Exterior

Concrete Calculator: Cubic Yards and Bags

Cubic yards for a slab, footing or column — and the honest bag count, so you know when to stop bagging and call a truck.

Your measurements

Concrete needed

3.26cu yd

20 × 12 ft at 4 in, including 10% over-order

VolumeBefore over-order
80.00 cu ft
With 10% allowance
88.00 cu ft
80 lb bags0.60 cu ft per bag
147
60 lb bags0.45 cu ft per bag
196
Ready-mix costPer cubic yard delivered, before short-load fees
$456 – $717
Bagged cost147 × 80 lb bags
$809 – $1,176
  • At 3.26 cubic yards this is 147 bags of ready-mix — order a truck instead. Most suppliers have a 1 yd minimum plus a short-load fee, and it is still cheaper and far better concrete than mixing this many bags by hand.
  • Order for the hole you actually dug, not the one you drew. Sub-grade rarely comes out level, and 10% over is the standard allowance.
  • A 4 in slab on grade wants a compacted base and either fibre mix or #3 rebar on 18 in centres. Concrete on soft fill cracks no matter how thick it is.

How this is calculated

  • Volume is length × width × thickness for a slab, length × width × depth for a footing, and π r² × depth × count for round columns and post holes — all converted to feet first.
  • A 10% over-order allowance is added, because sub-grade is never perfectly level and running short mid-pour creates a cold joint.
  • Bag counts use printed yields: 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb, 0.30 for 40 lb.
  • There are 27 cubic feet in a cubic yard. Ready-mix cost is typical 2025–2026 US delivered pricing before short-load fees.

Quantities are rounded up, because materials are sold in whole units and a second trip costs more than a spare box. Cost figures are typical US ranges and are not a quote — always price against your own supplier.

Frequently asked questions

How many bags of concrete are in a cubic yard?

45 bags of 80 lb, or 60 bags of 60 lb. That is the number that tells you bagging is the wrong choice for anything approaching a yard — a truck delivers better, more consistent concrete for less money and none of the mixing.

How much concrete do I need for a 20x12 patio?

A 20 × 12 ft slab at 4 in thick is 80 cubic feet, or about 3.3 cubic yards with a 10% allowance. That is ready-mix territory — roughly 148 bags of 80 lb otherwise.

How thick should a concrete patio or driveway be?

4 in for a patio or walkway on a compacted base. 5–6 in for a driveway, and 6 in with rebar if trucks or an RV will park on it. Base preparation matters more than the extra inch.

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