Concrete Calculator
Calculate how much concrete you need for a slab, footing, column, or circular slab. Results shown in cubic yards, cubic feet, and pre-mixed bags.
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Cubic yards
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Cubic feet
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Cubic metres
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Litres
Pre-mixed bags needed
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40 lb bags
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60 lb bags
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80 lb bags
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25 kg bags
Note: pre-mixed bag counts include the waste factor. Yields: 40 lb ≈ 0.30 ft³ · 60 lb ≈ 0.45 ft³ · 80 lb ≈ 0.60 ft³ · 25 kg ≈ 0.33 ft³
How Much Concrete Do I Need?
Concrete volume is calculated by multiplying the three dimensions of the area being filled — length × width × depth for slabs, or π × radius² × height for round footings and columns. The result is converted to cubic yards (the standard unit for ordering ready-mix concrete) or to pre-mixed bag counts for smaller jobs.
Slab Thickness Guide
| Use | Typical Thickness |
|---|---|
| Sidewalk / path | 3–4 inches |
| Residential driveway | 4 inches |
| Garage floor | 4–6 inches |
| Patio | 4 inches |
| Structural slab | 6+ inches |
When to Use Ready-Mix vs. Bags
Pre-mixed bags (40 lb, 60 lb, 80 lb) are cost-effective for small jobs under about 1 cubic yard. Mixing by hand is labour-intensive but works well for fence posts, small footings, and repair patches.
Ready-mix concrete (delivered by truck) is more economical for jobs over 1 cubic yard. Most suppliers have a minimum order of 1 yard, and short loads (under about 5 yards) may carry a surcharge. Call your local batch plant to confirm.
Reinforcement
Concrete slabs and walls often require steel reinforcement (rebar or wire mesh) to resist cracking. A standard residential slab typically uses #3 or #4 rebar on an 18-inch grid, or 6×6 wire mesh. The concrete calculator covers volume only; reinforcement quantity is separate.
Frequently Asked Questions
How do I convert cubic yards to bags? One cubic yard of concrete is 27 cubic feet. An 80 lb bag yields about 0.60 ft³, so you need approximately 45 bags per cubic yard. A 60 lb bag yields about 0.45 ft³ (about 60 bags per yard). Use the bag counts from this calculator for a quick answer.
Should I add extra concrete? Yes — always add a waste factor. Uneven subgrades, spillage, and forms that aren't perfectly dimensioned all consume concrete beyond the calculated volume. 10% extra is the standard recommendation for most slab jobs.
What is a "short load" charge? Ready-mix suppliers charge a "short load" or "small load" surcharge when you order less than a full truck load (typically 8–10 cubic yards). If you need only 1–4 yards, get a quote that includes any short-load fee before deciding between ready-mix and bags.
Can I mix concrete in cold weather? Concrete should not be poured when the ambient or ground temperature is below 40°F (4°C). Cold slows the curing reaction; freezing before cure is complete can ruin the slab. Heated enclosures and cold-weather admixtures are options for winter work. If your project also involves a gravel sub-base or drainage layer, the gravel calculator can help you estimate that material separately.
