Concrete bags calculator
Estimate how many bags of concrete mix a slab needs.
What this calculator covers
Estimate how many bags of concrete mix a rectangular slab needs based on slab size and bag size.
The walkthrough keeps the volume math visible so the bag estimate can be checked before buying material.
Frequently asked questions
- Which bag size should I choose — 40, 60, or 80 lb?
- For small pours or repairs, 40 lb bags are easiest to handle. Sixty-pound bags are the most common general-purpose choice. Eighty-pound bags reduce the number of bags to buy and mix but are heavy to move. The calculator supports all three and adjusts the bag count based on the yield of each size.
- Why does the calculator round up instead of giving an exact decimal?
- You cannot buy a fraction of a bag, so the result rounds up to the next whole number. This also builds in a small buffer for mixing waste, spillage, and measurement imprecision in the slab dimensions.
- Does the bag count include a waste allowance?
- No. The estimate is based on the theoretical slab volume divided by the bag yield with no extra waste factor. Adding 5–10% to the bag count as a buffer is common practice for outdoor pours where material loss is more likely.
- What is the bag yield figure based on?
- The calculator uses standard approximate yields published for bagged premix concrete: roughly 0.30 cu ft for a 40 lb bag, 0.45 cu ft for a 60 lb bag, and 0.60 cu ft for an 80 lb bag. Actual yield can vary slightly by brand and water ratio.
Tool
Run the calculation
Result
RESULT · CONCRETE BAGS
â„–012
Primary result
89 bags
A 12 ft by 10 ft slab at 4 in deep needs about 89 bags of 60 lb mix.
- Concrete volume
- 40 cu ft
- Bag yield
- 0.45 cu ft/bag
- Bags needed
- 89 bags
Step-by-step solution
- 1.Convert the slab depth to feet: 4 in ÷ 12 = 0.3333 ft.
- 2.Multiply length × width × depth to get a slab volume that displays as 40 cubic feet.
- 3.Divide the exact slab volume by the 60 lb bag yield of 0.45 cubic feet and round up to 89 bags.
Walkthrough
Visual walkthrough
Bag count planning starts with the same slab volume as ready-mix planning, then converts that volume into individual bags.
01
Convert slab depth into feet
4 ÷ 12 = 0.3333 ft
Length, width, and depth need the same base unit before you can compute volume.
02
Find total concrete volume
12 × 10 × 0.3333 = 40 cu ft
This is the slab volume as displayed after rounding.
03
Convert volume into bags
Each bag size yields a different amount of cured concrete, so the exact slab volume is divided by bag yield and rounded up.
89 bags of 60 lb mix