Square Footage Calculator for Rooms, Walls and Odd Shapes

How to use this tool

Each row is one area. Give it a name you will recognise later, pick the shape, and type the dimensions. In feet and inches mode every dimension takes two boxes, so a room that measures twelve foot six goes in as 12 and 6 rather than as 12.5. That is the input people most often get wrong: entering 12.6 for twelve foot six understates the room by about half an inch per foot, which on a whole floor is enough to leave you a box of tile short.

The quantity box repeats a row without you retyping it, which is what you want for two identical bedrooms or eight identical stair treads. Openings are taken off the total rather than added to it, so use them for doors and windows when you are measuring wall area to paint. Waste is applied last, to the net figure, because you do not buy material for a doorway and then add ten percent to the doorway. Put a depth in and the area turns into cubic feet and cubic yards, which is the number a concrete or gravel supplier actually wants.

What the calculation is doing

Area is length times width only for a rectangle. Everything else on the shape list is a way of getting back to rectangles. A triangle is half of the rectangle that boxes it in, which is why the formula carries the one half. A trapezoid is the rectangle you would get by averaging its two parallel sides, so half of the sum of A and B times the height. An L shape is a rectangle with a rectangle taken out of it. The circle is the odd one, since pi is not a rectangle trick, but it behaves the same way in that the answer scales with the square of the size.

When you give three sides rather than a base and a height, the tool uses Heron formula. Half the perimeter is s, and the area is the square root of s times s minus a, times s minus b, times s minus c. It is worth knowing why this can fail. If one side is longer than the other two put together, the triangle cannot close, the product under the root goes negative, and there is no area to report. The tool says so plainly rather than returning a number. Sides of 3, 4 and 9 are not a triangle no matter how carefully you measured them, and getting that result usually means one reading was taken from the wrong corner.

The conversions are exact by definition, not rounded approximations. One foot is exactly 0.3048 metres, so one square foot is exactly 0.09290304 square metres. A square yard is exactly nine square feet, and an acre is exactly 43560 square feet, which comes from a chain of 66 feet by a furlong of 660 feet. Those are the factors the tool uses and prints in the working, so you can check any line by hand.

Floor area and measured area are not the same question

This matters more than the arithmetic does. If you are buying flooring, you want the area of the surface you will cover, measured inside the walls, with the closet floor counted and the hearth taken off. If you are listing a house, you want gross living area measured to the outside face of the exterior walls, which includes the thickness of those walls and comes out noticeably larger. The two numbers describe the same house and can differ by five to eight percent on a typical single storey plan, more on a house with thick walls or many partitions.

Neither number is wrong. They answer different questions, and the mistake is quoting one when someone asked for the other. Listing conventions also cut out space with low ceilings, treat below grade rooms separately from above grade ones whatever the finish, and exclude the open void over a stairwell on the upper floor because the floor is not there. If your total disagrees with a listing or an appraisal, that gap is almost always the cause rather than a measuring error. Decide which question you are answering before you pick up the tape, and measure consistently for that one.

A worked check you can repeat on paper

The tool loads with three rows, so you can follow this through and see the same numbers on screen.

The living room is 12 ft 6 in by 16 ft 0 in. Twelve foot six is 12 plus 6 over 12, which is 12.5 ft exactly. So 12.5 times 16 is 200 sq ft.

The kitchen is the L shape from the diagram, 20 ft by 18 ft overall with an 8 ft by 9 ft notch. The full box is 360 sq ft, the notch is 72 sq ft, so the kitchen is 288 sq ft. Check it the other way: the bottom band runs the full 20 ft by 9 ft deep, which is 180 sq ft, and the top part is the remaining 12 ft by 9 ft, which is 108 sq ft. Those add to 288 sq ft, and the agreement is the point.

The bedroom is 10 ft 3 in by 11 ft 6 in, so 10.25 times 11.5, which is 117.875 sq ft. There are two of them, giving 235.75 sq ft.

Add the three rows: 200 plus 288 plus 235.75 is 723.75 sq ft. In metric that is 723.75 times 0.09290304, or 67.2386 sq m. Divide by nine for 80.4167 sq yd. Divide by 43560 for 0.01662 acres. At the ten percent waste the tool starts with, you would order for 796.13 sq ft, and rounding that up to whole boxes is the last step your supplier will do for you.

Now put 4 into the depth box with only the kitchen row present. Four inches is one third of a foot, so 288 times 0.3333 is 96 cubic feet, and 96 divided by 27 is 3.5556 cubic yards. That is a real slab quantity, and it is the same arithmetic whether the material is concrete, gravel or screened topsoil.

Mistakes that catch people out

• Typing 12.6 for twelve foot six. Six inches is 0.5 ft, not 0.6 ft. The error is small on one wall and compounds across a floor. Use the inches box and the problem disappears.
• Measuring the bounding box of an L shaped room. This is the error in the first panel of the diagram, and it runs 25 percent high on the room shown. Any room that is not a plain rectangle needs splitting or a notch taken off.
• Adding waste before taking off the openings. Waste covers offcuts and breakage on material you are actually laying. Applying it to a door opening you are not covering inflates the order. This tool deducts first, then applies waste, which is the right order.
• Mixing depth units with area units. A depth typed in inches against an area in square feet has to be divided by twelve before it means anything. The working line prints the depth in feet so you can see that conversion happen rather than trusting it.
• Measuring at waist height across a long room. A tape that sags or runs at a slight diagonal reads long every time, never short. Measure at floor level, along the wall, and if a room is longer than your tape mark the floor rather than guessing the overlap.
• Quoting flooring area as house size. Inside face and outside face measurements differ by the wall thickness all the way round. Say which one you used.

How this was checked

Every shape was tested against a hand calculation before the tool was published: a 3, 4, 5 triangle giving exactly 6 square units by both the base times height route and by Heron, an equilateral triangle of side 10 giving 25 root 3, a circle of diameter 12 giving 36 pi, and the L shape solved twice by the two independent routes shown in the diagram. The unit factors were checked against their exact definitions rather than against another calculator, so 43560 square feet returns 1.00000 acres and 1000 square feet returns 92.90304 square metres. The deduction, waste and cost chain was checked on a 40 by 8 wall with one 3 by 7 door and two 3 by 4 windows, which nets 275 square feet, becomes 302.5 at ten percent waste, and costs 756.25 at two dollars fifty a foot.

Related Calculators and Reference

• Tile and flooring calculator take the total from here and turn it into whole boxes, with the waste percentage set by how the tile is laid.
• Paint coverage calculator for wall area rather than floor area, once you have used the openings rows to take off the doors and windows.
• Mulch and soil calculator the natural next step when you have put a depth in and want bag counts rather than loose cubic yards.
• Area of 2D shapes calculator for a single shape worked in more detail, including ones this tool does not carry such as ellipses and regular polygons.
• Area converter when the answer needs to be in hectares, roods or perches rather than the four units shown above.
• Length converter for turning a dimension written in one system into another before it goes in a row.
• Deck board calculator for decking, where the board length and the joist spacing matter as much as the area does.
• BTU Calculator: air conditioner size for a room in BTU and tons.
• Carpet Calculator: square yards to order from a 12 or 15 ft roll.
• Grass Seed Calculator: pounds of seed for a new lawn or overseeding.
• Fertilizer Calculator: pounds per 1,000 sq ft from the numbers on the bag.
• Bathroom Fan CFM Calculator: exhaust fan size for any bathroom.
• Mortgage Calculator: the monthly payment on the home you are measuring.
• Dehumidifier Size Calculator: the floor area to enter for a dehumidifier size.
• Rainwater Harvesting Calculator: the roof footprint to enter as the rainwater catchment area.
• ANSI Z765 the American National Standard for measuring square footage in single family homes, which is the document that settles arguments about what counts.