Concrete Beam Calculator
Enter the length, width and depth of the beam. You get cubic yards, bags, weight and the ready-mix order for one beam or several.
Last updated November 13, 2026
How to calculate concrete for a beam
A beam is a long rectangle, so the volume is length × width × depth. Put all three in feet, multiply, and divide by 27 for cubic yards. The calculator above uses the footing shape, which is the same rectangular math, and lets you enter the width and depth in inches.
Worked example. A grade beam 30 ft long, 12 inches wide and 18 inches deep: 30 × 1 × 1.5 = 45 cubic feet. 45 ÷ 27 = 1.67 cubic yards. With 10% extra you would order 2 yards.
Concrete per foot of beam
Use this table when you know the cross-section and only need to multiply by the length. Bag counts are 80 lb bags for a 10 ft beam, before waste.
| Beam size (width × depth) | Cubic feet per foot | Cubic yards per 10 ft | 80 lb bags per 10 ft | Weight per foot |
|---|---|---|---|---|
| 8 × 12 in | 0.67 | 0.25 | 12 | 100 lb |
| 8 × 16 in | 0.89 | 0.33 | 15 | 133 lb |
| 12 × 12 in | 1.00 | 0.37 | 17 | 150 lb |
| 12 × 18 in | 1.50 | 0.56 | 25 | 225 lb |
| 12 × 24 in | 2.00 | 0.74 | 34 | 300 lb |
Weights use 150 lb per cubic foot, the usual estimating figure for reinforced concrete. See the concrete weight calculator for other densities.
Several beams, or beams with a slab
- Identical beams. Enter one beam and set the quantity to the number of beams.
- Different sizes. Add each beam as its own area and the calculator totals them.
- Beam and slab poured together. Count the slab at its full thickness, then count only the part of the beam that hangs below the slab. Otherwise the overlap is counted twice.
- Grade beams on piers. Work out the piers separately with the sonotube calculator and add the volumes.
What this calculator does not do
It finds the volume of concrete. It does not size the beam or its reinforcement. Beam depth, width and steel are structural decisions that depend on the span and the load, and they come from an engineer or from your approved plans.
Frequently asked questions
How do I calculate the volume of a concrete beam?
Multiply length × width × depth in feet to get cubic feet, then divide by 27 for cubic yards. A 30 ft beam that is 12 × 18 inches is 45 cubic feet, or 1.67 cubic yards.
How much does a concrete beam weigh?
About 150 lb per cubic foot. A 12 × 12 inch beam weighs about 150 lb per foot of length, so a 20 ft beam weighs about 3,000 lb.
How many bags of concrete do I need for a beam?
Divide the cubic feet by 0.60 for 80 lb bags. A 10 ft beam that is 12 × 12 inches is 10 cubic feet and needs 17 bags before waste.