Concrete Calculator

How the math works

Concrete is sold by volume. The calculator computes the volume of your shape, adds a waste allowance, then converts to the units you actually buy in: cubic yards for ready-mix delivery, or bags for hand-mixing.

For a rectangular slab, footing, or wall, volume is simply length × width × thickness. For a round column or sonotube it's π × radius² × height. A cubic yard is 27 cubic feet — the unit ready-mix is sold in — and each bag yields a published fraction of a cubic foot (an 80 lb bag makes about 0.60 ft³, so 45 bags fill a yard).

Bags vs. ready-mix — the real decision

The crossover is about one cubic yard. Below a yard, bags are usually cheaper and you avoid the short-load fee that ready-mix suppliers charge for partial loads ($60-150 is typical). Above a yard, you're looking at 45+ bags — that's a full day of mixing, and the bag price plus your time loses to a ready-mix truck. The calculator flags which side of the line you're on.

Common mistakes

• Forgetting waste. A perfectly screeded pour on a perfectly flat base might use 3% extra. Reality is 10%. Running short mid-pour means a cold joint.
• Entering thickness in inches. A 4-inch slab is 0.33 feet, not 4 feet. Use the unit toggle and the inch hint.
• Ignoring the short-load fee. A 0.5-yard order can cost more than a full yard once the small-load surcharge lands.
• Sizing rebar by spacing alone. This tool gives grid length, not bar diameter. Structural slabs need an engineer's call on bar size and cover.

When this calculator is the wrong tool

Use a structural reference for: suspended slabs and decks (loads and reinforcement governed by code), post-tensioned slabs, complex curved forms, and stamped or colored concrete (which changes the cost math entirely). This tool covers volume for flatwork and simple forms.

Density 150 lb/ft³ and bag yields from Quikrete/Sakrete public data sheets. Volume and rebar are plain math.