Concrete slab answer guide

How many concrete bags do I need?

Estimate concrete bag count from cubic feet, waste, bag size, bag yield, slab dimensions, post holes, footings, sonotubes, and known volume.

Reviewed - May 21, 2026

Short answer

Calculate the concrete volume in cubic feet, add waste, divide by the selected bag yield, then round up to the next whole bag.

For an 8 ft by 8 ft slab at 4 in thick with 10% waste, the calculator estimates 23.5 cu ft of concrete, or 40 standard 80 lb bags.

Under the current default, an 80 lb bag yields 0.60 cu ft, a 60 lb bag yields 0.45 cu ft, and a 40 lb bag yields 0.30 cu ft, so the same pour needs more bags when you choose smaller bag sizes.

Use the concrete bag calculator

Concrete bag count method

  1. Find project volume in cubic feet before waste.
  2. For slabs, multiply length x width x thickness in feet.
  3. For post holes or sonotubes, calculate cylinder volume from diameter, depth, and count.
  4. For continuous footings, multiply footing length by footing width in feet by footing depth in feet.
  5. If a drawing or supplier takeoff already gives known cubic feet, enter that volume before waste instead of converting it again.
  6. Add waste for uneven forms, over-excavation, spillage, and mixing loss.
  7. Divide the waste-adjusted cubic feet by the selected bag yield, then round up to a whole bag count.

Quick examples

Six post holes
22 bags
12 in diameter by 30 in deep, 10% waste
8 ft by 8 ft slab
40 bags
4 in thick, 10% waste
10 ft by 10 ft slab
62 bags
4 in thick, 10% waste
12 ft by 12 ft slab
89 bags
4 in thick, 10% waste
20 ft continuous footing
25 bags
12 in wide by 8 in deep, 10% waste
18 cu ft known volume
33 bags
Volume before waste, 10% waste

These examples use slab, cylinder, footing, and known-volume modes with the current U.S. default concrete assumptions: 0.60 cu ft per 80 lb bag and a 10% waste buffer.

Worked example

8 ft by 8 ft slab at 4 in thick.

80 lb bags
40 bags
0.60 cu ft per bag
Concrete volume
0.9 cu yd
23.5 cu ft with 10% waste
Slab area
64 sq ft
4 in thickness
DIY material total
$248–$536
80 lb bagged concrete plus forms/base
Contractor total
$453–$920
Materials plus labor

Starter shopping list

  • 80 lb concrete bags 40 bags
  • Forms, stakes, gravel base, reinforcement Project allowance
  • Mixer, water source, finishing and curing supplies As needed

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

Bag size

  • Use the bag yield printed on the product you will buy; this calculator scales 40 lb, 60 lb, and 80 lb bags from the current 80 lb yield assumption.
  • Switching from 80 lb bags to 60 lb or 40 lb bags increases the bag count because each bag produces less concrete.

Project volume

  • Convert slab thickness, hole diameter, and hole depth from inches to feet before doing volume math.
  • Measure the concrete depth only; gravel base, forms, and reinforcement are separate planning items.

Footings, tubes, and known volume

  • Use footing mode for continuous trench footings, sonotube mode for round piers, and known-volume mode when plans already provide cubic feet before waste.
  • Do not reuse a slab square-foot estimate for footings or tubes; bag count changes with width, depth, diameter, and count.

Mixing plan

  • Bagged concrete works well for small pads, post holes, tubes, and repairs.
  • When the estimate climbs into dozens of bags, compare ready-mix delivery, short-load fees, a rented mixer, and the time needed to place the pour consistently.

Common mistakes

  • Using slab thickness in inches as if it were feet.
  • Forgetting waste before dividing by bag yield.
  • Using the wrong bag yield after switching between 40 lb, 60 lb, and 80 lb bags.
  • Using slab square footage for a footing, pier, or tube instead of calculating cubic volume from the actual shape.
  • Rounding down because the decimal looks close to a whole bag.
  • Counting only bag price while forgetting mixer rental, forms, base material, reinforcement, finishing tools, and curing supplies.

FAQ

How do I calculate concrete bags?

Calculate the waste-adjusted concrete volume in cubic feet, divide by the selected bag yield in cubic feet, then round up to the next whole bag.

How much concrete is in an 80 lb bag?

This calculator uses 0.60 cu ft per 80 lb bag as the planning default. Check the bag you are buying because yield varies by product and water ratio.

How many 60 lb concrete bags equal 80 lb bags?

Under the current calculator assumptions, a 60 lb bag yields 0.45 cu ft and an 80 lb bag yields 0.60 cu ft. For the same volume, multiply the 80 lb bag count by about 1.33 and round up when switching to 60 lb bags.

How many 80 lb bags do I need for an 8x8 slab?

For an 8 ft by 8 ft slab at 4 in thick with 10% waste, the generated example estimates 23.5 cu ft of concrete and 40 standard 80 lb bags.

How many bags for a 20 ft footing?

For a 20 ft continuous footing that is 12 in wide by 8 in deep with 10% waste, the generated footing example estimates 25 standard 80 lb bags.

Should I buy extra concrete bags?

Yes. A waste buffer helps cover uneven forms, over-excavation, spillage, and small measuring errors. The calculator rounds up after adding waste.

When are concrete bags too many?

There is no single cutoff, but dozens of bags can be slow to mix and place consistently. Compare ready-mix or a rented mixer when timing, finish quality, or labor matters.

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