Concrete Volume Calculator: How Much Concrete You Actually Need in Cubic Yards
Concrete volume is calculated as length times width times depth, then divided by 27 to convert cubic feet into cubic yards — the unit ready-mix concrete is actually sold and delivered in.
Depth is the measurement most commonly estimated wrong, since it's often specified in inches (a 4-inch slab, for example) and needs to be converted to feet (4 inches = 0.333 feet) before multiplying with the length and width, both usually already in feet.
Add a Waste Buffer
Uneven subgrade, minor form movement during the pour, and spillage all mean the actual amount used typically runs slightly over the calculated volume. Adding 5-10% to the calculated total avoids running short mid-pour, which is a far bigger problem than ordering slightly extra.
Ready-Mix Minimums vs. Bagged Concrete
Ready-mix trucks usually have a minimum order (often around 1 cubic yard) and charge a short-load fee below that minimum, which makes ready-mix cost-effective mainly for larger pours. Smaller projects — a fence post base, a small patch — are usually more economical using bagged concrete mixed on site, even though the per-cubic-yard cost of bags is higher than a full truck.
Non-Rectangular Volumes
A round post hole uses a different formula (radius squared times π times depth) rather than length times width times depth, and calculating it separately from the main slab volume avoids underestimating the total needed for a project that includes both a slab and post footings.
Frequently Asked Questions
How much extra concrete should I order for waste?
A 5-10% buffer over the calculated volume is standard practice, covering uneven subgrade, minor form movement, and spillage during the pour. Running out mid-pour is a much bigger problem than having a small amount left over.
What's the minimum order for ready-mix delivery?
Most ready-mix suppliers set a minimum around 1 cubic yard and charge a short-load fee for orders below that, which is why ready-mix is most cost-effective for larger pours and bagged concrete is usually cheaper per-project for small jobs like a single post base.
How do I calculate volume for a round post hole versus a rectangular slab?
A round hole uses radius squared times pi times depth, while a rectangular slab uses length times width times depth — calculate each shape separately and add the volumes together if a project includes both, rather than applying one formula to the whole job.
Should I use bagged concrete or ready-mix for my project?
Bagged concrete is generally more economical for small volumes (well under a cubic yard) despite a higher per-yard cost, since it avoids ready-mix delivery minimums and short-load fees; ready-mix becomes more cost-effective as the volume needed grows closer to a full truck load.
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