How to calculate the square footage of a room.

1. Multiply length by width.

For a rectangular or square room, measure the length and the width of the floor, then multiply them. Measure wall to wall at floor level, in straight lines at right angles to each other. Take each measurement in two places. If the two readings differ, the walls are not parallel, and one length × width may not describe the room.

Write each dimension on a quick sketch next to the wall it belongs to. Use one unit system for the whole room. Feet in one direction and meters in the other give a number that means nothing.

2. Convert inches to decimal feet before you multiply.

Tape measures read in feet and inches, but a calculator needs decimal feet. Divide the inches by 12 and add the result to the whole feet. Six inches is half a foot, so 10 ft 6 in is 10.5 ft, not 10.6 ft.

• 3 in = 0.25 ft
• 4 in = 0.333 ft
• 6 in = 0.5 ft
• 9 in = 0.75 ft

If you type 13.9 × 11.4 instead, you get 158.46 sq ft, about 2.6 sq ft too much. Working in inches and dividing by 144 (12 × 12) is a good way to check your decimals.

3. Split L-shaped rooms into rectangles that do not overlap.

Draw a line from the inside corner across the room to make two rectangles. Measure each one, work out its area and add them. Count every part of the floor once. If your dividing lines put the same corner square in both rectangles, you have counted it twice.

The check works for any L-shape. Measure the full box around the room, then subtract the corner that is not floor. Both methods should give the same answer.

Treat closets, alcoves and bay areas as their own small rectangles. A 5 ft × 2 ft closet adds 10 sq ft. Label it on the sketch so nobody adds it again or leaves it out. The flooring measurement guide has a drawn L-shaped example.

4. Use the right formula for angled and round areas.

An angled corner or a 45° wall leaves a triangle. Its area is base × height ÷ 2, with the height measured at a right angle to the base. A corner cut at 45° that takes 4 ft off each wall is a triangle 4 ft by 4 ft. Subtract it from the full rectangle, or add it to the shapes next to it, depending on how you split the room.

For a circle, find the radius, which is half the diameter, and use π × radius × radius. A round area 8 ft across has a 4 ft radius. A half-round bay of the same width is half the circle.

5. Subtract areas that will not get new flooring.

Fixed kitchen islands, built-in cabinets and raised fireplace hearths usually stay in place, so their footprint is not part of the new floor. Measure each one and subtract it from the room.

Whether a floor can run under an island depends on the product and the installer, so keep both numbers in your notes: the full room and the area after subtraction. In I Measure Pro, a hole (no flooring) subtracts an area such as a hearth or island from the quantities. Holes are counted in the totals but have no position, so they are not drawn on the plan.

6. Convert between square feet, square meters and square yards.

One square meter is 10.764 square feet, and one square yard is 9 square feet. Use the square factor, not the length factor. One meter is 3.2808 feet, so one square meter is 3.2808 × 3.2808 = 10.764 square feet. Multiplying square meters by 3.28 is a common and costly slip.

Carpet is often priced by the square yard, while vinyl plank, laminate and tile are usually sold by the square foot in the US. Keep full decimals until the last step, then round. The floor area calculator adds rectangular sections and shows the total in square feet, square meters and square yards.

Common square footage mistakes, and a faster way to measure.

• Inches as decimals. 12 ft 6 in is 12.5 ft, not 12.6 ft.
• Counting a section twice. Overlapping rectangles, or a closet inside a larger box, inflate the total.
• Forgetting closets and alcoves. Small spaces add up across a house.
• Rounding too early. Round the final total, not each step.
• Treating area as the order. Waste, cuts, roll widths and carton sizes come later. See measuring for vinyl plank, laminate and tile.

On a supported Android phone, I Measure Pro measures the outline for you. Tap the room’s corners in order with the camera and close the shape to get the floor plan, wall lengths, floor area and perimeter, in square feet or square meters. Shape templates cover rectangles, circles, ovals, donuts and triangles when you would rather type the sizes. Confirm important sizes with a tape or laser before ordering. See how to measure a room on Android.