Square Footage Calculator

Nine calculators sit above this, one for each shape a real floor, wall, yard or slab tends to come in. Enter the measurements, pick the units you took them in, and each one returns the area in square feet along with the same figure in seven other units. Add a quantity if you have several identical pieces, and a price per unit area if you want the cost as well.

What square footage actually measures

Square footage is area: the amount of surface inside a boundary, counted in squares one foot on a side. It is not the distance around the edge, which is perimeter, and the two answer different questions. Perimeter tells you how much baseboard or fencing to buy. Square footage tells you how much carpet, paint, sod, tile or concrete.

For a rectangle it is length times width, and that single line covers most rooms. A bedroom 14 feet by 12 feet is 168 square feet. The reason the other eight calculators exist is that not every surface is a rectangle, and measuring a curved patio as though it were one throws the number out badly.

Every shape and its formula

Shape	What to measure	Formula
Rectangle	Length, width	L × W
Rectangle border	Length, width, border width b	L × W − (L − 2b)(W − 2b)
Circle	Diameter	π × (d / 2)2
Ring	Outer diameter, border width b	π × [(D / 2)2 − (D / 2 − b)2]
Triangle, three edges	All three sides	√[s(s − a)(s − b)(s − c)]
Triangle, base and height	Base, perpendicular height	½ × b × h
Trapezoid	Both parallel sides, height	[(b1 + b2) / 2] × h
Sector	Radius, angle	½ × r2 × θ
Parallelogram	Base, perpendicular height	b × h

Two of these are worth a note. The triangle with three edge lengths uses Heron's formula, where s is half the perimeter, and it is the one to reach for when you can stretch a tape along each side but cannot easily find a square height. The sector formula wants the angle in radians, so a figure entered in degrees is converted first, and that conversion is shown in the steps.

Splitting an awkward room into shapes

Most rooms that look impossible are two or three simple shapes pushed together. An L-shaped living room is two rectangles. A room with a bay window is a rectangle plus a trapezoid. A round patio with a straight edge cut off is a circle minus a sector.

The method is always the same. Draw the outline roughly to scale, cut it into shapes with straight pencil lines, measure each piece, run each through the matching calculator, then add the answers. A living room 20 by 14 with a 6 by 8 alcove is 280 plus 48, so 328 square feet.

Subtracting works the same way. For paint, measure the wall and take off the doors and windows: a wall 12 by 9 is 108 square feet, and removing a 3 by 7 door and two 3 by 4 windows leaves 63. For flooring, most installers leave the door and window openings in the total and rely on the waste allowance instead, since offcuts rarely reach far.

Two habits keep the arithmetic honest. Measure at floor level rather than at waist height, because walls are rarely plumb and skirting boards eat an inch. And measure every wall rather than assuming opposite walls match, since older houses in particular are often out by an inch or two across a room.

Converting between area units

Area units square up, which is where people slip. Three feet make a yard, but nine square feet make a square yard, and a hundred square feet make a hundredth of a square kilometer only if you also apply the factor twice. Multiply a length conversion by itself and the area conversion falls out.

1 of this	In square feet	In square meters
square inch	0.00694444	0.00064516
square foot	1	0.09290304
square yard	9	0.83612736
square meter	10.76391	1
acre	43,560	4,046.856
square kilometer	10,763,910	1,000,000
square mile	27,878,400	2,589,988

The acre is the odd one, at 43,560 square feet. That number is not arbitrary: an acre was a furlong by a chain, 660 feet by 66 feet, which is the strip a team of oxen could plough in a day. A square mile holds exactly 640 acres, which is why American land is still parcelled in halves and quarters of a section.

Two trade units turn up as well. Roofers buy in squares, where one square is 100 square feet, so a 2,400 square foot roof surface is 24 squares. Carpet was long sold by the square yard and some suppliers still quote that way, so a 500 square foot room is about 55.6 square yards.

Gross area against finished area

When square footage appears in a listing or an appraisal it follows a measuring standard rather than a tape measure alone. The one used across most of the United States is ANSI Z765, and its rules change the answer.

Houses are measured to the outside finished surface of the exterior walls, so the published figure includes the thickness of the walls and comes out larger than the sum of the rooms. Interior walls, stairwells and closets count. Garages, porches and unfinished space do not.

Ceiling height matters too. Finished area needs a ceiling of at least 7 feet, dropping to 6 feet 4 inches under beams and ducts. Where a ceiling slopes, at least half the finished floor area has to reach 7 feet, and nothing under 5 feet counts at all, which is why a converted attic often reports far less than its floor.

Below-grade space is the one that causes arguments. A finished basement is reported separately from above-grade living area rather than folded into it, however good the finish. If you are checking a listing against your own measurements, a gap of a few percent usually means one of these rules rather than an error.

Working out cost per square foot

Once the area is settled, cost is one multiplication, and the price box above does it for you. The care goes into matching the units. Flooring is quoted per square foot in the United States and per square meter almost everywhere else, and mixing the two overstates a bill by about eleven times.

Paint is the exception, since it is sold by volume rather than area. A US gallon covers roughly 350 to 400 square feet in one coat on a smooth surface, less on bare drywall or rough render, and most jobs need two coats. Divide the wall area by the coverage on the tin, multiply by the number of coats, then round up to whole tins.

For flooring, buy more than you measured. Cuts at the walls, damaged planks and the odd mistake all come out of the same box, and running short halfway through means a second batch that may not match.

Job	Add on top of the measured area
Plank or tile laid straight, square room	10%
Diagonal or herringbone layout	15%
Room with alcoves, angles or many doorways	15%
Patterned material that has to line up	20%

Keeping a spare box after the job is worth the shelf space, since a burst pipe or a dropped pan five years later is much easier to repair when the material still matches.

Building costs are quoted per square foot as well, and that figure covers a wide range because it depends on finish, site and region rather than on floor plan alone. It is a useful way to compare two quotes for the same house and a poor way to budget from scratch. Note also that a builder's price per square foot is usually based on the gross measured area, which includes the wall thickness, so it applies to a slightly larger number than the rooms add up to.

Reading your result

Each calculator shows the answer in square feet at the top, then repeats it in acres, square yards, square inches, square miles, square meters, square kilometers and square centimeters. Take whichever unit the next step wants, rather than converting by hand.

The quantity box multiplies the area, which is what you want for several identical rooms, panels or beds. The price box then applies to the multiplied total, so the cost line covers the whole job.

Round sensibly at the end and not before. A tape measure honest to a quarter of an inch does not support an answer carried to eight decimal places, so the extra digits are there for chaining one calculation into another rather than for the shopping list. For the volume of a slab or a pool rather than the area of a surface, use the Volume Calculator. For a concrete pour, the Concrete Calculator takes the depth as well, and for the same shapes measured in any unit you like, the Area Calculator covers the general case.