How to calculate cubic feet
Length × width × height, all three in feet. A room 12 × 10 ft with an 8 ft ceiling is 960.00 cubic feet.
When the sizes are in inches, multiply them as they are and divide by 1,728: there are twelve inches to a foot each way, and 12 × 12 × 12 is 1,728 cubic inches to a cubic foot.
Worked example. A box 24 × 18 × 16 in.
- In cubic inches: 24 × 18 × 16 = 6,912 cu in
- In cubic feet: 6,912 ÷ 1,728 = 4.00 cu ft
- In litres: 113.27 L
- In US gallons: 29.9 gal
The calculator reads each side in whatever unit you write. A number alone is in inches; 12 ft, 2 yd, 60 cm and 1.5 m are read as written, and one item can mix them: a shelf measured in inches against a room measured in feet. What each field understood is echoed beside it, so a slip shows before it is multiplied by two others.
Odd shapes
Not everything is a box.
- A cylinder is the area of its round end times its height: 3.1416 × the diameter squared ÷ 4, × the height. A tank 2 ft across and 3 ft high holds 9.42 cubic feet, which is 70.5 US gallons.
- A triangular prism is half the box it would fit in: the base of the triangle × its height ÷ 2, × the length. The space under a roof 4 ft wide at the base, 3 ft high and 10 ft long is 60.00 cubic feet.
- Anything else splits into these. Add one item for each piece, and the calculator adds the volumes before it rounds.
The drawing shows each item from above, to its own scale, with its sizes on it: a quick check that the diameter did not go in the height’s field.
Cubic feet for moving and storage
A moving truck and a storage unit are both rented by the cubic foot they hold, and boxes are sold by the cubic foot they contain. Enter a box once and say how many you have: ten boxes of 24 × 18 × 16 in and six of 18 × 18 × 16 in come to 58.00 cubic feet.
Boxes do not pack without gaps, and furniture packs worse. The figure here is the volume of the things themselves: ask the truck or storage company how full they expect a load to be.
Conversions
A cubic foot is a small unit for a big job and a big one for a small job, so the result is given six ways:
| One cubic foot, in | Amount |
|---|---|
| Cubic inches | 1,728 |
| US gallons | 7.48 |
| Litres | 28.32 |
And going up: a cubic yard is 27 cubic feet, a cubic metre 35.31. Soil, gravel and concrete are ordered by the cubic yard: the cubic yard calculator starts from an area and a depth, and the concrete calculator from the shapes concrete is poured in.
Questions about cubic feet
How many cubic feet in a cubic yard?
27. A yard is 3 ft, and 3 × 3 × 3 is 27. The room of 12 × 10 × 8 ft is 35.56 cubic yards.
How many gallons in a cubic foot?
7.48 US gallons, or 28.32 litres. A US gallon is 231 cubic inches (NIST).
How do I work out cubic feet from inches?
Multiply the three sizes in inches, then divide by 1,728. A box 24 × 18 × 16 in is 6,912 cubic inches, or 4.00 cubic feet.
Is a cubic foot the same as a square foot?
No. A square foot is an area, a square one foot each way; a cubic foot is a volume, a cube one foot each way. An area becomes a volume when it is multiplied by a height or a depth.
Sources
- National Institute of Standards and Technology, Guide to the SI (Special Publication 811), Appendix B.9: conversion factors. A foot is 0.3048 m and an inch 2.54 cm, exactly. A cubic foot is 0.02831685 m³, a cubic yard 0.7645549 m³, a US gallon 3.785412 L, a square foot 0.09290304 m² and an acre 4,046.873 m². Read on .
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