Concrete Calculator
Calculate concrete volume, bag count, and cost for slabs, footings, and columns.
Last reviewed: June 2026How the Concrete Calculator Works
The calculator converts project dimensions into cubic feet, then divides by 27 to convert cubic feet to cubic yards. A rectangular slab or footing uses length x width x depth. Because the depth input is in inches, the calculator first converts depth to feet by dividing by 12. Circular slabs and columns use the area of a circle, pi x radius squared, then multiply by thickness or height.
For the default 10 ft x 10 ft x 4 in rectangular slab, the volume is 10 x 10 x (4 / 12) = 33.33 cubic feet. Since one cubic yard is 27 cubic feet, the ready-mix volume is 33.33 / 27 = 1.23 cubic yards. The built-in ready-mix estimate uses $130 per cubic yard, so the default cost estimate is $160.49 before delivery fees, short-load fees, taxes, reinforcement, base material, or finishing tools.
Bag Yield Assumptions
The bag count uses common manufacturer yield assumptions: an 80 lb concrete mix bag yields about 0.60 cubic feet, and a 60 lb bag yields about 0.45 cubic feet. For the default 33.33 cubic-foot slab, that means ceil(33.33 / 0.60) = 56 bags of 80 lb mix, or ceil(33.33 / 0.45) = 75 bags of 60 lb mix. QUIKRETE training materials publish the same yield values for estimating bagged concrete.
Shape Inputs
Use rectangular slab for patios, pads, sidewalks, and driveways with straight sides. Use footing when the math is the same but the label better matches a trench or foundation strip. Use circular slab for round pads and column/post for cylindrical piers. For columns, the diameter and height are both entered in feet, while slab thickness remains in inches.
Why Real Orders May Be Higher
The calculator does not automatically add waste. Real sites often need a small extra allowance because excavated areas are rarely perfectly flat, forms may bow, sub-base thickness can vary, and some concrete is lost during placement. A 4 inch slab that is actually 4.5 inches thick uses 12.5% more concrete. If ordering ready-mix, suppliers may also have minimum loads or short-load fees.
Common Mistakes
The most expensive mistake is mixing inches and feet. A 10 ft x 10 ft slab at 6 inches thick is 50 cubic feet, not thousands of cubic yards. Another common issue is using the finished surface size but ignoring thickened edges, turndowns, piers, or footings. For structural work, the calculator can estimate quantity, but the slab design, reinforcement, subgrade, drainage, and local code requirements still need project-specific review.