rectangular prism volume calculator

Rectangular prism volume calculator: the formula

A rectangular prism volume calculator multiplies the three sides of a box to find how much space it holds. The formula is length × width × height. Because this tool takes feet, the answer comes in cubic feet, and it also converts to cubic yards, gallons, and liters.

Volume (ft³) = length × width × height. Cubic yards = ft³ ÷ 27. Gallons = ft³ × 7.48052. Liters = gallons × 3.78541.

Why the unit matters

Every side has to be in feet before you multiply. If your slab is 6 inches thick, enter 0.5, not 6. Mixing feet and inches is the most common reason for a wrong volume.

Inches	Feet to enter
2 in	0.167
3 in	0.25
4 in	0.333
6 in	0.5
8 in	0.667
12 in	1

Worked examples

Example 1, the default box. 8 ft long, 4 ft wide, 3 ft high.

• Cubic feet: 8 × 4 × 3 = 96.00 ft³
• Cubic yards: 96 ÷ 27 = 3.556 yd³
• Gallons: 96 × 7.48052 = 718.1 gal
• Liters: 718.1 × 3.78541 = 2,718.4 L

Example 2, a concrete slab. 12 ft × 10 ft, 6 in thick, so the height is 0.5 ft.

• Cubic feet: 12 × 10 × 0.5 = 60.00 ft³
• Cubic yards: 2.222 yd³
• Gallons: 448.8 gal

Example 3, a water trough. 6 ft long, 3 ft wide, 2 ft deep inside.

• Cubic feet: 36.00 ft³
• Cubic yards: 1.333 yd³
• Gallons: 269.3 gal

Quick read of the results

Result	What it tells you
Cubic feet	The raw space, good for bags of soil, mulch, or concrete mix
Cubic yards	The unit for bulk concrete, soil, and gravel orders
Gallons	How much water the box can hold
Liters	The same water amount in metric

Water is the only one of these that depends on the box being watertight and filled to the top, so use the real fill height.

Where a box shape shows up

• Concrete slabs, pads, and footings
• Trenches for pipes and conduits
• Raised garden beds and planters
• Sandboxes and mulch beds
• Rectangular tanks, cisterns, and fish tanks
• The air in a room, for sizing a fan or heater

How to measure

Use a tape measure on the inside dimensions if you want to know what the box holds, and use the outside dimensions if you want the space it occupies. Measure each side at two or three spots in case the box is out of square, and use the average. For an excavation, measure the finished depth, not the shovel depth at the start.

Adding an allowance

Real jobs rarely match the plan. Ground is uneven, forms bow, and material compacts. For concrete, soil, or gravel, order a bit extra over the exact volume. A common habit is to add a modest percentage, then confirm the amount with the supplier.

Common mistakes

• Typing inches into a feet field.
• Using outside dimensions for a tank that you want to fill.
• Forgetting that the volume of a slab depends on thickness as much as area.
• Treating the gallons figure as the amount a bin will hold when the bin has sloped sides.

A sanity check you can do in your head

Round each side to a whole number and multiply. For the default box, 8 × 4 × 3 is easy to do mentally, and you will see 96 before you trust the screen. Do the same for the slab: 12 × 10 is 120, and half of that is 60. If your mental figure and the tool differ by a lot, one of the inputs is probably in the wrong unit. Then divide by 27 for yards. A rough rule is that 27 cubic feet fits a box 3 ft by 3 ft by 3 ft, which is a handy picture when you are looking at a pile of soil or gravel.

Related calculators

For other shapes, see the volume calculator or the cube volume calculator. To go from cubic feet to the yards a supplier uses, try the cubic feet to cubic yards calculator, and the concrete calculator turns a slab size into an order.