Concrete bag estimates depend on the volume of the shape and the mixed yield of the product. A rectangular slab uses length, width and thickness. A round post hole containing a round post uses the hole volume minus the post volume before the concrete yield is applied.
The calculator turns those measured inputs into cubic feet, cubic yards or whole bags. It does not select a slab thickness, footing depth, reinforcement plan or structural concrete specification. Those requirements must come from the project design and the product instructions.
Identify the shape being filled
Choose the calculator mode that matches the work:
- For a slab, measure the length and width in feet and the thickness in inches.
- For a post hole, measure the hole radius, post radius and hole depth in feet. A radius is half the diameter.
- For a known concrete volume, enter cubic feet for the conversion mode instead of rebuilding the geometry.
- Separate openings, voids or embedded objects only when their dimensions are part of the calculator mode you are using.
Keep units consistent. The slab formula divides thickness in inches by 12 to convert it to feet. The post-hole mode uses radii in feet and subtracts the round post cross-section from the round hole cross-section.
Enter the product yield from the actual bag
Slab volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12. Bags = ceiling(volume (ft³) ÷ mixed yield (ft³/bag)). Post-hole volume (ft³) = π × [hole radius² − post radius²] × depth.
Concrete input checks by calculator mode
| Mode | Required inputs | Key check |
|---|---|---|
| Slab | Length, width, thickness | Thickness is in inches; divide by 12 before multiplying. |
| Post hole | Hole radius, post radius, depth | Use radii, and subtract the post volume from the hole volume. |
| Volume conversion | Concrete volume in ft³ | This mode converts a known volume in ft³ to yd³. |
| Bag purchase | Mixed yield in ft³/bag | Use the exact product yield; bag weight alone is not a volume yield. |
An 80 lb concrete mix may publish a yield near 0.60 ft³ per bag, but yield varies by product and package. Treat the displayed yield as a product-specific input and replace it when the bag label gives a different value.
Calculate the volume, then round the bag purchase
A slab and a round post hole use different shapes
Worked example: a 12 × 10 ft slab
- Inputs
- The slab is 12 ft long, 10 ft wide and 4 in thick. The selected concrete mix yields 0.60 ft³ per bag.
- Calculation
Volume: 12 × 10 × 4 ÷ 12 = 40 ft³. Bags before rounding: 40 ÷ 0.60 = 66.666666… bags. Final purchase: ceiling(66.666666…) = 67 bags. Independent check: 67 × 0.60 = 40.20 ft³ of stated mixed yield.- Result
- 40 ft³ of concrete; 67 bags
The calculator returns 67 bags for the stated yield. Keep 66.666666… unrounded until the final purchase step. The extra 0.20 ft³ represented by 67 whole bag yields is a package consequence, not a structural allowance.
Enter the slab dimensions and the actual yield in the Concrete Volume and Bag Calculator.
Use the yard measurement guide.
If a large order also needs delivery planning, compare the volume with the Landscape Material Delivery Load Calculator.
Conclusion
Select the correct geometric mode, convert every dimension to the units used by that mode, enter the mixed yield for the actual product and round bags only at the end. For the stated 12 × 10 ft, 4 in slab and 0.60 ft³ yield, the estimate is 40 ft³ and 67 bags.