals 9 square feet. To convert square feet to square yards, divide by 9.
• Square Meters (sq m): The standard metric unit of area. One square meter is approximately 10.764 square feet. To convert square feet to square meters, multiply by 0.0929 (or divide by 10.764).
• Acres: Used for large land plots. One acre contains exactly 43,560 square feet. To convert square feet to acres, divide the total area by 43,560.

Worked example: Calculating a living room and hallway

Let's say you want to install new vinyl plank flooring in a rectangular living room that measures 15 feet 4 inches by 12 feet, plus an adjoining hallway that measures 8 feet by 3 feet.

Common mistakes when measuring area

• Mixing units of measurement: Multiplying inches directly by feet (e.g., 10 feet × 6 inches = 60 square feet) leads to massive errors. Always convert inches to feet by dividing by 12 first.
• Forgetting the waste factor: Ordering the exact square footage of your space will leave you short. You must cut boards or tiles to fit borders, creating scrap. Always add a 10% waste buffer (15% for diagonal layouts).
• Measuring wall-to-wall instead of baseboard-to-baseboard: For flooring projects, ignoring thick wall recesses or door thresholds can leave you slightly short on materials. Always measure to the furthest boundaries where the flooring will actually sit.
• Confusing linear feet with square feet: Linear feet is a measure of straight distance (one-dimensional), while square feet represents surface area (two-dimensional). You cannot buy carpet or sod in simple linear feet without knowing the roll's constant width.

Six conventions, and this page computes none of them

The exterior-versus-interior question is only the first fork. There are at least six published ways to state the square footage of the same house, and the bodies that publish them say out loud that they disagree. This page multiplies the rectangle you type, which is the right number for materials and is not the number a listing, an appraisal, a tax bill or a lease is using.

1. ANSI Z765 finished square footage. Measured "at floor level to the exterior finished surface of the outside walls", with garages and unfinished areas excluded, openings to the floor below excluded, and stairs counted on the floor they descend from. Finished areas need a 7 foot ceiling, 6 ft 4 in under beams and ducts; in a sloped-ceiling room at least half the area must reach 7 feet and nothing under 5 feet counts at all. Its rule on basements is absolute: "No statement of a house's finished square footage can be made without the clear and separate distinction of above-grade areas and below-grade areas."
2. Fannie Mae's appraisal requirement, which is ANSI plus exceptions. Its Selling Guide says appraisers "must follow the American National Standard Institute (ANSI) Square Footage-Method for Calculating: ANSI Z765-2021", required for appraisals with effective dates of 1 April 2022 or later. Asked whether an appraiser may match local custom instead, its FAQ answers in one word: "No." But the same lender switches sides for apartment-style condominiums, where "Fannie Mae's policy of using interior perimeter measurements for the subject property should be applied", and exempts 2 to 4 unit dwellings entirely.
3. State listing standards, which can contradict ANSI outright. North Carolina measures exteriors but tells agents that where they can only reach an interior surface they must "add six inches for each exterior wall"; it requires the space be "heated by a conventional heating system", which ANSI does not; and it expressly permits what ANSI forbids, allowing agents "to report square footage of the dwelling as the total 'living area' without a separate distinction between 'above-grade' and 'below-grade' areas". A finished walk-out basement is therefore inside the advertised number and outside the appraised one, lawfully, in the same state.
4. Colorado goes further and makes you name your method. Its Real Estate Commission publishes a mandatory Square Footage Disclosure whose method checkboxes are "Exterior measurement / FHA / ANSI / Local standard / Other", with a separate list of sources including "Prior appraisal", "Building plans" and "Assessor's office". Its own disclaimer says measurement "is for the purpose of marketing ONLY" and "is not for loan, valuation or other purposes". A state regulator conceding on a mandatory form that five methods are in live use is the clearest evidence there is that no single number is the number.
5. The assessor, which is not one convention but many. Cook County says plainly: "The Assessor's Office calculates the square footage by measuring the exterior perimeter of the home. It is that measurement that your assessment is based upon, not what you might consider to be inside 'living space'." Miami-Dade does something different again, an Adjusted Square Feet figure that adds garages and porches at weighted factors, counting a carport at 0.25 of a square foot. That is a valuation index rather than a floor area, and ANSI excludes garages entirely. Fannie Mae confirms the gap: "GLA found in local MLS systems and assessor records may not be ANSI standard compliant."
6. Commercial rentable area, where the gap is deliberate. Under the ANSI/BOMA standards a tenant pays on more space than it can occupy. GSA's lease form defines rentable square feet as "the area for which a tenant is charged rent" and adds that it "is determined by the building owner and may vary by city or by building within the same city". Its own worked example: "11,500 RSF and 10,000 ABOA SF will have a CAF of 15%". At 30 dollars a rentable foot that tenant pays 345,000 a year for 10,000 usable feet, which is 34.50 per foot it can actually stand on. The 1,500 feet are not an error; they are the definition.

And the gap between the first two of these and the number this page gives you is not academic. A 40 by 30 foot footprint measured outside is 1,200 square feet. With 6.5 inch walls the floor inside those walls is about 1,125, roughly 6 percent less. That is several hundred dollars of flooring on a typical job, and enough to move a listing across a search filter.

ANSI itself declines to arbitrate. Its annex says the standard "makes no statement concerning differences between square footage calculations made by multiple parties for the same property", and on garages it tells practitioners to "heed common local convention". If your own number disagrees with a listing, a tax bill and an appraisal, the useful question is not which one is wrong. It is which convention produced each, because none of the four is answering the same question.

How well this page knows ANSI Z765. The 2021 standard is sold rather than published, and we have not bought it. The rules quoted above were read in the Z765-2020 public comment draft issued by Home Innovation Research Labs, the ANSI-accredited secretariat that develops the standard, and every substantive rule was then cross-checked against Fannie Mae's own restatement of Z765-2021, which is public. Where the two agree, they are quoted here. Where a section number appears, treat it as the standard's structure rather than as text read in the 2021 edition.

What this page does not know

It multiplies the dimensions you type and adds the results. It does not know whether you measured inside or outside, whether the ceiling clears seven feet, whether the space is heated, whether any of it is below grade, or which convention the number you are comparing against came from. Every one of those changes the answer, and none of them is an input here.

Nothing you type leaves this page. The arithmetic runs in your browser. Your dimensions are not sent to a server, are not written into the address bar, and are gone when you close the tab. There are no accounts and nothing to sign up for.

Related measurement and material calculators

Once you know the square footage of your space, use these specialized calculators to estimate exact materials and quantities for your project:

• Area Formula — step-by-step guide to calculating 2D areas.
• Flooring Calculator — box counts for hardwood and laminate.
• Tile Calculator — backing, grout, and tile counts.
• Paint Calculator — wall area paint coverage in gallons.
• Drywall Calculator — sheets, tape, and compound.
• Mulch Calculator — yard soil, compost, and mulch depth.
• Concrete Calculator — slab, footer, and column volume.
• Area Calculator — triangles, circles, and irregular shapes.
• Rectangle Calculator — math for perimeter and diagonals.