Why Concrete Bags Are Sold by Weight but You Need to Think in Volume

Two different units for two different purposes

Concrete mix is packaged and sold by weight (a 40, 60 or 80 pound bag) because that's how it's manufactured, shipped and priced — but a slab, footing or post hole is a three-dimensional space measured in volume (cubic feet or cubic yards). Figuring out how many bags to buy means converting between the two.

The published conversion

An 80 pound bag of pre-mixed concrete yields approximately 0.6 cubic feet once mixed and cured — a figure published on manufacturer data sheets (Quikrete, Sakrete and others) and governed by the ASTM C387 standard, so it's consistent across brands rather than being a rough guess.

Why smaller bags scale down proportionally

Since the mix ratio is the same regardless of bag size, yield scales down roughly linearly with weight: a 60 pound bag yields about 0.45 cubic feet, a 40 pound bag about 0.3 — all derived from the same per-pound yield rate as the 80 pound bag.

Why thickness matters as much as length and width

A slab's volume depends on all three dimensions, and thickness is usually specified in inches while length and width are in feet — forgetting to convert thickness from inches to feet before multiplying is an easy way to be off by a factor of 12.

Calculate your bag count

Our Concrete Calculator converts your slab dimensions into cubic feet, cubic yards, and the exact number of bags needed at any common bag size.