Use this free concrete calculator to estimate the volume of concrete needed for a rectangular slab, patio, pad, footing or trench. Enter length, width and thickness, then choose an extra allowance for site conditions and small measuring differences.
HomeCalc shows cubic yards, cubic feet and cubic meters and also estimates how many 40 lb, 60 lb or 80 lb bags of standard concrete mix would be required.
Concrete volume is length × width × thickness. For US projects, convert the result to cubic feet and divide by 27 to get cubic yards. HomeCalc performs those conversions and can add an optional allowance automatically.
FREE CONCRETE ESTIMATOR
Concrete Calculator
Estimate concrete volume for a rectangular slab, pad, footing or trench and see approximate 40 lb, 60 lb and 80 lb bag counts.
Estimated concrete needed
36.67 ft³ · 1.04 m³
Bag counts use approximate yields of 0.30 ft³ (40 lb), 0.45 ft³ (60 lb) and 0.60 ft³ (80 lb), matching published QUIKRETE and Sakrete product data. Verify the yield printed on your chosen mix.
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How to use the concrete calculator
- Choose Imperial or Metric.
- Enter project length and width.
- Enter concrete thickness. Imperial uses inches; Metric uses centimeters.
- Select an extra allowance. Choose 0% for pure geometric volume.
- Optionally enter a price per cubic yard for a rough ready-mix material estimate.
- Use cubic yards or bag count as a planning figure and verify it with your supplier or product label.
Concrete volume formula
For a rectangular slab, pad, footing or trench, the core calculation is straightforward:
length × width × thickness = volumeQUIKRETE uses the same method and gives a 10 ft × 6 ft × 4 in slab as an example. Converting 4 inches to about 0.33 ft gives roughly 20 cubic feet of concrete.
How to calculate cubic yards of concrete
There are 27 cubic feet in one cubic yard. With dimensions in feet, calculate cubic feet first and divide by 27.
cubic feet ÷ 27 = cubic yardsIf slab thickness is measured in inches, divide thickness by 12 before multiplying by length and width in feet.
How many bags of concrete do I need?
Bag count depends on the yield of the exact mix. Published QUIKRETE and Sakrete data give approximate yields of 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb bags.
| Bag size | Approx. yield | Approx. bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
These yields are approximate. Specialty or lightweight mixes can differ, so the package data for the product you buy takes priority.
Concrete calculator examples for common slab sizes
The table below shows geometric volume before any extra allowance.
| Slab size | Thickness | Volume | Cubic yards |
|---|---|---|---|
| 8 × 8 ft | 4 in | 21.33 ft³ | 0.79 yd³ |
| 10 × 10 ft | 4 in | 33.33 ft³ | 1.23 yd³ |
| 10 × 12 ft | 4 in | 40.00 ft³ | 1.48 yd³ |
| 12 × 12 ft | 4 in | 48.00 ft³ | 1.78 yd³ |
| 12 × 20 ft | 4 in | 80.00 ft³ | 2.96 yd³ |
Why add extra concrete?
Theoretical volume does not account for uneven substrate, waste and job-site differences. Some manufacturer calculators recommend about 10% extra on certain pours, so HomeCalc starts at a 10% planning allowance but lets you choose 0%, 5%, 10%, 15% or 20%.
The right allowance depends on how closely excavation and forms match the measurements. Choose 0% when you want pure geometric volume.
Bagged concrete or ready-mix?
Bagged mix can be practical for small pours and repairs. As volume increases the number of bags rises quickly: one cubic yard is about 45 standard 80 lb bags at 0.60 ft³ per bag before allowance.
For larger pours compare bag count, mixing effort and delivery with local ready-mix. The optional price-per-cubic-yard field is only a rough material comparison and excludes delivery, short-load charges, pumping, labor, tax and other supplier costs.
Before ordering concrete, check these details
- Thickness: make sure the chosen thickness suits the intended application and local requirements.
- Actual dimensions: measure the formed or excavated area rather than relying only on planned dimensions.
- Bag yield: verify the exact yield printed on the mix you plan to buy.
- Uneven subgrade: variations in excavation depth can increase required volume.
- Supplier minimums: ready-mix suppliers may have minimum loads or extra charges.
- Project requirements: structural or load-bearing work may require professional design, reinforcement or local permits.
Concrete calculator FAQ
How do I calculate how much concrete I need?
How much concrete is in one cubic yard?
How many 80 lb bags make a cubic yard?
How many 60 lb bags make a cubic yard?
How many 40 lb bags make a cubic yard?
How much for a 10 × 10 ft slab at 4 in thick?
Does HomeCalc add waste automatically?
Can I use metric measurements?
Sources and methodology
HomeCalc uses basic volume geometry and published bag yields. Bag estimates are not universal product specifications; always verify the yield of the concrete mix you choose.