Concrete Volume Calculator
Concrete is sold by volume and unforgiving once poured: order short and the slab gets a cold joint, order long and you pay for a truck to haul the extra away. The math itself is simple, length times width times thickness divided by 27, but the mistakes hide in the details: inches that never got converted to feet, no allowance for an uneven subgrade, and bag counts guessed instead of calculated from real yield figures.
This calculator handles slabs, footings and walls, and round columns or footings, then converts the result into cubic yards for ready-mix orders, bag counts for DIY pours, and total weight for hauling. It also tells you which side of the one-yard line your project falls on, because that line decides whether you call a plant or a hardware store. If you are pricing a whole concrete home rather than one pour, see our concrete house cost calculator for the bigger picture.
Concrete Volume Calculator
Quick Answer
Quick answer: a 20 by 10 ft patio slab at 4 inches thick needs 66.67 cubic feet of concrete, or 2.64 cubic yards with a 7 percent waste allowance. That is 119 eighty-pound bags, or about 10,700 lb, which puts it firmly in ready-mix territory: ordering a truck is cheaper and far less work than mixing 119 bags by hand.
The 324 shortcut
Every concrete volume formula is the same idea: multiply the three dimensions, then convert to the unit the supplier sells. Ready-mix is sold by the cubic yard, and one cubic yard is 27 cubic feet. When your thickness is in inches, the two conversions collapse into one memorable number:
Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 324
Why 324: one cubic yard spread one inch deep covers 27 × 12 = 324 square feet.
For the worked example: 20 × 10 × 4 ÷ 324 = 2.47 cubic yards before waste, matching the calculator. The 324 shortcut only fits rectangles, which is why the tool switches formulas for footings and round columns.
Formulas by shape
| Shape | Volume formula | Used for |
|---|---|---|
| Slab | L × W × (T in inches ÷ 12) | Patios, driveways, floors |
| Footing / wall | Length × width × depth, all in feet | Strip footings, stem walls |
| Round column | π × (diameter ÷ 2)² × height | Piers, sonotubes, round footings |
Steps and L-shaped slabs are the two shapes this tool does not model directly. For an L-shape, split it into two rectangles and add the results. For steps, treat each step as a block from the ground to its tread and add the landing separately.
How far does one cubic yard go?
This is the fastest sanity check on any ready-mix order: divide your slab area by the coverage figure for its thickness. If the plant quotes something wildly different, recheck your measurements before you recheck their math.
| Thickness | Sq ft per cubic yard | Yards per 100 sq ft | Typical use |
|---|---|---|---|
| 3 in | 108 | 0.93 | Thin pad on firm ground |
| 4 in | 81 | 1.23 | Walkway, patio, shed floor |
| 5 in | 64.8 | 1.54 | Residential driveway |
| 6 in | 54 | 1.85 | Garage floor, heavy driveway |
| 8 in | 40.5 | 2.47 | Machinery pad, footings |
Bag yields: what each bag actually makes
Bags are sold by weight but concrete is used by volume, and the conversion is the number most DIY estimators get wrong. Per the manufacturer's own data sheet for standard 4000-psi mix, the yields are:
| Bag size | Yield (cu ft) | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 | 45 |
| 60 lb | 0.45 | 60 |
| 40 lb | 0.30 | 90 |
Yields come straight from the bag: the dry weight includes air that collapses when water hits it, so an 80 lb bag makes 0.60 cubic feet, not the 0.8 you might guess from the weight ratio. The calculator divides your waste-adjusted volume by these yields and rounds up, because suppliers do not sell fractional bags.
Bagged or ready-mix: the one-yard line
Bagged mix: under ~1 cubic yard
- No truck scheduling, no minimums, no washout area needed
- Consistent small batches you control
- Downside: 45 eighty-pound bags per yard, mixed one wheelbarrow at a time
Ready-mix: over ~1 cubic yard
- Cheaper per yard, uniform mix, placed in minutes
- One delivery beats a full day of bag mixing
- Watch for: delivery minimums, short-load fees, and site access for the truck
Right around one yard, price both: a plant's minimum delivery charge can exceed the cost of 45 bags, while a big-box bulk discount can undercut a short load. The calculator's verdict badge flags which side you are on, but the final call is a phone quote.
What competing calculators miss
Most concrete calculators compute one rectangle and stop. They skip the waste allowance entirely, which the National Ready Mixed Concrete Association pegs at 4 to 10 percent over plan dimensions for spillage, over-excavation, and form deflection, and a short order mid-pour is the most expensive mistake in concrete work. They also tend to offer one bag size or none, forcing you to redo the division yourself, and almost none convert to weight, which matters the moment you haul bagged mix in a pickup with a payload limit. This tool bakes the NRMCA allowance into an adjustable input, carries all three standard bag yields from the manufacturer's data sheet, and reports weight alongside volume so the number you take to the supplier is the number you actually need.
One more conversion that trips people up: suppliers quote some materials by the ton and others by the yard. Our weight to volume calculator handles that translation for aggregates and soils. And if the slab is part of a larger project, the house build cost calculator puts the concrete line in context with every other trade.
Frequently asked questions
10 × 10 × 4 ÷ 324 = 1.23 cubic yards before waste. With a 7 percent allowance that is 1.32 yards, or about 60 eighty-pound bags. You are just over the one-yard line, so get a ready-mix quote before committing to bags.
Order 4 to 10 percent over your calculated volume, per NRMCA guidance. Use the low end for simple rectangles on well-prepared subgrade and the high end for irregular shapes, sloped sites, or hand-dug footings. Running short mid-pour costs far more than the extra half yard.
Forty-five. An 80 lb bag of standard mix yields 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 bags. Sixty-pound bags yield 0.45 cubic feet (60 per yard) and 40 lb bags yield 0.30 cubic feet (90 per yard).
Four inches suits walkways, patios, and shed floors. Use five to six inches for residential driveways and garage floors that carry cars, and six to eight inches for heavy vehicles or machinery pads. Thickness is a structural decision: when in doubt, ask your local building department what the code requires.
Use π × radius² × height with all dimensions in feet, then divide by 27 for cubic yards. A 12 inch diameter pier 3 ft tall is π × 0.5² × 3 = 2.36 cubic feet, or 0.087 cubic yards: about four 80 lb bags.
About 4,000 lb, or 2 tons. Normal-weight concrete runs roughly 150 lb per cubic foot, so 27 cubic feet comes to just over two tons. That is why ready-mix trucks carry 8 to 10 yards and not more: the legal weight limit arrives before the drum is full.
Under about one cubic yard, bags usually win because ready-mix plants charge delivery minimums and short-load fees. Over one yard, ready-mix is cheaper per unit and enormously less labor. Near the boundary, price both: 45 eighty-pound bags at retail can still beat a minimum delivery charge.
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References
- National Ready Mixed Concrete Association, Concrete in Practice CIP 8: Discrepancies in Yield: order 4 to 10 percent over plan dimensions. Accessed October 2026.
- QUIKRETE, Concrete Mix No. 1101 Specification Data: bag yields of 0.60, 0.45, and 0.30 cu ft; ASTM C387 compliance. Accessed October 2026.
- Calculating Concrete, Types of Quikrete: Which Bag and Mix to Use for the Job: mix selection by set time and strength. Accessed October 2026.
- Engineer Fix, How Much Extra Concrete Should You Order?: 5 to 10 percent standard safety margin. Accessed October 2026.
- Rocket Concrete Company, How to Order Ready-Mix Concrete: Quantity, PSI, Slump and Delivery Basics: ordering and delivery guidance. Accessed October 2026.