Concrete Volume and Bag Calculator

Estimate concrete volume and bag count for a defined project shape.

Inputs

Preset: 12x10 ft concrete slab

A 4 in slab with 80 lb concrete mix yielding about 0.6 ft³ per bag. Hole mode separately assumes a 12 in diameter hole, a 4 in diameter round post and 2 ft depth. Geometry is illustrative, not a footing design.

Bag count - Length (ft), Width (ft), Slab thickness (in), Yield per bag (ft³/bag)
Enter length in ft.
Enter width in ft.
Enter slab thickness in in.
Use the rate or yield for the exact product and conditions.
Formula, inputs and conditions

Formula: ceil(Length*Width*Slab thickness/12/Yield per bag)

Decimal results are displayed to two places when needed.

Inputs
Member dimensions, any volume to subtract, and manufacturer finished yield per bag.
Outputs
Slab and annular post-hole volume in cubic feet, cubic-yard conversion, and whole bags.
Conditions and scope
The supported slab and post-hole methods estimate the geometry you enter. They do not select footing depth, structural strength or reinforcement. Results are planning estimates, not engineering, permit, code or contractor-price decisions.

Concrete volume before bag count

Concrete demand starts with the shape being filled. A slab uses length, width and thickness. A round hole containing a round post uses the hole volume minus the post volume. Bag count then depends on mixed concrete yield, not dry bag weight alone.

Measure and calculate

  1. Choose slab, post-hole or volume-conversion mode.
  2. Enter dimensions in the displayed units; a radius is half the diameter.
  3. Use the mix yield for the exact bag weight before rounding upward.
Calculation method

Slab bags = ceiling(length in feet × width in feet × thickness in inches ÷ 12 ÷ bag yield).

A worked example

Inputs
A 12 by 10 ft slab is 4 in thick. Each 80 lb bag yields about 0.6 ft³.
Calculation
Slab volume: 12 × 10 × 4 ÷ 12 = 40 ft³. Bags: ceiling(40 ÷ 0.6) = 67.
Result
67 bag

The result is 67 bags before any separately specified allowance. This estimates a chosen geometry; it does not determine slab thickness, footing depth or structural suitability.

Read the result

Output interpretation and next steps
Output or checkWhat it means for your project
Slab volume and bagsSlab thickness is entered in inches; length and width are in feet. Convert volume to bags using the exact product yield before rounding upward.
Post-hole volumeThe annular model subtracts the post’s cross-section from the circular hole, then multiplies by hole depth. Enter radii, not diameters.
Geometry versus designThe tool estimates the dimensions you enter. It does not select slab thickness, hole depth or a structurally suitable concrete specification.

Common questions

Is hole radius the same as hole diameter?

No. A 12 in diameter hole has a 6 in radius, entered here as 0.5 ft. Using the diameter as a radius quadruples the circular area.

Why can different bag weights change the answer?

Bag count uses the product’s mixed volume yield. Select the yield for the actual product and package size, not a general weight-to-volume conversion.