How to Measure Square Feet

Measuring square footage is simple once you know the trick: every space, however awkward, can be broken into rectangles, triangles and circles. Measure each piece, work out its area, and add them up. This guide walks through the process with diagrams, then covers the mistakes that most often throw the answer off.

What you need

• A 25 ft (7.5 m) tape measure, or a laser distance measurer for long rooms
• Paper and a pencil, ideally graph paper, to sketch the space
• A helper to hold the other end of the tape (handy, but not essential)
• The square footage calculator to do the maths and add the pieces together

Step 1: Sketch the space

Draw a rough outline of the room or area from above. It doesn’t need to be to scale, but it should show every wall, corner, alcove and doorway. You will write each measurement onto this sketch, which makes it easy to spot anything you have missed.

Step 2: Measure the length and width

For a rectangular room, measure the longest wall from corner to corner, at floor level, then measure the wall that meets it. Hold the tape straight and tight. Write both numbers on your sketch, to the nearest inch.

Step 3: Multiply

Multiply the length by the width. For the room above, 12 ft × 10 ft = 120 square feet. If your measurements include inches, divide the inches by 12 to get a decimal (for example, 10 ft 9 in = 10.75 ft) or use the Feet + inches option in the calculator.

Step 4: Split irregular spaces into simple shapes

An L-shaped room is two rectangles. Draw a line that splits the L, measure each rectangle, and add the two areas. Make sure the pieces don’t overlap, so that no part of the floor is counted twice.

Angled walls create triangles. For a triangle, measure the base and the height at a right angle to it, then multiply and halve: base × height ÷ 2. If measuring the height is awkward, measure all three sides instead and use the three-sided triangle option, which uses Heron’s formula.

For a circular area such as a round patio, measure the diameter through the centre. Halve it to get the radius, then multiply π (3.1416) by the radius squared. A 12 ft circle has a radius of 6 ft: 3.1416 × 6 × 6 = 113.1 sq ft.

Step 5: Add and subtract

Add the areas of all the pieces to get the total. Subtract anything inside the outline that you don’t want to include: a fireplace hearth, a fitted kitchen island or a staircase opening. The calculator’s running total does this for you, with a Subtract option for cut-outs.

Measuring in inches or metres

If you measure in inches, multiply the two dimensions and divide by 144 to get square feet. If you measure in metres, multiply to get square metres, then multiply by 10.764. A 4 m × 3 m room is 12 m², or 129.2 sq ft.

Measuring walls for paint or wallpaper

Wall area works the same way. Multiply each wall’s width by the ceiling height, add the walls together, then subtract doors (about 20 sq ft each) and windows (measure them). A 12 × 10 ft room with 8 ft ceilings has 2 × (12 + 10) × 8 = 352 sq ft of wall before openings.

Common mistakes to avoid

• Mixing units: one measurement in inches and the other in feet gives a wildly wrong answer.
• Treating inches as decimals: 10 ft 6 in is 10.5 ft, not 10.6 ft.
• Assuming the room is square: measure both pairs of opposite walls.
• Forgetting closets and alcoves that the flooring will cover.
• Counting overlapping pieces twice when you split an irregular room.
• Forgetting waste when buying flooring or tile; add 5–15%.