Use this free concrete calculator to estimate the volume of concrete needed for a rectangular slab, patio, pad, footing or trench. Enter the length, width and thickness, then choose an extra allowance for site conditions and small measuring differences.
HomeCalc gives the result in 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.
How to use the concrete calculator
- Choose Imperial or Metric.
- Enter the project length and width.
- Enter the concrete thickness. In Imperial mode this field uses inches; in Metric mode it uses centimeters.
- Select an extra allowance. Set it to 0% when you only want the geometric volume.
- Optionally enter a price per cubic yard for a rough ready-mix material estimate.
- Use the calculated 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 = volume
QUIKRETE's concrete training material uses the same method and gives a 10 ft × 6 ft × 4 in slab as an example. Converting 4 inches to approximately 0.33 ft gives about 20 cubic feet of concrete. See QUIKRETE's training example.
How to calculate cubic yards of concrete
There are 27 cubic feet in one cubic yard. When your dimensions are in feet, calculate the volume in cubic feet first and divide by 27.
cubic feet ÷ 27 = cubic yards
If the slab thickness is measured in inches, divide the thickness by 12 before multiplying it by the length and width in feet.
How many bags of concrete do I need?
Bag count depends on the yield of the exact mix you buy. QUIKRETE's current Concrete Mix data sheet gives approximate yields of 0.30 ft³ for a 40 lb bag, 0.45 ft³ for a 60 lb bag and 0.60 ft³ for an 80 lb bag. Sakrete publishes the same approximate yields for its High-Strength Concrete Mix.
| Bag size | Approx. yield | Approx. bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 bags |
| 60 lb | 0.45 ft³ | 60 bags |
| 80 lb | 0.60 ft³ | 45 bags |
These values are approximate. Different formulations, lightweight mixes and specialty products can have different yields, so the package data for the product you actually 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?
Manufacturer calculators warn that theoretical yields do not account for uneven substrate, waste and similar job-site differences. Sakrete's concrete calculator explicitly recommends adding 10% for safety on some pours. HomeCalc therefore defaults to a 10% planning allowance but lets you choose 0%, 5%, 10%, 15% or 20%.
The correct allowance depends on how accurately the excavation and forms match your measurements. Set the calculator to 0% when you want to see the pure geometric volume.
Bagged concrete or ready-mix?
Bagged mix can be practical for small pours and repairs. As project volume increases, the number of bags rises quickly: one cubic yard is approximately 45 standard 80 lb bags at 0.60 ft³ per bag before extra allowance.
For larger pours, compare the total number of bags, mixing effort and delivery requirements with ready-mix concrete from a local supplier. HomeCalc's optional price-per-cubic-yard field provides only a rough material comparison and does not include delivery fees, short-load charges, pumping, labor, tax or other supplier-specific costs.
Before ordering concrete, check these details
- Thickness: make sure the project thickness is appropriate for 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 the required volume.
- Supplier minimums: ready-mix suppliers may have minimum loads or additional charges.
- Project requirements: structural and 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 of concrete make a cubic yard?
How many 60 lb bags of concrete make a cubic yard?
How many 40 lb bags of concrete make a cubic yard?
How much concrete do I need for a 10 × 10 slab at 4 inches thick?
Does HomeCalc add waste automatically?
Can I use metric measurements?
Sources and methodology
HomeCalc uses basic volume geometry and published bag yields. The bag estimates are not universal product specifications; always verify the yield of the concrete mix you choose.