Beam Concrete Calculation: Volume Formula, Worked Examples & Deductions

How Do You Calculate Concrete Volume for a Beam?

A beam's concrete volume is length × width × depth, divided by 27 for cubic yards:

Cubic yards = (L ft × b ft × d ft) ÷ 27

where b is the beam width and d is its overall depth. Convert any inch dimensions to feet first (÷ 12). For a 20 ft beam that is 12 in wide and 18 in deep, that's 1.11 cubic yards before waste.

Quick start: Drop the dimensions into the Beam Concrete Calculator for instant volume, bag count, and waste allowance.

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Worked Example

Beam: 20 ft long, 12 in wide, 18 in deep.

1. Width: 12 ÷ 12 = 1.0 ft
2. Depth: 18 ÷ 12 = 1.5 ft
3. Volume: 20 × 1.0 × 1.5 = 30 ft³
4. Cubic yards: 30 ÷ 27 = 1.11 yd³
5. Add 10% waste: 1.22 yd³ to order

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Multiple Beams at a Glance

For a grid of identical beams, compute one and multiply. Volumes below are per beam, net of waste (12 in wide):

Length	Depth	Volume (ft³)	Volume (yd³)
12 ft	12 in	12.0	0.44
16 ft	16 in	21.3	0.79
20 ft	18 in	30.0	1.11
24 ft	24 in	48.0	1.78

Sum all beams, then apply a single 5–10% waste allowance to the total.

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The Slab-Overlap Deduction (Monolithic Pours)

When beams are cast monolithically with a slab, the top of the beam is already counted in the slab volume. To avoid double-counting, measure beam depth below the slab soffit, not the full structural depth.

Example: an 18 in deep beam under a 6 in slab contributes only 18 − 6 = 12 in (1.0 ft) of independent beam depth. Using the full 18 in would over-order by a third.

If you also estimate the slab, see how to calculate concrete for a slab and keep the boundary consistent between the two.

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T-Beams, L-Beams & Haunches

• Rectangular beam: straight L × b × d — the default above.
• T-beam / L-beam: the flange is normally part of the slab; count only the web (the stem below the slab).
• Haunches and fillets: add the small triangular wedges where the beam meets columns or slabs if they're formed — they're minor but real on long spans.

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Reinforcement Note

Steel barely changes concrete volume, but heavily reinforced beams need attention during the pour:

• Maintain ≥ 1.5 in clear cover before measuring usable dimensions.
• Congested stirrups and bottom bars trap air — vibrate to prevent honeycombing.
• Size bars, spacing, and cover with the rebar sizing, spacing & cover guide.

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Frequently Asked Questions

What is the formula for concrete beam volume?

Concrete beam volume is length × width × depth. Keep every dimension in feet and divide by 27 to get cubic yards. For example, a 20 ft × 1 ft × 1.5 ft beam is 30 ft³, or 1.11 cubic yards.

Do you include the slab when calculating a monolithic beam?

No — measure the beam depth below the slab so you don't double-count concrete. An 18-inch beam under a 6-inch slab contributes only 12 inches of independent beam depth.

How much waste should I add for beam concrete?

Add 5–10% to the combined beam total. Use the lower end for clean formwork on simple rectangular beams and the higher end for deep beams, haunches, or congested reinforcement.

How do you handle a T-beam or L-beam?

Count only the web (stem) below the slab, since the flange is part of the slab pour. This prevents overlap between your slab and beam estimates.

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Summary

• Beam concrete = L × b × d ÷ 27 (cubic yards).
• A 20 ft × 12 in × 18 in beam = 1.11 yd³ (order ~1.22 yd³ with waste).
• For monolithic beams, measure depth below the slab to avoid double-counting.
• Total all beams first, then apply one waste allowance.

Check any beam — including waste and bag counts — with the Beam Concrete Calculator.

Related Guides

• Column Concrete: Rectangular vs Circular
• Rebar Sizing, Spacing & Concrete Cover
• Concrete Footing Calculations

Visit Concrete Calculator Max for the full suite of structural concrete tools.