Concrete Calculator
Calculate cubic yards, bag count, and ready-mix cost for slabs, footings, columns, and stairs.
Concrete Needed (with 10% waste)
1.36 CY
33.33 cubic feet net
| Volume (net) | 33.33 ft³ |
| Cubic yards (net) | 1.23 CY |
| With 10% waste | 1.36 CY |
| 80 lb bags needed | 62 bags |
| 60 lb bags needed | 82 bags |
| Rebar suggestion | #4 rebar at 18" OC |
Methodology
Volume = length × width × depth. Cubic yards = cubic feet ÷ 27. Ready-mix price based on national BLS PPI average of $165/CY. Always add 5–10% for waste, spillage, and form seating. Bag counts use 80 lb = 0.60 cf and 60 lb = 0.45 cf of finished concrete. Column formula uses V = πr²h. Stair volume uses a triangular prism approximation: each step occupies half a rectangular block.
Frequently Asked Questions
How many cubic yards do I need for a 10×10 slab at 4 inches?
What is the formula for concrete volume?
How much does ready-mix concrete cost?
80 lb vs 60 lb bags — which should I buy?
How the Concrete Calculator works
The concrete calculator converts your slab, footing, column, or stair dimensions into cubic yards, bag counts, and a ready-mix cost estimate. Every shape reduces to a volume in cubic feet, which is then divided by 27 to get cubic yards, since there are 27 cubic feet in a cubic yard.
For a rectangular slab the volume is length × width × thickness, with thickness converted from inches to feet by dividing by 12. A footing uses length × width × depth in feet. A column or pier uses the cylinder formula V = π × r² × height, where the radius is the diameter in inches divided by 24, then multiplied by the number of columns. Stairs use a triangular-prism approximation: each step occupies half a rectangular block, so the volume is steps × (rise ÷ 12) × (run ÷ 12) × stair width ÷ 2.
Once the net volume is known, the tool adds a 10% waste factor (× 1.10) to cover spillage, over-excavation, and form seating. Bag counts assume an 80 lb bag yields 0.60 cubic feet and a 60 lb bag yields 0.45 cubic feet of finished concrete. The ready-mix estimate multiplies the cubic yards (with waste) by $165 per cubic yard. When the pour exceeds 2 cubic yards the calculator flags ready-mix as the practical choice.
Worked example: a 24 ft × 20 ft slab poured 6 inches thick works out to 24 × 20 × 0.5 = 240 cubic feet, or 240 ÷ 27 = 8.89 cubic yards net. Adding 10% waste gives 9.78 cubic yards. At $165 per cubic yard that is about $1,613 of ready-mix concrete, and because the slab is 6 inches thick the tool suggests #5 rebar at 12 inches on center.
Frequently Asked Questions
How many 80 lb bags of concrete are in a cubic yard?
An 80 lb bag yields 0.60 cubic feet, and a cubic yard is 27 cubic feet, so it takes 27 ÷ 0.60 = 45 bags of 80 lb concrete to fill one cubic yard. A 60 lb bag yields 0.45 cubic feet, so you would need 60 bags per cubic yard. This is why ready-mix becomes far more practical once you pass roughly 2 cubic yards.
How much does the 10% waste factor add?
The calculator multiplies your net volume by 1.10, adding 10% for spillage, uneven subgrade, and concrete that seats into the forms. On a 5 cubic yard net pour that adds 0.5 cubic yards. Ordering short and running out mid-pour causes a cold joint, so a waste allowance of 5 to 10% is standard practice.
When should I order ready-mix instead of bags?
The tool flags ready-mix once the pour with waste exceeds 2 cubic yards. At 45 bags per cubic yard, a 2 cubic yard pour is about 90 eighty-pound bags, or roughly 3,600 lbs of material to mix by hand. Beyond that threshold, truck-delivered ready-mix is faster, cheaper, and gives a more consistent mix.
How does the calculator find the volume of a round column?
It uses the cylinder formula V = π × r² × height. The radius is the column diameter in inches divided by 24, which both halves the diameter and converts it to feet. That volume is multiplied by the number of columns. For example, a 12-inch diameter column 8 feet tall holds π × 0.5² × 8 = 6.28 cubic feet each.