THE NUMORIX GUIDE
How to use the Concrete Calculator
Last reviewed September 14, 2026
What this calculator does
The engine treats the project as a rectangular prism.
Formula and method
The engine treats the project as a rectangular prism. It converts depth from inches to feet, calculates cubic feet as length x width x depth / 12, divides by 27 for cubic yards, and estimates bags using 0.6 cubic ft per 80 lb bag or 0.45 cubic ft per 60 lb bag. Bag counts are rounded up to whole bags.
Variables and inputs
Enter length and width in feet and slab or pour depth in inches. The result includes cubic feet, cubic yards, and whole 80 lb and 60 lb bag counts. The UI emphasizes cubic yards, cubic feet, and 80 lb bags, but the engine also calculates the 60 lb alternative.
Worked example
For a 10 ft x 10 ft area at 4 in deep, volume is 10 x 10 x (4 / 12) = 33.3333 cubic ft. Using that unrounded volume, cubic yards are 33.3333 / 27 = 1.235 yd3. The estimates are ceil(33.3333 / 0.6) = 56 80 lb bags and ceil(33.3333 / 0.45) = 75 60 lb bags.
How to interpret the result
Cubic yards are the bulk volume to discuss with a ready-mix supplier. Bag counts are purchase estimates based on the engine's assumed bag yields, so rounding up gives a whole-bag minimum under those assumptions.
Common mistakes to avoid
Enter depth in inches, not feet, because the engine divides the depth by 12. Keep both plan dimensions in feet, and do not confuse a bag's weight with its yield; 80 lb and 60 lb bags use different assumed volumes.
Assumptions and limitations
The calculation covers only a uniform rectangular volume. It does not add waste, account for sloped or uneven subgrade, reinforcement, forms, finishing losses, moisture, or the actual yield printed on a product bag. Verify the supplier's yield and order a practical reserve for a real pour.