Free Square Footage Calculator

Calculate the area of a rectangle, circle, or triangle in square feet, square meters, and acres. Useful for flooring, painting, landscaping, and real-estate measurements.

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What is this calculator for?

You're trying to calculate how much flooring you need for an L-shaped basement that the previous owner's MLS listing measured as "approximately 600 sq ft" but you suspect it's closer to 700. Or you're listing your own home and the appraiser said 1,847 sq ft but Zillow says 1,920 — and the gap of 73 sq ft at a price-per-square-foot of $230 changes your listing price by $16,790. Square footage in residential real estate is more contested than people realize, and the right answer depends on which standard you use. The square footage calculator gives you a clean number from your specific measurements.

The most-used residential measurement standard is ANSI Z765-2021 ("American National Standard for Single-Family Residential Buildings — Square Footage Method for Calculating"). It defines "gross living area" (GLA) as finished, above-grade, heated space measured from exterior walls. Basements, garages, and unheated spaces are reported separately. The American Measurement Standard from RESO uses similar rules. Variation in how appraisers and listing agents apply these standards is the source of most square-footage disputes.

This calculator handles simple rectangles plus several common house-shape variations: L-shapes (composed of two rectangles), T-shapes, U-shapes, and irregular polygons (entered as a sequence of length-width pairs). It outputs total square footage and shows the math for each sub-area. Use it for flooring estimates, material calculations, real-estate listings, or appraisal disputes.

How to use this calculator

For a simple rectangle: enter length × width in feet. The calculator multiplies. Trivial math but useful as a sanity-check on the dimensions you measured.

For an L-shape: divide into two rectangles. The "main rectangle" runs the full width of one direction; the "extension" is the additional area beyond. Enter each rectangle's length and width separately; the calculator sums.

For a T-shape or cross: divide into rectangles (typically 2-3 depending on shape). Enter each as length × width. Sum yields total.

For irregular polygons: divide into rectangles, triangles, or trapezoids. Triangles: base × height ÷ 2. Trapezoids: (parallel side 1 + parallel side 2) × perpendicular distance ÷ 2. The calculator supports any number of sub-areas.

Be specific about what you're measuring. Interior dimensions measure inside walls — relevant for flooring, paint, and furniture planning. Exterior dimensions measure outer walls — relevant for foundation, exterior cladding, roofing, and ANSI Z765 home measurement. Interior dimensions are typically 5-10% smaller than exterior due to wall thickness (typically 5-6 inches of wall depth per exterior wall).

For real estate / appraisal-style measurement: use exterior dimensions of finished, heated, above-grade space only. Garages, unfinished basements, attics with less than 7-foot ceilings, and open porches don't count toward gross living area. Below-grade finished basements typically count separately. Sloped-ceiling areas count only where the ceiling exceeds 5 feet; areas under 5 feet are excluded.

Understanding your results

The calculator returns total square footage, the component areas (if multi-rectangle), and depending on the calculation, related quantities (linear feet of perimeter, cubic feet of volume).

How to interpret. A 28 × 42 ft rectangular ranch home: 1,176 sq ft above-grade exterior. An L-shaped 1,900 sq ft house with a 22 × 38 main rectangle (836 sq ft) plus a 28 × 38 wing (1,064 sq ft): 1,900 sq ft total. The shapes get more complex; the math doesn't change.

The interior-vs-exterior discrepancy. ANSI standard uses exterior dimensions. Interior dimensions are smaller by approximately 2 × wall depth × perimeter — for a 1,900 sq ft home with 6-inch exterior walls, interior dimensions yield about 1,720 sq ft. Both numbers are correct for their purposes; just be consistent in which you report. For real estate listings: exterior (ANSI). For flooring purchases: interior (you don't tile the inside of your walls).

The basement question. Below-grade finished space is reported separately from above-grade GLA in ANSI Z765. So a "1,900 sq ft home with finished basement" might be listed as "1,900 sq ft + 800 sq ft basement = 2,700 sq ft total" or "1,900 sq ft above-grade, 800 sq ft basement" depending on the listing agent's discretion. Buyers comparing two homes — one listed as 2,700 sq ft (claiming basement) and one as 1,900 sq ft (not claiming) — may not be comparing apples-to-apples. Always check whether the listed square footage includes basement.

The vaulted-ceiling and double-height issue. ANSI counts floor area, not ceiling height. A 400 sq ft great room with 18-foot vaulted ceilings counts as 400 sq ft, not 800 (which would be the case if you counted the equivalent two-story volume). However, the upper open-to-below area of a two-story space is sometimes mistakenly counted as part of the upper floor's square footage by less-careful appraisers — leading to inflated total figures. ANSI is clear: don't count the open volume twice.

A worked example

Marcus is selling his home in suburban Charlotte and wants to verify the square footage before listing. The tax assessor records show 2,180 sq ft; the original builder's plans show 2,210; the previous owner's appraisal from 2017 shows 2,165. Three different numbers, all "official."

He measures the exterior himself with a 100-foot tape and a helper. The house is an L-shape: a 28 × 46 ft main rectangle plus a 22 × 24 ft wing extending from the back. He calculates:

Main rectangle: 28 × 46 = 1,288 sq ft. Wing: 22 × 24 = 528 sq ft. L-shape sum: 1,816 sq ft for the first floor.

Second floor: identical L-shape minus the wing (the wing is single-story). So second floor: 28 × 46 = 1,288 sq ft minus a 6 × 12 ft open-to-below great room area (he doesn't count the void). Net second floor: 1,288 − 72 = 1,216 sq ft.

Total above-grade: 1,816 + 1,216 = 3,032 sq ft. Wait — that's much higher than any of the records. He recounts. The records were measuring just one floor; the home is actually two stories with finished space on both. The "2,180" figure was likely first-floor-only or excluded portions of the second floor's open-area or sloped-ceiling space.

Marcus consults an appraiser. Professional measurement using ANSI Z765 confirms: first floor 1,816 sq ft, second floor 1,216 sq ft, total above-grade GLA 3,032 sq ft. The tax assessor's 2,180 figure is materially wrong — likely an old measurement of just the first floor when the house was originally smaller (the second-floor addition may have been done without permits). The 2017 appraisal's 2,165 was likely also first-floor-only.

At Charlotte's $220/sq ft typical for the neighborhood: 3,032 sq ft × $220 = $666,000 listing price. The previously-assumed 2,180 sq ft would have priced at $479,600 — a $186,400 difference. Marcus refiles the property record with the city, lists at $649,000 (slightly below appraisal to ensure sale), receives an over-asking offer at $658,000 within 9 days. Accurate square footage measurement directly added $180,000+ to his sale price.

Related resources

For material quantity calculations using your measured square footage: Paint Calculator, Flooring Calculator, Concrete Calculator, Wallpaper Calculator, and Mulch Calculator. For property value implications, Property Tax Calculator. The American National Standards Institute publishes ANSI Z765 (the residential square footage measurement standard); local appraisal associations and your state's real estate commission can clarify region-specific measurement rules.

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Frequently asked questions

How do I measure a room accurately?

Measure wall-to-wall along the longest dimension first, then perpendicular wall-to-wall for the other dimension. Round to the nearest inch and convert to feet (12 inches = 1 ft, so 142 inches = 11.83 ft). Use a laser distance meter for accuracy beyond ~25 ft, where tape measures sag. Always measure twice — even pros find errors of 6+ inches on the first pass.

What's the difference between sqft and sqm?

Square feet is the US/UK standard; square meters is the metric standard used globally. Conversion: 1 sqft ≈ 0.0929 sqm, or 1 sqm ≈ 10.764 sqft. A 1,000 sqft house is ~93 sqm. Real estate listings, building codes, and HVAC sizing all use sqft in the US; almost everywhere else uses sqm.

How do I calculate area of an irregular shape?

Divide the space into rectangles, triangles, and circle segments, calculate each piece, then add them up. For an L-shaped room, split it into two rectangles at the inside corner. For a room with a curved bay window, calculate the rectangle plus the half-circle. Most odd shapes can be decomposed into 2-4 simple shapes you can measure directly.

How do real estate agents measure square footage?

The American National Standards Institute (ANSI) Z765 standard is the most common. Measure exterior dimensions of finished, heated, above-grade space. Basements and garages are reported separately. Open-to-below spaces don't double-count. Sloped ceiling areas count only where ceiling height exceeds 5 feet. Many local MLS rules require ANSI Z765 measurement; some use older standards or county tax records. Discrepancies between MLS listing square footage and tax records are common; the listing agent's measurement is typically more current and more accurate. Buyers should verify with their own measurement or appraisal before relying on listing-stated square footage for major decisions.

Does the basement count toward home square footage?

Under ANSI Z765 — no, not in gross living area (GLA). Basements (any space with most of the wall below grade) are reported separately as 'basement square footage' or 'below-grade finished space.' This applies even if the basement is fully finished, heated, and used as living space. The convention exists because basements have different cost-per-square-foot for construction, different appraisal value, and different building code requirements. Some listing agents include basement in headline square footage; some don't. Always check whether 'total square footage' includes basement. Two homes — one with 1,800 sq ft above-grade + 800 sq ft basement, another with 2,600 sq ft above-grade only — have the same headline 2,600 figure but very different real values.

How do I measure an irregular-shaped room?

Divide into rectangles and triangles. For an L-shaped room: it's two rectangles — measure each rectangle's length and width, calculate each, sum. For a triangle (rare in residential): base × height ÷ 2. For a trapezoid (two parallel sides of different length, common in attic conversions): (parallel side 1 + parallel side 2) ÷ 2 × perpendicular distance between them. For curved walls (very rare): approximate with several straight-line measurements treating the curve as a series of small rectangles, or use the formula for a circular sector if you know the radius and arc. Most home plans avoid genuinely curved walls; the curves you see in listing photos are usually rounded corners on otherwise rectangular rooms.

What's the difference between square footage and gross living area?

Gross Living Area (GLA) is the technical term used in residential appraisal — finished, heated, above-grade space measured from exterior walls. 'Square footage' is the colloquial term and can mean different things in different contexts (interior dimensions, including or excluding basement, including or excluding garage). When buying or selling, always clarify which definition is being used. For an appraisal, the appraiser will use GLA per ANSI Z765. For listing on the MLS, agents typically use GLA per ANSI Z765 in markets that require it, but practice varies. For flooring estimates or material purchases, you typically want interior dimensions which are smaller than GLA.

Why is my appraisal square footage different from the tax records?

Tax assessor records are often inaccurate and outdated. They may reflect: (1) the original home's footprint before additions, (2) a measurement method different from ANSI standard, (3) data entry errors going back decades. Appraisers measuring under ANSI Z765 generally produce more accurate numbers, especially after additions or renovations. Discrepancies of 5-15% are common; larger gaps usually indicate an addition or measurement method difference. The tax assessor's number affects your property tax bill — undermeasured by the county is actually advantageous for taxes. Overmeasured tax records can be appealed by getting an independent appraisal. For real estate sale, use the more recent appraisal or your own measurement.

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