How to measure
Use these guides to identify the measurements the calculator needs. Drawings are not to scale.
Round post: D is the diameter through the center, not the radius.
Check the unit beside every field. For example, 10 ft 6 in is 10.5 ft. A layer depth may use inches while length uses feet.
Turn project dimensions into concrete quantities
Choose a slab, rectangular footing, or round post. Enter its dimensions and how many identical shapes you have. The main answer gives bulk volume; the breakdown compares 80, 60, and 40 lb bags side by side.
For a 10 × 10 ft slab, 4 inches thick, the measured volume is 33.33 cubic feet. With 10% extra it is 36.67 cubic feet, about 1.36 cubic yards. That is 62 bags of 80 lb, 82 bags of 60 lb, or 123 bags of 40 lb. These are alternatives, not quantities to add together.
How it works
Slabs and rectangular footings use length × width × depth. A round post uses π × (diameter ÷ 2)² × height. The calculator multiplies by the number of identical shapes, then adds the extra percentage. It converts 27 cubic feet to one cubic yard, or uses the exact foot-to-meter conversion for metric results.
Bag counts use approximate yields from the QUIKRETE Concrete Mix No. 1101 data sheet (PDF): 0.60 cubic feet for 80 lb, 0.45 for 60 lb, and 0.30 for 40 lb. Each final count rounds up from the full calculated volume. A different mix or regional package may have a different yield; check the label before buying.
A full cylinder 12 inches in diameter and 4 ft high has about 3.14 cubic feet of volume. With 10% extra it needs about 3.46 cubic feet, or 6 bags of 80 lb at this yield.
What the estimate leaves out
This estimates material only. It does not design reinforcement, footings, load capacity, thickness, or foundations. Follow the project specifications and applicable building requirements. The round mode is a solid cylinder; it does not subtract a timber or steel post inside a hole.
Use decimal measurements rather than fractions. For different-sized pours, calculate each separately and combine volumes before ordering. Ask your supplier about ready-mix delivery for larger quantities. The extra allowance starts at 10% and accepts 0–50%; it is a planning assumption you can change.