Masonry & Concrete
Concrete Volume Calculator
Stop guessing and over-buying at the hardware store. Calculate exactly how many cubic yards or pre-mixed bags you need for your slab, patio, footing, or fence post holes.
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Estimating Concrete Like a Pro
Concrete is heavy, expensive, and cures fast. Getting your order right the first time is critical.
Order Extra
Never order the exact mathematical volume. Forms bow, subgrades settle, and spills happen. A 10% waste factor is standard in the industry to ensure you don't run short.
Bags vs. Delivery
Mixing 45 bags of concrete by hand for a single cubic yard is back-breaking work. If your project is larger than a yard, call a ready-mix truck. The delivery fee is worth the saved labor.
The Math Behind Concrete Volume
Concrete is always ordered by volume. For large deliveries, it is measured in Cubic YardsCubic Meters. For small jobs, you figure out the raw volume and divide it by the yield of the bag you're buying.
| Project Type | Formula | Notes |
|---|---|---|
| Rectangular Slab | Length × Width × Depth | Convert all measurements to the same unit (e.g. feet or meters) before multiplying to get the cubic volume. |
| Cylindrical Post Hole | π × Radius² × Depth × Count | The radius is half the diameter. Multiply the volume of one hole by the total number of holes you are digging. |
| Waste Factor | Total Volume × 1.10 | Always add 10% to the perfect geometric volume to account for uneven subgrades and spillage. |
| Bags Needed | Total Volume / Bag Yield | An 80lb bag yields 0.60 cu ft. A 60lb bag yields 0.45 cu ft.A 20kg bag yields 0.0094 cubic meters. |
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Frequently Asked Questions
How do I calculate how many bags of concrete I need?
Calculate the total cubic volume of your pour, add a 10% waste factor, and divide by the yield of the bag size you are using. This calculator does all the math for you for slabs and post holes.
Should I buy bags or order a concrete delivery?
A common rule of thumb is that if your project requires more than 1 cubic yard (or roughly 45 80lb bags)1 cubic meter (or roughly 53 40kg bags), it is usually cheaper and much less labor-intensive to order a ready-mix concrete delivery rather than mixing bags by hand.
Why do I need a 10% waste factor?
Uneven subgrades, slight bowing in forms, spillage, and settling all consume more concrete than perfect mathematical geometry suggests. Adding 10% ensures you do not run short in the middle of a pour.
How much volume does an 80lb bag of concrete yield?
A standard 80lb bag of premixed concrete yields approximately 0.60 cubic feet. It takes 45 bags to make one cubic yard.
How much volume does a 20kg bag of concrete yield?
A standard 20kg bag of premixed concrete yields approximately 9.4 liters (or 0.0094 cubic meters). It takes about 107 bags to make one full cubic meter.
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