How to convert linear feet to cubic feet
Linear feet describes a single length only. To get volume in cubic feet, you also need the cross-section: a width and a height (both in feet). Multiply length × width × height for cubic feet.
ft³ = linear ft × width (ft) × height (ft)
Worked examples
Example 1: 50 ft trench, 1 ft wide × 2 ft deep
50 × 1 × 2 = 100 ft³ of soil to remove (or fill).
Example 2: 20 ft beam, 6 in × 6 in
20 × 0.5 × 0.5 = 5 ft³.
Example 3: 100 ft of footings, 8 in wide × 12 in deep
100 × 0.667 × 1 ≈ 66.67 ft³, or about 2.47 yd³ of concrete.
| Volume per linear foot (by cross-section) | Conversion factor | Worked example |
|---|---|---|
| 6″ × 6″ | × 0.25 | 0.25 ft³ per linear ft |
| 6″ × 12″ | × 0.5 | 0.5 ft³ per linear ft |
| 12″ × 12″ | × 1 | 1 ft³ per linear ft |
| 12″ × 24″ | × 2 | 2 ft³ per linear ft |
| 24″ × 24″ | × 4 | 4 ft³ per linear ft |
Common uses
- Trench excavation and fill volume
- Concrete footings and continuous strip foundations
- Beams, posts, and structural lumber
- Pipes, conduits, and ducts (volume of contents)
- Rebar curing trenches and decorative borders
Where this conversion comes up
Linear feet is one-dimensional — board length, pipe length, fence run. To get cubic feet you multiply by the cross-sectional area.
- Lumber and firewood — linear feet of board × cross section in square feet = cubic feet.
- Concrete footings — linear feet of trench × width × depth.
- Gravel trench drains and french drains.
- Pipe and conduit fill calculations in plumbing and electrical work.
Common mistakes to avoid
- Forgetting cross-section dimensions. Linear feet measures length only. To get volume, you also need cross-sectional width and height (or radius for round cross-sections).
- Confusing linear feet with square feet. Linear feet is 1D; square feet is 2D. Don't multiply two linear-feet measurements and call the result cubic feet — that gives square feet.
- Using nominal vs actual lumber dimensions. A “2×4” that is 8 linear feet long has cross-section 1.5 × 3.5 inches, not 2 × 4 inches. Use actual dimensions.
Expert tips
- Multiply linear feet × cross-section area (in ft²) for cubic feet. A 2×4×8 board: cross-section 1.5 × 3.5 in = 5.25 in² = 0.0365 ft². Volume: 8 × 0.0365 = 0.292 ft³.
- For pipes, use π × radius² × length. Half the inside diameter gives the radius. See the cylinder calculator for the formula.
- For lumber, this conversion gives volume — but lumber is usually priced in board feet. 1 board foot = 1/12 ft³. See the cubic feet to board feetconverter for the next step.
- For trench drains and french drains, this gives gravel volume.Linear feet of trench × width × depth = cubic feet of gravel needed. See the gravel calculator.
- For fence posts, the cubic feet is the concrete in each hole, not the fence run. A 4-foot deep, 12-inch diameter hole holds about 3.14 ft³ of concrete.
Frequently asked questions
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