How to Calculate Concrete
Estimating concrete starts with a simple formula: length × width × depth gives you the volume of the pour, and that volume gets converted into cubic yards — the standard unit ready-mix is sold by in the United States. One cubic yard equals 27 cubic feet, so a slab that's 10 feet long, 10 feet wide, and 4 inches deep works out to about 1.23 cubic yards before adding waste factor.
The math changes slightly for footings and columns. Footings are usually long, narrow trenches, so length runs in feet, width in inches or feet, and depth in inches. Columns are calculated by their cross-section area times height — round columns use π × radius² × height, and our calculator handles the conversion for you.
Why Waste Factor Matters
Concrete pours never use exactly the calculated volume. Spillage, over-excavation, uneven subgrade, and pump-line losses all eat into your order. A 10% waste factor is the industry default for slabs and patios, while complex pours with rebar, embeds, or irregular formwork can need 15% or more. Ordering short means a second short-load delivery fee and a cold joint in your pour — both expensive. Ordering long is almost always cheaper than ordering short.
Bags vs. Ready-Mix Truckloads
For small jobs — fence posts, small slabs, repair work — bagged concrete from a home center is the practical choice. An 80-pound bag yields roughly 0.6 cubic feet, so 45 bags equal one cubic yard. Anything over half a cubic yard usually pencils out cheaper as ready-mix delivery, and the quality of a single batched mix beats the inconsistency of mixing 45 bags by hand. Most ready-mix producers have a one-yard minimum, with short-load fees applied for orders under three or four yards.
Using the Estimate to Order
When you call your local ready-mix producer, you'll need three pieces of information: total cubic yards (with waste factor included), the mix design you want — typically specified by PSI (3,000 PSI for residential slabs, 4,000 PSI for driveways, 4,500+ PSI for structural work) — and the delivery window. The price-per-yard field in our calculator gives you a quick total so you can budget the job before you call.
Want to learn more about how ready-mix is dispatched, ticketed, and tracked from plant to pour? See our Dispatchopedia for industry terminology, or learn how eTicketing is replacing paper delivery tickets on modern jobsites.