Everyday Tools
Concrete Calculator
Estimate how much concrete a slab, footing or round column needs, in cubic yards, cubic feet and premixed bags.
Calculation inputs
Slabs and footings use length, width and depth. Columns use diameter and height.
Results
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Complete the fields and select Calculate to show results here.
Estimates only. Confirm quantities and structural requirements before ordering.
About this calculator
This calculator estimates the volume of concrete needed for a rectangular slab, patio or footing, or for round columns and post holes. It reports cubic yards, cubic feet and the number of 60-pound or 80-pound premixed bags to buy.
How the calculation works
- Slab: length × width × (depth in ÷ 12) = ft³
- Column: π × (diameter in ÷ 24)² × height = ft³
- Yards = ft³ ÷ 27 · Bags = ft³ ÷ bag yield, rounded up
Notes and assumptions
Bag yields are typical label values; check the yield printed on the mix you buy.
How it works in plain English
Concrete is ordered and sold by volume, so the job is to find how many cubic feet a pour will fill. For a rectangular shape, multiply length by width by depth. Because slab thickness is usually measured in inches while length and width are in feet, the tool divides the depth by 12 to keep every dimension in feet.
Round columns and post holes are cylinders. Their volume is pi times the radius squared times the height. The tool takes the diameter in inches, halves it and converts it to feet in a single step by dividing by 24, then squares it, multiplies by pi and multiplies by the height in feet.
The single-pour volume is multiplied by the number of identical pours, then increased by a waste allowance to cover uneven ground, spillage and a little left in the mixer. Ten percent is a common default. Cubic feet are divided by 27 to give cubic yards, the unit ready-mix suppliers use. For bagged concrete, the total is divided by the typical yield of each bag size and rounded up, since you cannot buy part of a bag. Check the yield printed on the brand you choose, because mixes vary slightly.
The formula
- Slab: length (ft) × width (ft) × depth (in) ÷ 12 = ft³
- Column: π × (diameter (in) ÷ 24)² × height (ft) = ft³
- Yards = ft³ ÷ 27 · Bags = ft³ ÷ bag yield, rounded up
Worked example
Imagine pouring a 10-by-10-foot patio slab that is 4 inches thick. Converting the depth gives 4 ÷ 12, or about 0.333 feet.
The volume is 10 × 10 × 0.333, which equals 33.33 cubic feet. Adding a 10% allowance for waste brings the total to about 36.67 cubic feet. Dividing by 27 gives roughly 1.36 cubic yards.
For bagged mix, an 80-pound bag yields about 0.60 cubic feet, so 36.67 ÷ 0.60 is about 61.1, rounded up to 62 bags. With 60-pound bags at about 0.45 cubic feet each, you would need 82 bags. Because the job is over one cubic yard, comparing the cost and effort of a small ready-mix delivery against mixing that many bags by hand would be sensible.
Typical premixed concrete bag yields
| Bag size | Approx. yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Yields printed on standard premixed concrete bags from major U.S. manufacturers; 1 cubic yard = 27 cubic feet.
Frequently asked questions
How thick should a concrete slab be?
Four inches is a common thickness for patios, walkways and shed floors that carry foot traffic. Driveways and areas supporting vehicles are often poured at five to six inches. Local building codes and the expected load should guide the final choice, and thicker slabs need proportionally more concrete, which this tool will reflect.
Why add extra for waste?
Ground is rarely perfectly flat, forms can bow slightly, and some concrete is always lost to spillage or left in the mixer. Running short in the middle of a pour can create a weak joint, so ordering a little extra is standard practice. Five to ten percent covers most small projects.
Should I use bags or ready-mix delivery?
Bags are practical for small jobs such as post holes, steps or pads under about one cubic yard. Beyond that, the number of bags and the mixing effort climb quickly, and a ready-mix truck often becomes more practical. Some suppliers also offer short-load deliveries for jobs between those sizes.
How many 80 lb bags are in a cubic yard?
An 80-pound bag of standard premixed concrete yields roughly 0.60 cubic feet once mixed, and a cubic yard holds 27 cubic feet. Dividing 27 by 0.60 gives 45 bags per cubic yard. Smaller 60-pound bags yield about 0.45 cubic feet, which works out to 60 bags per cubic yard.
Can I use this for footings and post holes?
Yes. Rectangular footings work exactly like a slab, so enter their length, width and depth. For round post holes or columns, switch to the round option and enter the diameter and height. Set the quantity to the number of identical holes, and the tool totals the volume for all of them.
Helpful next steps
- Volume Calculator — Check the volume of other shapes, such as cylinders for round footings.
- Volume Converter — Turn cubic feet into cubic yards or cubic meters when ordering.
- Length Converter — Convert slab thickness from inches to feet or centimeters.
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