Concrete & Slab Volume Estimator

Standard Bag Yields

Pre-mixed concrete is sold in fixed-weight bags, and each bag covers a predictable volume once mixed with the water specified on the label. Knowing the yield per bag lets you convert a cubic-yard figure into a shopping list you can load into a cart.

80 lb bag yield

0.60 ft³

60 lb bag yield

0.45 ft³

Cubic feet per cubic yard

27 ft³

As a rule of thumb, a single 80 lb bag fills about 0.6 cubic feet, so you need roughly 45 bags to make one cubic yard. Above about 60 bags, most contractors switch to ready-mix delivery, since mixing that many bags by hand is slower and yields a less consistent blend.

Why Waste Margins Matter

Concrete does not forgive a shortage. Because a pour has to be placed continuously, running out mid-slab means a cold joint: a permanent seam where the first batch has already begun to cure. Fixing it costs far more than the extra material would have.

• Sub-grade unevenness means real depth often exceeds the nominal thickness you designed.
• Roughly 5-10% of material clings to chutes, mixers, and forms and never reaches the slab.
• Over-excavated footings and trench edges absorb volume that your math never accounted for.
• A small surplus is useful for filling low spots, patching edges, or forming a curb lip.

A 5% buffer is a sensible default for a form-set slab on prepared ground. Push toward 10% when pouring onto an excavated trench, over an uneven sub-base, or when the pour is small enough that a single bag or two makes up a meaningful share of the total.