What is the formula for the cubic feet of a rectangle?

Multiply length × width × height with all three numbers in feet. The result is volume in cubic feet (ft³). For other units, convert each measurement to feet first.

How do I calculate cubic feet from inches for a box?

Multiply length × width × height in inches, then divide by 1,728. For example, a 36 × 24 × 12-inch box: 36 × 24 × 12 = 10,368 ÷ 1,728 = 6 ft³.

How do I find cubic feet of a room?

Measure the floor length and width plus the ceiling height in feet, then multiply all three. A 12 × 14 × 9 ft bedroom is 1,512 ft³.

How many cubic feet are in a 4 × 8 × 1 ft raised bed?

Exactly 32 cubic feet. The math: 4 × 8 × 1 = 32 ft³, which equals about 1.19 cubic yards or 16 of the standard 2 ft³ bagged soil products.

Can I calculate cubic feet with mixed units?

Yes — the calculator above lets you set a different unit on each input (e.g., 12 ft × 14 ft × 96 in). It converts everything to feet internally before computing.

What is the cubic feet of a 5 × 5 × 5 cube?

125 cubic feet. Any cube where all three sides are 5 ft has the same volume since 5 × 5 × 5 = 125. A cube with 10 ft sides is 1,000 ft³.

Cubic Feet Calculator (Rectangle) — L × W × H Volume for Boxes & Rooms

How to calculate cubic feet of a rectangle

The cubic foot of a rectangular shape — also called a cuboid or rectangular prism — is the simplest volume calculation in geometry. Multiply the three perpendicular dimensions, and as long as they're all in the same unit, the result is volume cubed in that unit. For an answer in cubic feet, every measurement must be in feet.

Cuboid / rectangular prism

V = Length × Width × Height

= For 5 ft × 3 ft × 2 ft → V = 30 ft³

Worked examples

Example 1: A 4 × 8 ft raised garden bed, 12 inches deep

Convert depth to feet: 12 in ÷ 12 = 1 ft. Then multiply: 4 × 8 × 1 = 32 ft³. That's about 1.19 cubic yards of soil, or 16 standard 2 ft³ soil bags.

Example 2: A 12 × 12 ft patio slab, 4 inches thick

Depth in feet: 4 ÷ 12 ≈ 0.333 ft. Multiply: 12 × 12 × 0.333 ≈ 48 ft³. That's 1.78 cubic yards of concrete — round up to 2 yd³ when ordering.

Example 3: A 36 × 24 × 12-inch shipping box

Either convert each side to feet first (3 × 2 × 1 = 6 ft³), or multiply in inches and divide by 1,728: 36 × 24 × 12 = 10,368 ÷ 1,728 = 6 ft³. Both methods give the same answer.

Example 4: A 12 × 14 × 9 ft bedroom

Multiply: 12 × 14 × 9 = 1,512 ft³. At 4 air changes per hour, the HVAC requirement is (1,512 × 4) / 60 ≈ 101 CFM of supply air.

Calculating from different units

The calculator accepts any combination of units, but if you're doing it by hand, the conversion factors are:

Inches → feet: divide by 12 (or multiply final volume in in³ by 1/1,728)
Yards → feet: multiply by 3
Centimeters → feet: multiply by 0.0328084
Meters → feet: multiply by 3.28084
Millimeters → feet: multiply by 0.00328084

Expert tip

Always convert each linear measurement to feet first, before you multiply. If you compute volume in a mixed-unit form (like ft × in × ft) the units will not cancel and your answer will be off by the conversion factor.

Common uses for the rectangular calculator

Concrete slabs, footings, and pads
Soil for raised garden beds, planters, and trenches
Mulch, gravel, and topsoil for landscaping plots
Shipping boxes, pallets, and freight containers
Refrigerator, freezer, oven, and microwave interiors
Storage units, moving trucks, and walk-in closets
Room volume for HVAC sizing and air-changes calculations
Aquarium, pool, and tank water capacity

Common mistakes to avoid

Mixing units in the same multiplication. Converting all three to feet before multiplying is the cleanest path.
Confusing area with volume. Square feet × depth in feet = cubic feet. If you stop after L × W, you have area, not volume.
Using outside dimensions for an interior measurement. Refrigerators, boxes, and storage units have walls — measure the inside.
Rounding too early. Round only at the final result. Rounding in every intermediate step compounds the error.

Expert tips

Measure inside dimensions for containers, outside for boxes. A refrigerator's “20 cubic feet” rating uses interior space; a moving box's rating uses exterior. Match the convention to your purpose.
Note the units on every measurement. Three numbers without units attached is a recipe for the inches-vs-feet mistake. Write “L = 84 in, W = 24 in, H = 30 in” before you start multiplying.
Sanity-check the order of magnitude. A typical bedroom is 800 to 1,500 ft³. A typical fridge is 18 to 25 ft³. A 20-foot moving truck is about 1,015 ft³. If your answer is 10× off, you probably forgot to convert inches.
For floor-to-ceiling rooms, exclude built-ins. Door swings, closets, and built-in shelving reduce usable volume. Subtract their volumes if your purpose is HVAC sizing or storage capacity.
Round up for practical use. When ordering a container, truck, or storage unit, round the volume up to the next standard size. The cost of an extra 10–15% capacity is far less than the cost of a second trip.