Square Footage Calculator

Length times width, with the units kept straight

Square footage is the area of a flat surface measured in feet. For a rectangle the whole formula is one multiplication: length times width. A room 12 feet long and 14 feet wide measures 12 x 14 = 168 square feet.

The unit is square feet, not feet, and that matters when the answer feeds a price. A floor that costs $6.50 per square foot costs 168 x 6.50 = $1,092 to cover, while $6.50 per foot of wall length would describe something else entirely. Whenever a number looks wildly off in a material estimate, the first thing to check is whether the price was quoted per foot, per square foot or per square yard.

Inches have to be converted before multiplying. Twelve feet six inches is 12.5 feet, because 6 / 12 = 0.5. A 10 foot 3 inch wall is 10.25 feet. Rounding 10.25 to 10 loses 3 inches of length, which on a 10 foot wall is about 2.5 percent of the area, enough to leave a visible gap at the end of a flooring run.

Odd fractions on a tape measure are the same idea. A measurement of 8 feet 9 inches is 8.75 feet, since 9 / 12 = 0.75. Measure twice, write the decimal, then multiply. Keeping every measurement in feet before any arithmetic is the habit that keeps a whole project consistent.

Worked example: a 16 by 20 deck

A deck measures 16 feet by 20 feet. The decking is priced per square foot, the stain is priced per square foot, and the joists are priced per board, so the area is the first number to get right.

• Area: 16 x 20 = 320 square feet
• Square yards: 320 / 9 = 35.56
• Roofing squares, if this were a roof: 320 / 100 = 3.2
• Square meters: 320 x 0.0929 = 29.7
• With a 10 percent waste factor: 320 x 1.10 = 352 square feet
• Decking at $6.50 per square foot: 352 x 6.50 = $2,288

The gap between 320 and 352 is 32 square feet, which is the difference between a job that finishes with a little left over and one that stops two boards short. On decking that runs at $6.50 a square foot, the extra material costs $208, and a second delivery for two boards usually costs more than that.

The table also shows why square yards trip people up. A carpet quoted at $22 per square yard on a 320 square foot deck is 35.56 square yards, which comes to 35.56 x 22 = $782.22, while a competitor quoting $2.60 per square foot looks cheaper at first glance and comes to 320 x 2.60 = $832. Same carpet, different unit, $50 apart.

Shapes that are not rectangles

Most rooms are rectangles and most yards are not. Four shapes cover the cases that come up, and each one reduces to a formula with two or three numbers in it.

An L shaped living room is two rectangles. A 20 by 15 main space measures 300 square feet, and a 6 by 8 alcove adds 48, for 348 square feet. Draw the outline on paper, cut it into rectangles with a pencil, and label each piece with its own two dimensions. The same method handles a T shape and a room with a bay window.

A triangle needs the height measured perpendicular to the base, not the length of a slanted side. A gable end 12 feet wide with a 9 foot rise from the base to the peak measures 0.5 x 12 x 9 = 54 square feet. A circular patio 12 feet across has a 6 foot radius, so 3.1416 x 36 = 113.10 square feet.

For an irregular lawn, the fast approach is to break it into rectangles and triangles, add the pieces, then compare the total against a bounding box drawn around the whole yard. A total that exceeds the box is a sign that a measurement was written down twice or a piece was counted in two places.

Square yards, squares, square meters

The same area gets quoted in four different units depending on who is selling. Carpet and concrete come in square yards, roofing comes in squares, tile often comes in square meters, and lumber and paint come in square feet.

The square meter factor is exact rather than approximate. A foot is defined as 0.3048 meters, so a square foot is 0.3048 squared, which is 0.09290304 square meters. The NIST guide to the SI lists the same factor, and it is the one to use when a tile order is written in metric.

Roofing has its own arithmetic because slopes add area. A roof with two 16 by 30 foot slopes measures 2 x 480 = 960 square feet of surface, and the material order adds 10 percent for hips, valleys and waste, which brings it to 1,056 square feet, or 10.56 squares. Shingle bundles are sold by the square, so that order is 11 squares once it is rounded up to whole bundles.

An acre is 43,560 square feet, which is a square about 208.7 feet on a side. A lot 100 feet wide and 200 feet deep is 20,000 square feet, or 0.459 of an acre. That conversion matters when a listing quotes acreage and a fence quote is priced per linear foot of boundary.

How much extra to buy: the waste factor

The area you measure is the area you have. The material you buy is a little larger, because cuts, breaks and pattern matching eat into every job. The percentage added is called the waste factor, and it is a trade rule of thumb rather than a code requirement.

The size of the room matters as much as the layout. A 12 by 14 room needs 5 percent waste for straight lay, while a hallway 3 feet wide with many doorways can waste 15 percent because every cut leaves a short offcut that fits nowhere. Small rooms and complex outlines sit at the top of the range.

Boxes have to be rounded up after the waste is added, and that is a separate step. Tile sold in 15 square foot boxes at 352 square feet of demand needs 352 / 15 = 23.47 boxes, which rounds up to 24 boxes and 360 square feet of tile. The flooring calculator handles that box rounding and spare-box advice; this page stops at the square footage.

How a listed house area is measured

The number on a listing is not the same as the number a tape measure gives for each room, and the difference is a published standard. The ANSI Z765 square footage standard sets the method appraisers and most listing services follow, and Fannie Mae requires appraisers to use the 2021 edition of it for gross living area.

Those rules explain a common argument. A homeowner measures 1,400 square feet of rooms, and the listing says 1,620. The gap is usually wall thickness, closets counted inside the exterior footprint, and a garage that was converted with a permit. It can also run the other way when a finished attic has a ceiling that slopes below 5 feet over half its floor.

This calculator measures surfaces, not houses, and it uses your numbers as given. For a paint, flooring or roofing order that is exactly what you want. For a listing or an appraisal, use the standard, and note that a figure computed from exterior dimensions alone has to be stated as such. Room size and paint coverage are a different job, which is what the room size calculator covers.

Measuring mistakes that cost money

Five errors account for most of the gap between an estimate and a final bill, and each one has a fix that costs a minute.

Adding feet to inches. A wall of 10 feet 6 inches written as 10.6 feet is 10 feet 7.2 inches, and it is wrong by 1.2 inches. Divide the inches by 12 every time: 6 / 12 = 0.5, so the wall is 10.5 feet.

Measuring one room and multiplying. Three bedrooms of 11 by 12 are not the same as one open space of 33 by 12, because walls, closets and hallways take area out. Measure each room and add the pieces: 3 x 132 = 396 square feet of rooms does not include the hall.

Forgetting the closet. A 10 by 12 bedroom with a 2 by 6 closet is 120 + 12 = 132 square feet of floor, and flooring has to cover the closet too. Skipping it leaves a visible strip of bare subfloor.

Using the room height for wall area. Floor area and wall area are different multiplications. A 12 by 14 room with 8 foot ceilings has 168 square feet of floor and 2 x (12 + 14) x 8 = 416 square feet of wall before doors and windows come out.

Trusting a phone app outdoors. Satellite and augmented reality measures drift by a foot or more on a deck or a lawn. For anything bought by the square foot, a tape measure and a second person holding the far end is still the standard that holds up when the material arrives short.