Concrete Slab Calculator — Yards & Bags
Estimate slab and thickened-edge volume, cubic yards, full concrete bags, ordering allowance, and ready-mix or bag cost.
Concrete slab volume is footprint area × finished thickness, plus any additional volume below the main slab at a thickened perimeter edge. The planning order then adds an allowance and compares ready-mix cubic yards with full bags using the selected product yield.
240 sq ft of slab · 80 cu ft before 10% allowance.
- Bagged concrete alternative147 80-lb bags
- Ready-mix alternative3.26 cubic yards
The thickened-edge option adds only the depth below the main slab, so the slab volume is not counted twice.
Confirm subbase, drainage, reinforcement, joints, thickness, mix specification, frost conditions, access, local code, and supplier rounding before ordering.
Bag yield varies by product and mixing consistency. Replace the default with the yield printed on the selected bag.
Step by step
- 1
Enter the finished slab length, width, thickness, and number of matching slabs.
- 2
If the approved design has a thickened edge, enter its total width and depth; the tool adds only the depth below the main slab.
- 3
Replace the typical bag yield with the exact label value, add an appropriate allowance, and confirm ready-mix rounding, short-load fees, subbase, reinforcement, joints, drainage, and local requirements.
Check the estimate before you buy
Estimate a rectangular slab plus an optional thickened perimeter edge, then compare ready-mix cubic yards with full concrete bags and cost.
A 20 × 12 ft slab at 4 in thick contains 80 cubic feet, or about 2.96 cubic yards. A 10% allowance raises the planning order to about 3.26 cubic yards, equal to 147 80-lb bags at a typical 0.60-cubic-foot yield.
How the calculation works
The main slab uses length × width × thickness. When selected, the perimeter calculation adds edge width × only the extra depth below the main slab, avoiding double counting. Allowance is applied to the combined volume before bags are rounded up.
Concrete Slab Calculator — Yards & Bags FAQ
Should I buy exactly the calculated amount of concrete?
Usually no. Cuts, spillage, breakage, uneven surfaces, batch matching, and package rounding can increase the order. Use the editable allowance and confirm the product label.
Why does the result round up?
Stores sell full bags, boxes, cans, rolls, panels, or pieces. The tool keeps the mathematical amount visible, then rounds the purchasing quantity up separately.
Does the thickened edge double-count the slab?
No. The calculator includes the main slab over the full footprint, then adds only the edge depth below that slab.
How much extra concrete should a slab order include?
There is no universal percentage. Grade variation, forms, measurement confidence, pump or chute losses, and supplier rounding matter. Use a project-appropriate allowance and confirm it with the installer or supplier.
Reference: QUIKRETE Concrete Mix product data and yield guidance. Results are practical estimates; complete labels, package directions, product specifications, and qualified professional advice take priority.