How to measure a room for square footage and volume

To find a room’s square footage, measure each wall-to-wall distance, change feet and inches to decimal feet (divide the inches by 12), split any odd-shaped floor into rectangles, multiply length by width for each and add them up. For volume, multiply that area by the height or depth in the same unit. Then convert with a squared or cubed factor: 9 ft² in a square yard, 27 ft³ in a cubic yard, and about 7.4805 US gallons in a cubic foot.

Watch for three common slips: reading 4 ft 10 in as 4.10 ft, using a length factor on an area, and multiplying a depth in inches by lengths in feet.

How do you measure a room for square footage?

Multiply the floor’s length by its width, both in feet: a room 12 ft 6 in by 10 ft is 12.5 × 10 = 125 ft². Measure the floor you actually plan to cover, wall to wall, and write every number on a rough sketch as you go.

1. Sketch first. Draw the outline and mark doorways, closets and fixed items such as cabinets or a hearth. Each straight wall gets a measurement.
2. Hold the tape straight and level along the floor. For flooring or paint, reading to the nearest ¼ in is plenty (the precision section below shows why).
3. Measure each dimension near both ends of the wall. If the two readings differ, the room isn’t a true rectangle. If the end walls beside those two readings are parallel to each other, the floor is a trapezoid: its area is the average of the two readings times the distance between those end walls. For buying material, you can use the larger reading instead.
4. Check the corners. If opposite walls are equal in length, compare the two diagonals as well; if they are equal too, the corners are square.
5. Decide what’s in. Include a closet if the flooring runs into it. Measure the footprint of a fixed island or cabinet run and subtract it if the flooring stops at its base.

How do you convert feet and inches to decimal feet?

Divide the inches by 12 and add the result to the feet:

$$\text{decimal feet} = \text{feet} + \frac{\text{inches}}{12}$$

So 12 ft 6 in is 12.5 ft, not 12.6 ft, and 4 ft 10 in is 4.8333 ft, not 4.10 ft. Fractions of an inch work the same way: 7½ in is 7.5 ÷ 12 = 0.625 ft.

Many of these are repeating decimals, so carry at least four decimal places until the end (below, … marks a repeating decimal). Another way is to change every measurement to inches, multiply, and then divide square inches by 144 or cubic inches by 1,728. That route involves no rounding until the last step.

How do you find the area of an L-shaped or irregular room?

Split the floor into rectangles, find each area and add them. Or take the rectangle that encloses the whole room and subtract the missing piece. Both methods must give the same total, which makes a useful check.

Take an example L-shaped living and dining area. The overall length is 22 ft 3 in and the overall width 15 ft 6 in. One corner is cut out by a notch 8 ft 9 in long and 4 ft 10 in deep, where a closet from the next room intrudes.

In decimal feet, those measurements are 22.25, 15.5, 8.75 and 4.8333… ft.

Method 1, enclosing rectangle minus the notch:

1. Whole rectangle: 22.25 × 15.5 = 344.875 ft².
2. Notch: 8.75 × 4.8333… = 42.2917 ft².
3. Floor area: 344.875 − 42.2917 = 302.5833, or 302.58 ft².

Method 2, two rectangles:

1. A full-length strip 22 ft 3 in by 10 ft 8 in (15 ft 6 in − 4 ft 10 in): 22.25 × 10.6666… = 237.3333 ft².
2. The remaining piece, 13 ft 6 in (22 ft 3 in − 8 ft 9 in) by 4 ft 10 in: 13.5 × 4.8333… = 65.25 ft².
3. Total: 237.3333 + 65.25 = 302.58 ft².

Check in inches: 267 × 186 − 105 × 58 = 49,662 − 6,090 = 43,572 in², and 43,572 ÷ 144 = 302.58 ft².

The same floor is 302.58 ÷ 9 = 33.62 yd², or 302.58 × 0.09290304 = 28.11 m².

Its perimeter, which is what you need for baseboard, equals the perimeter of the enclosing rectangle: 2 × (22.25 + 15.5) = 75.5 ft. That’s because the notch’s two inside walls replace exactly the lengths of outer wall it removes. This only works for a notch at a corner; a notch in the middle of a wall adds twice its depth. Subtract door openings from the total.

Other common shapes fit into the same method:

• Bay window or angled end wall: a trapezoid, with area ½ × (side 1 + side 2) × depth, where the two sides are the parallel ones.
• Cut-off corner: a right triangle, with area ½ × base × height. The guide to solving triangles covers triangles without a right angle.
• Half-round alcove: ½ × π × r², where the radius is half the width across the flat side.

How do you convert square and cubic feet to other units?

Apply the length conversion factor twice for an area and three times for a volume. With 1 ft = 0.3048 m:

$$\begin{array}{rcl} 1\ \text{ft}^2 & = & (0.3048\ \text{m})^2 \\ & = & 0.09290304\ \text{m}^2 \\[6pt] 1\ \text{ft}^3 & = & (0.3048\ \text{m})^3 \\ & = & 0.028316846592\ \text{m}^3 \end{array}$$

That’s because a foot measures length, while a square foot is the area of a 1 ft × 1 ft square and a cubic foot is the space inside a cube 1 ft on each side. A 12 ft × 12 ft room is 144 ft². “Ten feet square” describes a 10 ft × 10 ft square of 100 ft², while “ten square feet” is only 10 ft².

Multiplying the 302.58 ft² room by 0.3048 gives 92.23, which isn’t its area in any unit. The room is 28.11 m². Dividing square feet by 3.28 fails the same way.

The gallon factor is 1,728 ÷ 231, because a US gallon is exactly 231 in³. That division never ends, so the table rounds it.

How do you find the volume of a room?

For a flat ceiling, multiply the floor area by the ceiling height in the same unit. With an 8 ft ceiling, the L-shaped room holds 302.5833 × 8 = 2,420.67 ft³, or 68.55 m³.

Under a sloped ceiling, treat the space as a box plus a triangular prism (the triangle’s area times the room’s length). Take an example room 14 ft long and 12 ft wide, with the ceiling rising across the 12 ft width from 8 ft at one wall to 11 ft at the other:

1. Box: 14 × 12 × 8 = 1,344 ft³.
2. Prism: the triangle is 12 ft across and 11 − 8 = 3 ft tall, so its area is ½ × 12 × 3 = 18 ft², and 18 × 14 = 252 ft³.
3. Total: 1,344 + 252 = 1,596 ft³.

As a check, the floor area times the average ceiling height, 168 × 9.5, also gives 1,596 ft³.

How do you convert cubic feet to cubic yards?

Divide by 27, because a cubic yard is 3 ft × 3 ft × 3 ft. You need cubic yards whenever material such as ready-mix concrete, gravel or mulch is priced by the cubic yard. The depth is usually measured in inches, so convert it to feet before multiplying.

Take an example concrete pad for a shed, 10 ft 8 in long, 8 ft 4 in wide and 4 in thick:

1. Convert: 10.6666… ft, 8.3333… ft and 4 ÷ 12 = 0.3333… ft.
2. Area: 10.6666… × 8.3333… = 88.8889 ft².
3. Volume: 88.8889 × 0.3333… = 29.63 ft³.
4. Cubic yards: 29.63 ÷ 27 = 1.10 yd³ (1.0974 before rounding), which is 0.84 m³.

Working in inches gives the same result: 128 × 100 × 4 = 51,200 in³, and a cubic yard is 27 × 1,728 = 46,656 in³, so 51,200 ÷ 46,656 = 1.10 yd³.

That is the exact volume of the drawing. How much extra to order for spillage and an uneven base is a separate decision, covered in the guide to waste allowances.

For layers of even depth, one cubic yard covers 324 ÷ depth in inches square feet. The 324 is 27 ft³ × 12 in per foot.

The pad checks out: 88.89 ft² at 4 in deep is 88.89 ÷ 81 = 1.10 yd³.

How many gallons does a rectangular tank or space hold?

Find the volume in cubic inches and divide by 231, since a US gallon is defined as exactly 231 in³. In cubic feet, multiply by 7.4805.

Take an example tank 24 in tall, measured on the inside at 48 in long and 20 in wide, and filled to a water depth of 22 in:

1. Volume: 48 × 20 × 22 = 21,120 in³ (12.22 ft³).
2. US gallons: 21,120 ÷ 231 = 91.43 US gal.
3. Liters: 91.4286 × 3.785411784 = 346.09 L.

Two measuring choices change the answer:

• Measure inside, to the water line. Using outside dimensions of 49 × 21 in (½ in walls) and the full 24 in height gives 106.91 gallons, 17% more water than the tank will actually hold at that depth.
• Say which gallon you mean. The same water is 76.13 imperial gallons, because the imperial gallon is 4.54609 L. The volume converter keeps the two apart.

For a round tank or column, use $V = \pi r^2 h$ with the radius, which is half the diameter. Putting the diameter in place of the radius gives four times the true volume.

How precise do your measurements need to be?

For room lengths and widths, the nearest ¼ in is enough. The smallest dimension, usually a depth or thickness, needs the most care. When dimensions are multiplied, their percentage errors roughly add, and a fixed error of ¼ in is a much larger percentage of 4 in than of 128 in. Here is the shed pad with ¼ in more on each measurement in turn:

A quarter inch in the thickness changes the volume more than ten times as much as the same slip in the length and width combined. For slabs, beds and fills, check the depth in several places. The plan dimensions need less fuss.

Rounding works the same way. If you round every dimension of the L-shaped room to the nearest foot (22 × 16 − 9 × 5), you get 307 ft², 1.5% too high. Round only the final answer.

Which unit slips cause the biggest errors?

What these numbers leave out

• Waste and packaging. Everything here is the exact geometric quantity. Flooring comes in boxes, paint in cans and concrete in bags or part-yard loads, each with an allowance for cuts and spills. The flooring and tile calculator and the waste guide handle that step.
• Walls. Paint needs wall area (perimeter × height, minus doors and windows), not floor area. The paint calculator works from wall measurements.
• Uneven surfaces. A single depth reading assumes a flat base. Average several readings when the ground or floor varies.
• Official square footage. Appraisals follow a measuring standard with its own rules, covered in the questions below.

Try it

You can reproduce each worked example. The converters show more digits than the rounded figures here.

• Area converter: choose Room size (length × width), set To to square feet, and enter 22 ft 3 in by 10 ft 8 in (237.33 ft²) and then 13 ft 6 in by 4 ft 10 in (65.25 ft²). The two add up to 302.58 ft². Choose An area and enter 302.58 square feet to see 28.11 m² and 33.62 yd².
• Area and perimeter calculator: choose a rectangle in feet with length 22.25 and width 15.5. The perimeter, 75.5 ft, is also the L-shaped room’s perimeter. The area, 344.88 ft², is the enclosing rectangle before the notch comes out.
• Concrete calculator: press Reset to clear the example’s patio and post holes, then enter one slab, 10 ft 8 in long, 8 ft 4 in wide and 4 in thick, quantity 1, waste allowance 0%. The volume is 29.63 ft³, 1.10 yd³ or 0.84 m³.
• Volume and surface area calculator: choose a cuboid in inches with length 48, width 20 and height 22. The volume is 21,120 in³, which is 91.43 US gallons or 346.09 L.