Square Footage Calculator

How to use the square footage calculator

This square footage calculator (sq ft calculator for short) finds the floor area of anything flat: a bedroom, a whole house, a patio, a garden bed or a building lot. It works for one simple rectangle and for a floor plan made of many parts, and it shows its working so you can check every number.

1. Sketch the space. Draw a rough plan on paper and split it into simple shapes: rectangles, triangles, circles. Mark anything inside the outline that should not count, such as a fireplace hearth, a kitchen island or a stair opening.
2. Measure each part. Measure wall to wall at floor level with a tape measure or laser measure. Write down feet and inches, or meters and centimeters, exactly as you read them.
3. Choose the unit. Pick the unit you measured in under Measure in. You can also type a unit with any number, such as 12' 6", 3 m 20 cm or 150 cm, and mix units from field to field.
4. Enter each area. Give each part a name and a shape, then fill in its dimensions. Use Add area for every extra room or part, and How many for identical rooms.
5. Subtract the cut-outs. Set Add or subtract to Subtract for anything that should come off the total.
6. Read the total. The results show the total in square feet and square meters, and in square inches, square yards, acres and hectares, with a breakdown by area. Copy the results, download them as a CSV file or print them.

Two more tabs answer the next questions. Flooring & cost turns the total into the amount of flooring to buy, with a waste allowance, whole boxes and a cost estimate from your own prices, so it doubles as a flooring calculator. The Unit converter converts any area between ten units, from square inches to square kilometers.

How to calculate square footage

Square footage is area measured in square feet. One square foot is a square one foot on each side. For a rectangle, which is what most rooms are, multiply the length by the width, both in feet:

square footage = length (ft) × width (ft)

A 12 by 12 foot bedroom is 12 × 12 = 144 sq ft. A 15 by 12 foot living room is 15 × 12 = 180 sq ft, which is 16.72 m² or 20 square yards. Both measurements must be in the same unit before you multiply: multiplying feet by inches gives a number that means nothing.

For a room that is not a rectangle, split it into shapes you can measure, find each area and add them up. Anything inside the outline that you don't want to count is worked out the same way and subtracted. The calculator does exactly this: each area card is one shape, and the total is the sum of the cards set to Add minus the sum of the cards set to Subtract.

Area formulas for every shape

Each shape in the calculator uses one of these formulas. The letters match the labels on the shape drawings in the tool. Every example is worked out in feet.

Two of these deserve a word. Four sides + diagonal is for a four-sided room whose corners are not square: four side lengths alone don't fix the shape, but one diagonal does, because it splits the room into two triangles whose three sides are all known. Measure the diagonal from the corner where side D meets side A to the corner where side B meets side C. Any shape handles any outline with straight sides. Pick one corner as 0, 0, then for each corner in order around the room measure how far it is along one wall (x) and along the wall at right angles to it (y). The tool draws the outline as you type, so a misplaced corner shows up at once. The outline must not cross itself.

The tool also shows the perimeter where the inputs fix it, which is handy for baseboard, trim or edging. An L-shape has the same perimeter as its overall rectangle, 2 × (L + W), so the 20 × 16 ft example needs 72 ft of baseboard before doorways are taken off. An ellipse's perimeter has no exact formula; the tool uses Ramanujan's approximation and marks it with ≈.

Square footage from feet and inches

Tape measures read in feet and inches, but the area formula needs one unit. Turn the inches into a fraction of a foot by dividing by 12, then multiply as usual. A room that measures 12 ft 6 in by 10 ft 3 in is 12.5 ft by 10.25 ft, so its area is 12.5 × 10.25 = 128.13 sq ft.

You don't have to convert anything in the calculator: type 12' 6", 12'6, 12 ft 6 in or 12 1/2 and it reads all of them as 12.5 ft. If you measured everything in inches, choose Inches under Measure in; a 150 by 120 inch room is 18,000 square inches, and because a square foot is 12 × 12 = 144 square inches, that is 125 sq ft. Metric works the same way: a 4.5 by 3.6 m room is 16.2 m², or 174.38 sq ft.

Worked examples

An L-shaped living room

The room is 20 ft along its longest wall and 16 ft deep, but an 8 by 6 ft corner is missing where the entry hall cuts in. As an L-shape: 20 × 16 = 320 sq ft for the whole rectangle, less 8 × 6 = 48 sq ft for the missing corner, leaves 272 sq ft. Split into two rectangles instead, it is 12 × 16 = 192 sq ft plus 8 × 10 = 80 sq ft: the same 272 sq ft. Use whichever split is easier to measure.

A room with a cut-out

A family room is 18 by 14 ft, 252 sq ft, with a 6 by 2 ft raised hearth that the new floor will not cover. Add a second area for the hearth, set it to Subtract, and the total becomes 240 sq ft. Closets that get the same flooring are added the same way, as extra areas set to Add.

A round patio and the path around it

A round patio 10 ft across covers π × 10² ÷ 4 = 78.54 sq ft. A 3 ft gravel path around it makes a ring 16 ft across on the outside and 10 ft on the inside: π × (16² − 10²) ÷ 4 = 122.52 sq ft of path. For the gravel itself, the cubic yard calculator turns an area and a depth into volume.

An irregular room traced corner by corner

When walls meet at odd angles, the corner-point shape is the most reliable method. For the L-shaped room above, starting at one corner and going around, the corners are at 0, 0; 20, 0; 20, 10; 12, 10; 12, 16 and 0, 16 (in feet). The shoelace formula gives 272 sq ft, matching the other two methods, and the same approach works for a room with a bay window or an angled wall, a pie-shaped lot or a garden bed.

A building lot

A rectangular lot 100 ft wide and 150 ft deep is 100 × 150 = 15,000 sq ft. Dividing by the 43,560 sq ft in an acre gives 0.344 acres, which is 1,393.55 m² or 0.139 hectares.

How to measure a room for square footage

• Measure at floor level, wall to wall. Baseboards and walls are rarely perfectly straight. Measure each dimension in two or three places and use the largest figure if you are buying material, the typical one if you are describing the room.
• Hook the tape or use a laser. For long rooms a laser measure held against one wall is quicker and more accurate than a sagging tape.
• Write the numbers on a sketch. Note which wall each figure belongs to. Most mistakes come from swapping two numbers, not from misreading the tape.
• Treat alcoves and bays as their own shapes. A bay window is often a trapezoid; a closet is a small rectangle; a curved wall can be a circle sector or a set of corner points.
• Check the corners. If the two diagonals of a four-sided room differ, its corners are not square, and length × width will be slightly off. Use Four sides + diagonal for an exact figure.

Measuring inside each room gives the floor you can actually cover, which is what flooring, tile and paint need. It is smaller than the square footage on a listing or an appraisal, which is measured differently.

How a house's square footage is measured

Adding up the rooms inside a house leaves out the walls, so it always comes to less than the figure on a listing. In the United States, the usual method for single-family homes is the American National Standard Square Footage – Method for Calculating, ANSI Z765-2021, published by Home Innovation Research Labs. It is a voluntary standard for detached and attached single-family homes; it does not apply to apartments or other multifamily buildings and does not cover room dimensions. Since 2022, major U.S. mortgage guidelines have required appraisers to use it for many loans. The full text is sold by its publisher, so here is a summary in our own words of the points that matter most, as described by the publisher and by the North Carolina Real Estate Commission:

• Measured from outside. Finished area is taken from the exterior of the house, to the outside finished surface of the outer walls, so wall thickness is included. Figures obtained any other way have to be explained.
• Above grade and below grade are reported separately. A level with any part of its walls below the ground counts as below grade, so a finished basement is not simply added to the main floors.
• Ceiling height. Finished area generally needs a ceiling of at least 7 feet, with allowances under beams and ducts, and special rules for sloped ceilings, under which at least half of a room's finished area must reach 7 feet.
• Openings don't count. A two-story foyer or the opening above a staircase is not floor area on the upper level; the stairs themselves count on the floor they come down from.
• Garages and unfinished space are not finished square footage. They are reported separately, if at all.

State real estate rules, local listing services, tax assessors and appraisers can each have their own conventions, so two honest measurements of the same house can differ. This calculator works out plan area from the dimensions you give it. For a sale, a listing, an appraisal or a tax appeal, the figure that counts is the one a licensed appraiser or your local assessor produces under the rules that apply there.

Square feet to square meters, square yards and acres

All of these conversions are exact. Since 1959 the foot has been defined as exactly 0.3048 meter, and since January 1, 2023 the U.S. survey foot is deprecated, so this "international foot" is now the foot for all but historic and legacy uses. Squaring it gives the area factors listed by NIST in Special Publication 811 and Handbook 44, Appendix C:

So to convert square feet to square meters, multiply by 0.09290304, or divide by about 10.7639. To go from square meters to square feet, multiply by about 10.7639. Square feet to square yards is a division by 9, because a yard is 3 feet and 3 × 3 = 9. Here are some common sizes:

Carpet is still often priced by the square yard, so divide square feet by 9 before comparing a per-yard price with a per-foot one. The Flooring & cost tab takes prices per square foot, square yard or square meter and does the conversion for you.

Lot size: square feet, acres and hectares

Land is measured in acres in the United States. An acre is exactly 43,560 sq ft, and it has no fixed shape: a square acre is about 208.71 ft on each side, while a lot 66 ft wide and 660 ft long is also exactly one acre. To turn a lot's square footage into acres, divide by 43,560; a quarter acre is 10,890 sq ft and a half acre 21,780 sq ft. In metric countries land is measured in hectares: a hectare is 10,000 m², about 2.471 acres.

Lots are rarely perfect rectangles. For an irregular lot, use the corner-point shape with distances read from your plat or survey, or split the lot into triangles and use the three-sided triangle. The result is the plan area of the lot; a land survey remains the authority for its legal boundaries and size.

How much flooring to buy

Flooring is cut to fit, and the offcuts at the end of each row are not always usable, so you buy more than the floor area. The Flooring & cost tab works it out in three steps:

area with waste = floor area × (1 + waste % ÷ 100)
boxes           = area with waste ÷ coverage per box, rounded up
material cost   = boxes × coverage per box × price per sq ft

For the 15 by 12 ft room, 180 sq ft plus a 10% allowance is 198 sq ft. If the planks come in boxes covering 20 sq ft, that is 198 ÷ 20 = 9.9, so you buy 10 boxes, or 200 sq ft. At an example price of $4.00 per square foot, the material comes to $800.00, or $4.44 per square foot of floor once the extra is paid for.

There is no single correct waste allowance. Installers commonly add some extra for cuts and mistakes; a diagonal or herringbone pattern, large tiles or planks, and rooms with many corners and doorways need more than a simple rectangle laid straight. The calculator starts at 10% and offers quick picks from 0% to 20%; the product's installation instructions and the coverage printed on the carton are what to go by. Some people keep a spare box for future repairs, since a later batch may not match.

For walls and ceilings, use the paint calculator and the drywall calculator; for tile, the tile calculator counts tiles and grout from the tile size.

Price per square foot

Price per square foot is the price divided by the area. It works in both directions: divide a total price by the square footage to compare offers of different sizes, or multiply a quoted price per square foot by your square footage to get a total. For example, a home listed at $350,000 with 1,750 sq ft is $200.00 per square foot. A flooring quote of $4.00 per square foot for 180 sq ft is $720.00 before waste. Price per square foot comparisons only work when both areas were measured the same way, which is one reason listings say how their figure was obtained.

How we calculate

The tool and every example on this page use the same code, and the examples are checked against it automatically.

• One internal unit. Every length is converted to feet as you type it, using 1 ft = 0.3048 m exactly, 1 yd = 3 ft and 1 ft = 12 in. Areas are computed in square feet and converted once, at the end, with exact factors (1 acre = 43,560 sq ft; 1 m² = 1 ÷ 0.09290304 sq ft), so no rounding builds up.
• Exact formulas. The formulas in the table above, with π to full double precision. The three-sided triangle uses a numerically stable form of Heron's formula; impossible triangles, notches larger than their outline and inner circles larger than the outer one are rejected with a message instead of giving a wrong area.
• Corner points. The outline is closed automatically from the last corner back to the first, and outlines that cross themselves are rejected, because the shoelace formula is only valid for an outline that doesn't.
• Totals. Each area is multiplied by its count. The total is the sum of the areas set to Add minus the sum of those set to Subtract; the cut-outs cannot be larger than what they come out of.
• Rounding. Results are rounded only for display: two decimals in the cards and totals, six significant digits in the unit tables, money to the cent. Box counts are rounded up.
• Not included: wall thickness, the height rules for finished area, and the differences between measuring standards. The result is the plan area of the shapes you enter.

Your measurements stay on your device

• Nothing is uploaded. Every calculation runs as JavaScript in your browser tab. The page's Content-Security-Policy only allows connections back to this site, and the tool makes none: open your browser's developer tools, choose the Network tab and type a few numbers, and no request appears.
• It keeps working offline. Once the page has loaded, you can disconnect and keep calculating.
• No account, no cookies. The tool remembers your areas, unit and last tab in your browser's local storage, on your device only.

Sources

• National Institute of Standards and Technology (NIST): U.S. Survey Foot (the international foot, 0.3048 m exactly, for all applications from January 1, 2023).
• NIST Special Publication 811, Guide to the SI, Appendix B.8: Factors for units listed alphabetically.
• NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement, from the current edition (1 acre = 43,560 square feet exactly).
• Home Innovation Research Labs: Square Footage Method (ANSI Z765) FAQ.
• North Carolina Real Estate Commission: NCREC Residential Square Footage Guidelines vs. ANSI Standard, April 2022.

Reviewed by microplugins · Last updated October 2026

This is an educational tool, not appraisal, surveying, legal or construction advice. Results are the plan area of the shapes and dimensions you enter. For a sale, listing, appraisal, tax assessment or the legal size of a property, rely on a licensed appraiser or surveyor. Example prices are illustrations, not market rates.