Concrete Calculator
Volume in cubic feet, divided by 27, gives cubic yards.
Compiled by DIY Repair Guide · reviewed 2026-11-03 ·how we research these
Length in feet times width in feet times thickness in inches, divided by 12, gives concrete volume in cubic feet, so a 10 ft by 10 ft slab 4 inches thick is 33.33 cubic feet, or 1.23 cubic yards. At DIY Repair Guide, we compiled the yield figures below from the manufacturer's data sheet.
The calculator sits directly under this intro. For a rectangle it asks for length in feet, width in feet and thickness in inches. For a round column or post hole it asks for diameter and depth or height in inches. Both shapes take a count of identical pieces, an extra percentage for waste (10 by default), and a bag size of 40, 50, 60, 80 or 90 lb (80 lb by default). It loads with a 10 ft by 10 ft by 4 in rectangle, one piece, 10 percent extra and 80 lb bags. The results show volume with no extra and with extra, in cubic feet and cubic yards, the bag count rounded up for the size you chose, and the dry mix weight, which is bags times bag weight. The tool has Copy result and Print buttons. The tool makes no network calls.
How to Calculate Concrete
Volume comes first, because both the cubic yard figure and the bag count are built from it. Measure each side with a tape measure.
Take a 10 ft by 10 ft slab 4 inches thick. Multiply 10 by 10 by 4, then divide by 12, and the volume is 33.33 cubic feet. Divide 33.33 by 27 for 1.23 cubic yards. Divide 33.33 by 0.60 and the bag count is 55.6, which rounds up to 56 bags of 80 lb with no extra.
Add 10 percent extra and the order amount becomes 36.67 cubic feet, or 1.36 cubic yards. Divide 36.67 by 0.60 and the count is 61.1, so the order is 62 bags of 80 lb. The tool shows the volume with no extra in the first results row and the order amount with extra in the second.
We ran the same formulas on seven example pours with the manufacturer's bag yields and saved the output. No pour was made and no bag was tested.
The bag count always rounds up. A result of 0.60 cubic feet needs one bag of 80 lb, and 0.61 cubic feet needs two.
The same arithmetic in three steps:
- Cubic feet. Multiply length in feet by width in feet by thickness in inches, then divide by 12.
- Cubic yards. Divide cubic feet by 27, because one cubic yard is 27 cubic feet.
- Bags. Divide the order amount in cubic feet by the bag yield, then round up to a whole bag. An 80 lb bag yields 0.60 cubic feet according to the Quikrete Concrete Mix No. 1101 data sheet.
Bags or Ready-Mix by the Numbers
One cubic yard is 45 bags of 80 lb, and those 45 bags weigh 3,600 lb.
The table shows three example pours at 4 inches thick with 10 percent extra.
| Example pour | Cubic yards with 10 percent extra | Bags of 80 lb |
|---|---|---|
| 4 ft x 4 ft x 4 in pad | 0.22 | 10 |
| 10 ft x 10 ft x 4 in slab | 1.36 | 62 |
| 12 ft x 24 ft x 4 in slab | 3.91 | 176 |
Bag Yield by Size
The Quikrete data sheet lists five bag sizes and gives each yield as approximate. The yield is the volume one bag produces, in cubic feet.
Every bag size adds up to 3,600 lb of bags per cubic yard: 90 bags of 40 lb, 72 of 50 lb, 60 of 60 lb, 45 of 80 lb and 40 of 90 lb. Only the count changes with bag size.
The data sheet notes that the 90 lb size has regional availability.
For the 10 ft by 10 ft by 4 in slab with 10 percent extra, the bag counts by size are:
- 40 lb: 123 bags
- 50 lb: 98 bags
- 60 lb: 82 bags
- 80 lb: 62 bags
- 90 lb: 55 bags
| Bag size | Yield per bag (cubic feet) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
| 90 lb | 0.675 | 40 |
How Much Extra to Order
The calculator adds 10 percent extra by default. That figure is our own planning assumption with no published standard behind it, and the extra percentage can be changed in the tool.
Yield is approximate and depends on how much water goes in, so every bag count here is an estimate.
On the 10 ft by 10 ft by 4 in example, the extra moves the count from 56 bags to 62, a difference of 6 bags of 80 lb.
These conditions call for more than 10 percent extra:
- An uneven subgrade under the pour.
- A hole dug past its planned size.
- Forms that leak.
How Thick to Pour
The 2025 California Residential Code, section R506.1, requires slab-on-ground concrete floors to be not less than 3 1/2 inches thick. Section R506.3.2 calls for a 4-inch base course under below-grade slabs, with exceptions. That is one state's code, effective January 1, 2026, and adoption and edition vary by locality.
Check your local code for the thickness a patio, driveway or garage floor needs.
Thickness drives the bag count. The 10 ft by 10 ft slab at 3.5 inches needs 1.08 cubic yards, or 1.19 with 10 percent extra, and 54 bags of 80 lb. At 4 inches the same slab needs 62 bags. Dropping from 4 inches to 3.5 inches cuts the order by 8 bags in this example.
The data sheet says the mix is designed for pouring concrete 2 inches thick or more. A pour thinner than 2 inches falls outside the thickness the mix is designed for.
One cubic yard covers a different area at each thickness. The coverage table follows the list below.
The data sheet's slab method runs in four steps:
- Dampen the sub-grade.
- Fill the form to full depth.
- Strike off and float.
- Cut control joints 1 inch deep every 6 to 8 feet.
| Thickness (inches) | Area one cubic yard covers (sq ft) |
|---|---|
| 3 | 108 |
| 3.5 | 92.57 |
| 4 | 81 |
| 5 | 64.8 |
| 6 | 54 |
Round Columns and Post Holes
For a round column or post hole, the tool uses pi times (diameter in inches divided by 24) squared times (height in inches divided by 12), which gives cubic feet. Dividing the diameter in inches by 24 converts it to a radius in feet.
A post hole 10 inches in diameter and 36 inches deep is 1.64 cubic feet. With 10 percent extra it needs 3 bags of 80 lb.
A post hole 12 inches in diameter and 42 inches deep is 2.75 cubic feet, or 0.10 cubic yards. With 10 percent extra it needs 6 bags of 80 lb.
A round hole's real volume is bigger than the formula when the hole is wider than its tube or form. The formula uses only the diameter you enter, so a wider excavation adds volume the calculation does not count.
Mixing Quantities
The data sheet says the mix needs only water. For each 80 lb bag, add about 6 pints of water to start. The final water is about 6 to 9 pints per 80 lb bag.
For one cubic yard, which is 45 bags, that works out to 270 pints of water to start and 270 to 405 pints in the final mix.
The data sheet also says to allow at least 1 cubic foot of mixer capacity for each 80 lb bag mixed at one time. A cubic yard in 80 lb bags therefore needs at least 45 cubic feet of mixer capacity if all 45 bags go in together. The 62-bag order for the 10 ft by 10 ft slab needs at least 62 cubic feet if mixed in one batch.
The yields are approximate, so the water and mixer figures are estimates tied to the bag count.
When the Concrete Calculator Result Is Wrong for You
The calculator covers two shapes, a rectangle and a round column or post hole. A pour in any other shape needs a calculation the tool does not provide.
A supplier is the better route in these cases:
- A pour with an unusual shape, since the tool models only the rectangle and round shapes.
- A structural or load-bearing pour, because the mix design and strength for those loads are not known to us.
- Any count past 90 bags, where the tool suggests ready-mix delivery or a rented mixer.
The tool shows its ready-mix note above 90 bags. Delivery costs, truck capacity and mixer rental terms are not in our sources, so the note does not size either option.
The Short Answer
Multiply length in feet by width in feet by thickness in inches, divide by 12 for cubic feet, divide by 27 for cubic yards, then divide the order amount in cubic feet by the bag yield and round up. Add extra for waste. The 10 percent default is a planning assumption.
Below one cubic yard, the 80 lb bag count stays under 45 before extra. At one cubic yard it is 45 bags, which weighs 3,600 lb to mix and move, so ask a supplier about delivered concrete before you buy bags. The tool's own ready-mix note appears above 90 bags. We cannot price it.
Basis: calculated. Yields are approximate. Price and truck minimums are not known to us. Odd shapes and structural loads need a supplier or the local building department.
Common questions
How many 80 lb bags of concrete are in a yard?
There are 45 bags of 80 lb in one cubic yard. One cubic yard is 27 cubic feet and an 80 lb bag yields 0.60 cubic feet, so 27 divided by 0.60 is 45. Those 45 bags weigh 3,600 lb.
How much does a yard of concrete cover?
At 4 inches thick, one cubic yard covers 81 square feet. The same yard covers 108 square feet at 3 inches and 54 square feet at 6 inches. We calculated these from the 27 cubic foot yard. No published coverage figure is behind them.
How many bags of concrete do I need for a 10x10 slab?
For a 10 ft by 10 ft slab 4 inches thick, you need 62 bags of 80 lb with 10 percent extra, or 56 bags with no extra. At 3.5 inches thick the count is 54 bags with extra. The 10 percent figure is our planning default.
How much does a yard of concrete weigh?
About 3,780 lb, using the data sheet unit weight of about 140 lb per cubic foot multiplied by 27 cubic feet. Bags give a different figure: 45 bags of 80 lb weigh 3,600 lb.
How much concrete is in a truck?
Our sources give no truck capacity or minimum load, so we state neither. The cubic yard result from the calculator is the figure to set against whatever truck size a supplier quotes.
What is the right mix for concrete?
The right mix depends on the load, and our sources do not cover mix design for structural loads. The Quikrete Concrete Mix No. 1101 data sheet describes a pre-blended mix for general structural use that needs only water, designed for pours 2 inches thick or more. Its typical compressive strength results are 2,500 psi at 7 days and 4,000 psi at 28 days.