Beam Volume Calculator

Short answer: one 20 ft beam, 12 in wide and 18 in deep, holds 30.00 cu ft — 1.11 cubic yards net (1.22 with 10% waste). Enter your beam below.

Fresh concrete being poured into wooden formwork from a chute
Concrete being placed for a beam in its forms.
Width 12 inDepth 18 inLength 20 ft · 1.11 cubic yards net
Diagram — not to scale. Dimensions are from the worked example on this page.

Calculate your beam volume

Enter measurements in the selected units. Results are shown in cubic yards and bags, the units suppliers use.

The full formed length, end to end.

10% is the site default for formed work. Net and order volumes are shown separately.

Enter your local quote. Example only: $165.

Example only: $6.50. Enter your store price for a real comparison.

How the calculation works

Volume = cross-section × length ÷ 27. Cross-section = width × depth (both in feet). Multiply by the count when several identical beams are poured. Bag counts use QUIKRETE published yields: 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 50 lb = 0.375 cu ft, 40 lb = 0.30 cu ft.

Worked example: 20 ft long × 12 in wide × 18 in deep, count 1 = 1.50 sq ft cross-section × 20 ft = 30.00 cu ft = 1.11 cubic yards net (1.22 with 10% waste). Bags from the order volume: 55 × 80 lb, 74 × 60 lb, 88 × 50 lb, 110 × 40 lb.

Method note: rectangular beams only, volume only. The volume of a rectangular beam is its cross-sectional area multiplied by its length, divided by 27 for cubic yards — the method shown in Estimating in Building Construction (Example 10-6, grade beams) and in the Pennsylvania Department of Education career-and-technical-education volume chart (cubic yards = length × width × height ÷ 27). The volume shown is the gross volume of the formed rectangle. This calculator does not size a beam: beam sizes, and what goes inside them, follow the structural design and the local building department where one is required. The Brick Industry Association (Technical Note 17B) warns against rule-of-thumb methods for beam design.

Lintels and headers: a lintel is a beam over an opening. Enter the full formed length, end to end — the length of the formed beam itself, not the width of the opening below it. T-shaped and L-shaped sections are not covered: their flange widths come from the structural design, and adding the flange and the stem separately counts their overlap twice. For any other cross-section shape, multiply its cross-section area by its length for cubic feet, then use the volume converter on the cubic yard calculator.

How to use this calculator

  1. Enter your measurements in the calculator above, in the units shown.
  2. Check the inputs against what you measured — depth is the number most often entered wrong.
  3. Press Calculate, then use the order amount (with waste, where shown) when you buy or order.

Frequently asked questions

How many cubic yards are in a concrete beam?

It depends on the section and the length: one 20 ft beam, 12 in wide and 18 in deep, holds 30.00 cu ft — 1.11 cubic yards net (1.22 with 10% waste).

How do you calculate the volume of a rectangular beam?

Multiply the cross-section (width × depth, in feet) by the length, in feet, for cubic feet, then divide by 27 for cubic yards. Multiply by the count when several identical beams are poured.

Does this calculator work for T-shaped or L-shaped beams?

No — rectangular sections only. T-shaped and L-shaped sections need their flange widths from the structural design, and adding the flange and the stem separately counts their overlap twice.

Can I use this calculator for a lintel over a door or window?

Yes, for volume only: a lintel is a beam over an opening, so enter the full formed length, end to end — the length of the formed beam itself, not the width of the opening below it.

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