Square footage and area calculator

An area calculator for rooms, floors and land

Measuring a room for new flooring, checking the size of a lot, comparing a listing in square feet with one in square meters: this calculator handles all three. The first block finds the area of a shape and draws it, the second adds up several rooms, the third turns an area into materials. Everything runs in your browser.

How to use it

1. In “1. Area of a shape”, pick a shape: rectangle, square, triangle, circle, semicircle, trapezoid, parallelogram, ellipse, circle sector or L-shape.
2. Set the “Unit of measurement”: inches, feet, yards, millimeters, centimeters, meters or kilometers. For triangles and circles, “Known measurements” lets you switch between base and height or three sides, radius or diameter.
3. Type your measurements. The area appears in square meters, with the formula filled in with your numbers, and in square feet, acres, hectares, ares, square centimeters and square kilometers.
4. “Add to rooms” sends the shape to the room list; “Work out materials” sends its area to block 3.

Area formulas, each with a worked example

• Rectangle: length × width. A bedroom of 12 ft by 10 ft is 120 sq ft, which the tool also shows as 11.148 m².
• Square: side². A 3 m square is 9 m².
• Triangle: base × height ÷ 2, so 4 × 3 ÷ 2 = 6. When you only know the sides, Heron's formula does the job: for sides of 3, 4 and 5, half the perimeter is 6 and the area is √(6 × 3 × 2 × 1) = 6.
• Circle: π × radius². A patio with a 2 m radius covers 12.566 m²; a semicircle of the same size covers half, 6.283 m².
• Trapezoid: (long base + short base) × height ÷ 2. With 6, 4 and a height of 3: 10 × 3 ÷ 2 = 15.
• Parallelogram: base × height, measured square to the base, not along the slanted side. 5 × 2 = 10.
• Ellipse: π × a × b, using the semi-axes. 3 and 2 give 18.85.
• Circle sector: π × radius² × angle ÷ 360. A quarter circle (90°) with a 2 m radius is 3.142 m².

Measuring an irregular room

Split the floor into simple shapes, measure each one and add them up. An L-shaped living room breaks into two rectangles: 5 × 4 + 3 × 2 = 20 + 6 = 26 m², which is exactly what the “L-shape” option does. In “2. Area of several rooms”, each line has a name, a length and a width, in the “Unit of length” you choose. Tick “Subtract” to take out a closet or a stairwell opening: a 1 m × 0.6 m closet brings the room down to 25.4 m². “Copy the summary” copies every line and the total. The list stays saved on your device.

Square footage in listings is not always the same number

The geometric area of a floor is not always the figure an appraiser or an agent will publish. In the United States, many appraisers follow the ANSI Z765 standard for single-family homes, which has its own rules on finished space, stairs and low ceilings. In the United Kingdom, agents often quote the floor area from the Energy Performance Certificate, and surveyors follow the RICS measuring rules. For an official figure, ask a qualified surveyor or appraiser.

Allowing for waste, and buying paint

In “3. How much material”, the “Floor: tiles, wood, laminate” option adds a “Waste allowance” (10% by default) and divides by the “Coverage of one box”. For 14 m²: 14 + 10% = 15.4 m², then 15.4 ÷ 1.44 = 10.69, rounded up to 11 boxes. Tiles laid on the diagonal need more cuts; with 15% waste you get 16.1 m², so 12 boxes.

The “Walls: paint” option starts from the perimeter and the ceiling height, then takes out doors (1.6 m² each by default) and windows (1.2 m² each). A 4 × 3.5 m room with a 2.5 m ceiling has 15 m × 2.5 m = 37.5 m² of wall, minus 2.8 m² of openings, so 34.7 m². Two coats at 10 m² per liter: 34.7 × 2 ÷ 10 = 6.94 liters, or three 2.5 L cans. This block works in meters and liters; check the coverage printed on the can.

Handy area conversions

Frequently asked questions

How do I calculate square footage from feet and inches?

Convert the inches to a decimal of a foot first: 10 ft 6 in is 10.5 ft. Or set the unit to inches and type 126. A room of 10.5 × 12 ft is 126 sq ft.

How do I find the area of a plot of land?

Take the side lengths from the survey or the deed plan and split the plot into rectangles, triangles and trapezoids. For a triangle with three known sides, choose “Three sides (Heron's formula)”. The result is also shown in acres and hectares.

Can I type commas in large numbers?

Yes. 1,250 is read as one thousand two hundred fifty and 3.5 as three and a half. Spaces are ignored.

Why does the result show a dash?

A measurement is missing or zero, or the three sides cannot form a triangle. The message under the fields tells you which.