Concrete slab answer guide

How much concrete do I need?

Estimate concrete volume from slab length, width, thickness, waste, cubic yards, bag count, and ready-mix planning.

Reviewed - May 20, 2026

Short answer

For a rectangular slab, multiply length x width x thickness in feet to get cubic feet, add waste, then divide by 27 to convert cubic feet to cubic yards.

For a 12 ft by 10 ft slab at 4 in thick with 10% waste, the calculator estimates 44.0 cu ft of concrete, or 1.7 cu yd when rounded for ordering.

Use the matching project shape: slab rectangles, cylinders for post holes and tubes, length x width x depth for footings, or known-volume mode when a takeoff already gives cubic feet before waste.

Use the concrete calculator

Concrete volume method

  1. Measure the project shape and convert thickness or depth from inches to feet.
  2. For a slab, calculate cubic feet as length x width x thickness in feet.
  3. For post holes or tubes, calculate cylinder volume as pi x radius x radius x depth, then multiply by the count.
  4. For continuous footings, multiply footing length by footing width in feet by footing depth in feet; for known-volume takeoffs, enter the cubic feet before waste.
  5. Add waste for over-excavation, uneven forms, spillage, and a little placement margin.
  6. Divide waste-adjusted cubic feet by 27 for cubic yards, or divide by bag yield for bag count, then round up.

Quick examples

10 ft by 10 ft slab
1.4 cu yd
4 in thick, 10% waste
12 ft by 12 ft slab
2.0 cu yd
4 in thick, 10% waste
20 ft by 20 ft slab
5.5 cu yd
4 in thick, 10% waste
Six post holes
0.5 cu yd
12 in diameter by 30 in deep, 10% waste
20 ft continuous footing
0.8 cu yd
16 in wide by 8 in deep, 10% waste

These examples use the current U.S. default concrete assumptions: 27 cu ft per cubic yard, 0.60 cu ft per 80 lb bag, and a 10% waste buffer across slab, cylinder, and footing modes.

Worked example

12 ft by 10 ft slab at 4 in thick.

Concrete volume
1.7 cu yd
44.0 cu ft with 10% waste
80 lb bags
74 bags
0.60 cu ft per bag
Slab area
120 sq ft
4 in thickness
DIY material total
$411–$920
Ready-mix concrete plus forms/base
Contractor total
$795–$1,640
Materials plus labor

Starter shopping list

  • Ready-mix concrete 1.7 cu yd
  • Forms, stakes, gravel base, reinforcement Project allowance
  • Finishing tools and curing supplies As needed

This example is generated from the same calculator logic used on the Concrete slab calculator page.

Slabs and pads

  • Use the actual finished slab thickness, not the gravel base depth, when calculating concrete volume.
  • Small changes in thickness matter: moving from 4 in to 5 in adds 25% more concrete before waste.

Footings, piers, and known volume

  • Footing and pier dimensions are structural inputs, so settle code, frost-depth, and load requirements before using the calculator for quantity.
  • Estimate separate groups of post holes, sonotubes, footing runs, or known-volume sections separately when dimensions or waste conditions differ.

Bags vs ready-mix

  • Bagged concrete can work for small pads, post holes, and repairs, but a slab with dozens of bags is slow to mix consistently.
  • Ready-mix is usually easier to compare once the estimate is several cubic yards or site timing matters.

Ordering margin

  • Keep a waste buffer because forms are rarely perfect and running short during a pour is expensive.
  • Before ordering, confirm sub-base prep, reinforcement, access, slump, delivery minimums, and short-load fees with the supplier or contractor.

Common mistakes

  • Using inches as feet when calculating slab thickness.
  • Forgetting to add waste before converting to cubic yards or bags.
  • Ordering exact volume with no margin for uneven excavation or form variation.
  • Using slab square footage for footings or piers instead of volume by width, depth, and count.
  • Comparing bagged material price with ready-mix without including delivery, short-load fees, tools, and mixing time.
  • Treating a volume estimate as structural design for footings, piers, reinforcement, or load-bearing slabs.

FAQ

How do I calculate how much concrete I need?

Calculate volume in cubic feet, add waste, then divide by 27 for cubic yards or by the selected bag yield for bag count. For slabs, cubic feet are length x width x thickness in feet.

How much concrete do I need for a 12x10 slab?

For a 12 ft by 10 ft slab at 4 in thick with 10% waste, the generated example estimates 44.0 cu ft of concrete, 1.7 cu yd for ready-mix ordering, or 74 standard 80 lb bags.

Should I order concrete by cubic feet or cubic yards?

Ready-mix concrete is normally ordered by cubic yard. Bagged concrete is usually planned from cubic feet and bag yield, then rounded up to whole bags.

How much concrete do I need for a footing?

For a 20 ft continuous footing that is 16 in wide by 8 in deep with 10% waste, the generated example estimates 19.6 cu ft of concrete, 0.8 cu yd for ready-mix ordering, or 33 standard 80 lb bags.

How much waste should I add for concrete?

A 10% waste buffer is a practical early planning default for many small projects. Increase it when excavation is uneven, forms are rough, access is difficult, or the pour cannot run short.

When should I use ready-mix instead of bags?

Compare ready-mix once the bag count gets high, the slab must be placed quickly, or consistent mixing is important. Delivery fees and minimum loads can still make bags reasonable for small pours.

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