Concrete Calculator
Ordering concrete is unforgiving — under-order and you cold-joint the pour, over-order and you pay for a half-truckload you cannot use. This calculator converts slab, footing, column and wall dimensions into cubic yards (or cubic metres), the number of 60 lb or 80 lb pre-mix bags, and an estimated material cost. It also adds a waste factor so you do not come up short.
How to calculate concrete
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1
Pick the shape
Rectangular slab, circular column, strip footing, or retaining wall. Each has its own formula.
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2
Enter dimensions
Length, width, thickness (or diameter and height for columns). Feet and inches or metres, your pick.
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3
Add a waste factor
10% is standard for slabs, 15% for columns and awkward pours. The calculator adds it to the base volume.
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4
Read the output
Cubic yards (or m³), number of 60 lb / 80 lb bags if pouring from pre-mix, and estimated cost at a per-yard rate.
The formulas
- Slab or rectangular footing:
Volume = length × width × thickness - Circular column:
Volume = π × (diameter / 2)² × height - Strip footing:
Volume = length × width × depth
Always use the same units. Feet × feet × feet gives cubic feet; divide by 27 to get cubic yards. Metres give m³ directly.
Typical concrete volumes
| Job | Approximate volume |
|---|---|
| 10 x 10 ft slab, 4“ thick | 1.23 cubic yards |
| 20 x 20 ft driveway, 4“ thick | 4.94 cubic yards |
| 12“ diameter column, 8 ft tall | 0.23 cubic yards |
| 30 ft footing, 12“ wide, 18“ deep | 1.67 cubic yards |
Bags vs. ready-mix truck
A standard 80 lb bag yields about 0.6 cubic feet of cured concrete. A 60 lb bag yields about 0.45 cubic feet.
- Under 1 cubic yard (27 cubic feet): bag it. That is about 45 bags of 80 lb, roughly one pallet.
- 1 to 2 cubic yards: still bag-able but tough — consider a small-batch delivery.
- Over 2 cubic yards: call a ready-mix truck. Minimum delivery is usually 1 cubic yard, most trucks hold 8-10.
Waste factor — why 10%
Real pours always use more than the geometric volume:
- Spillage at the chute.
- Uneven subgrade (thicker in low spots).
- Over-dig on footings.
- Sample cylinders if required by inspection.
10% is standard. For deep footings on rough ground or thin overlays, 12-15% is safer.
PSI and mix design
Pre-mix bags are labelled by strength at 28 days:
- 2,500 PSI: sidewalks, non-structural pads.
- 3,000 PSI: driveways, patios, most residential slabs.
- 4,000+ PSI: structural footings, columns, freeze-thaw exposure.
Ready-mix from a plant is specified by PSI plus slump and aggregate size. Your inspector or engineer will dictate this — the calculator only handles volume.
Frequently Asked Questions
A 10 x 10 ft slab at 4“ thick is 33.3 cubic feet, or about 56 bags of 80 lb pre-mix. At that volume most people order ready-mix instead.
Always up. Concrete trucks usually deliver in 0.25 cubic yard increments with a 1-yard minimum. Ordering 2.1 cubic yards means paying for 2.25. Running short means an expensive cold joint.
10% for flat slabs on well-prepared subgrade. 12-15% for footings over rough ground, deep forms, or when the subgrade has not been compacted precisely.
No — it only computes concrete volume. Rebar and wire mesh are usually sized from the plan: #4 bar at 18“ centres each way for residential slabs is typical.
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