Cubic Feet Calculator: Formula & Easy Steps

This calculator works out the volume of a rectangular space in cubic feet (ft³) when you enter its length, width, and height. It also converts the result to cubic yards, cubic inches, and liters, so you can use the answer for shipping, concrete, soil, or storage without doing extra math. Just type in your three dimensions in feet (or inches, which the calculator converts automatically) and press Calculate.

Formula

The basic formula for volume of a rectangular box is:

Volume (ft³) = Length (ft) × Width (ft) × Height (ft)

Where:

• Length is the longest side of the base, in feet.
• Width is the shorter side of the base, in feet.
• Height is the vertical dimension, in feet.

If your measurements are in inches, first divide each by 12 to get feet. If they are in yards, multiply by 3 to get feet. The calculator above does those conversions for you when you use the unit selector.

Worked Example

Using the default values from the calculator: length = 10 ft, width = 5 ft, height = 2 ft.

1. Multiply the three dimensions: 10 × 5 × 2 = 100.
2. The volume is 100 cubic feet.
3. To get cubic yards, divide by 27 (since 1 yd³ = 27 ft³): 100 ÷ 27 ≈ 3.70 yd³.
4. To get cubic inches, multiply by 1728 (since 1 ft³ = 1728 in³): 100 × 1728 = 172,800 in³.
5. To get liters, multiply by 28.3168 (since 1 ft³ ≈ 28.3168 L): 100 × 28.3168 ≈ 2,831.68 L.

The calculator shows 100.00 ft³ as the main answer, which matches our manual calculation.

When to Use Cubic Feet

Cubic feet is a standard unit of volume in the United States and Canada, used for:

• Shipping and freight: Carriers charge by dimensional weight, which is based on cubic feet (or cubic meters for international).
• Construction materials: Concrete, gravel, sand, and mulch are often sold by the cubic yard (27 ft³). Knowing cubic feet helps you order the right amount.
• Home appliances: Refrigerators, freezers, and ovens are sized in cubic feet.
• Storage units: Rental prices are often quoted per square foot, but the actual space you get is cubic feet.
• Aquariums and pools: Water volume is sometimes expressed in cubic feet before converting to gallons.

How to Measure Correctly

For a rectangular space, measure the inside dimensions (not the outside) using a tape measure. For a room, measure from wall to wall, floor to ceiling. For a box, measure the internal dimensions if you plan to fill it with something.

If the shape is not a perfect rectangle, break it into rectangular sections, calculate each volume, and add them together. For a cylinder (like a pipe or tank), use the formula π × radius² × height, where radius and height are in feet.

Common Mistakes and How to Avoid Them

• Mixing units: The most frequent error is entering inches as if they were feet. Always convert to the same unit before multiplying. To convert inches to feet, divide by 12.
• Forgetting to convert when ordering materials: Concrete is sold by the cubic yard, but many people calculate in cubic feet and then forget to divide by 27. This leads to ordering 27 times too much or too little.
• Rounding too early: If you round each dimension to the nearest foot, you can lose accuracy. Keep at least one decimal place until the final answer.
• Using outside dimensions for a container: If you need to know how much a box holds, measure the inside, not the outside, or you’ll overestimate by the wall thickness.

Limitations of the Simple Formula

The formula works only for rectangular prisms (boxes). For irregular shapes, spheres, cones, or cylinders, you need different formulas. For example, a sphere’s volume is (4/3)πr³, and a cylinder’s is πr²h. Also, if you’re measuring something like a moving truck, the cargo space may have wheel wells or sloped ceilings that reduce the usable volume—always check the manufacturer’s specs for actual capacity.

For soil or mulch, note that the volume changes when compacted. A cubic foot of loose soil may settle to less after watering. Order a little extra (typically 10%) for landscaping projects to account for settling and spillage.

Frequently Asked Questions

How many cubic feet are in a cubic yard?

There are 27 cubic feet in 1 cubic yard (3 ft × 3 ft × 3 ft = 27 ft³). To convert cubic feet to cubic yards, divide by 27.

How do I convert cubic feet to gallons?

One cubic foot equals approximately 7.48052 US gallons. Multiply the cubic feet by 7.48 to get gallons. For imperial gallons, multiply by 6.23.

Can I use this calculator for a cylinder?

No, this calculator is only for rectangular boxes. For a cylinder, use the formula π × radius² × height, where all measurements are in feet. The result will be in cubic feet.

What is a board foot and how does it relate to cubic feet?

A board foot is a lumber measurement equal to 1 inch thick × 12 inches wide × 1 foot long, which is 144 cubic inches, or 1/12 of a cubic foot. To convert cubic feet to board feet, multiply by 12.

Why do I need to convert inches to feet?

Because the formula requires all dimensions in the same unit. If you multiply inches by feet, you get a meaningless mix. Convert inches to feet by dividing by 12, then multiply.

How accurate is this calculator for irregular rooms?

It’s only accurate for rectangular rooms. For L-shaped rooms, divide the room into two rectangles, calculate each volume, and add them. For rooms with sloped ceilings, use the average height.

Related

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in length | Length | ft | 10 | 0
in width | Width | ft | 5 | 0
in height | Height | ft | 2 | 0
out cubic_ft | Cubic feet | ft³ | length * width * height | 2
out cubic_yd | Cubic yards | yd³ | (length * width * height) / 27 | 2
out cubic_in | Cubic inches | in³ | (length * width * height) * 1728 | 0
out liters | Liters | L | (length * width * height) * 28.3168 | 2