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Concrete slab calculator

Patios, floors, driveways and pads — length × width × thickness.

4 in is typical for a patio; 5–6 in for a driveway.
Most pours order 5–10% extra for spillage and over-excavation.

Estimated volume

1.23cubic yards· 0.944 m³· 33.33 ft³
Bag sizeYield per bagBags needed
40 lb0.3 ft³112
60 lb0.45 ft³75
80 lb0.6 ft³56
25 kg0.413 ft³81
30 kg0.496 ft³68
Planning estimate. Bag counts assume the yields above; check the yield printed on your bag, and allow extra for over-excavation and spillage. Structural and load-bearing work must follow your local building code and an engineer's specification.

Premium · cost estimator

What this pour costs

Bagged
$364.00
56 × 80 lb
Ready-mix
$214.50
1.3 yd³ ordered

Prices are yours to set and stay in this browser. Ready-mix suppliers usually have a minimum load and a short-load surcharge — check before comparing.

Premium · project

Multi-pour project & order sheet

Add this pour to a saved project to total several pours into one order, then print a clean order sheet with the bag counts and yardage to hand your supplier.

Projects are stored in this browser only — nothing is uploaded and there is no account.

Formwork

How the volume is measured

lengththicknesswidth runs into the pageSLAB · L × W × T

How this calculator works

A slab is a simple rectangular prism: multiply length × width × thickness. Enter the thickness in inches (it is usually the smallest dimension) and the plan size in feet. Four inches suits patios and walkways; drives and garage floors are usually five to six.

The volume is then converted to cubic yards (27 cubic feet to the yard) and cubic metres, and divided by each bag's yield to give a bag count — always rounded up, since you cannot buy part of a bag. Bag yields follow the published premix figures: 0.30 ft³ for a 40 lb bag, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb, with metric bags scaled from the same mixed density.

Add a waste allowance for anything real: over-excavated trenches, uneven sub-grade, spillage and what stays in the mixer. Five to ten percent is typical; a rough excavation can need more.

Worked examples

Common slab sizes, already calculated

Concrete for a 4×4 ft slab, 4 in thick0.2 yd³9 × 80 lb bagsConcrete for a 4×4 ft slab, 5 in thick0.25 yd³12 × 80 lb bagsConcrete for a 4×4 ft slab, 6 in thick0.3 yd³14 × 80 lb bagsConcrete for a 4×8 ft slab, 4 in thick0.4 yd³18 × 80 lb bagsConcrete for a 4×8 ft slab, 5 in thick0.49 yd³23 × 80 lb bagsConcrete for a 4×8 ft slab, 6 in thick0.59 yd³27 × 80 lb bagsConcrete for a 5×5 ft slab, 4 in thick0.31 yd³14 × 80 lb bagsConcrete for a 5×5 ft slab, 5 in thick0.39 yd³18 × 80 lb bagsConcrete for a 5×5 ft slab, 6 in thick0.46 yd³21 × 80 lb bagsConcrete for a 6×6 ft slab, 4 in thick0.44 yd³20 × 80 lb bagsConcrete for a 6×6 ft slab, 5 in thick0.56 yd³25 × 80 lb bagsConcrete for a 6×6 ft slab, 6 in thick0.67 yd³30 × 80 lb bags

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