Concrete Calculator
Calculate cubic yards, cubic feet, and number of 80lb bags needed for slabs and columns. A 10% waste factor is automatically included. Select shape, enter dimensions, and get your material estimate instantly.
Slab (Rectangle)
Note: 10% waste factor is included in all estimates. One 80lb bag of concrete mix covers approximately 0.6 cubic feet. For ready-mix concrete in large projects, 1 cubic yard = 27 cubic feet.
How to Calculate Concrete Volume
Concrete volume is calculated from the dimensions of the form being filled. The result in cubic feet is divided by 27 to get cubic yards (the standard ordering unit for ready-mix) or divided by 0.6 to get the number of 80lb bags needed.
Slab Formula
Volume (cu ft) = Length (ft) × Width (ft) × Thickness (ft) Volume (cu yd) = Volume (cu ft) ÷ 27 With waste = Volume (cu yd) × 1.10
Example: A 12 ft × 10 ft driveway section, 4 inches thick:
Thickness = 4 in ÷ 12 = 0.333 ft
Volume = 12 × 10 × 0.333 = 40.0 cu ft
= 40.0 ÷ 27 = 1.48 cu yd
With 10% = 1.48 × 1.10 = 1.63 cu yd
80lb bags = (40.0 × 1.10) ÷ 0.6 = 44.0 ÷ 0.6 = 74 bags Cylinder (Column) Formula
Volume (cu ft) = π × (Diameter/2)² × Height
= π × Radius² × Height Example: Four concrete deck piers, each 12 inches diameter and 36 inches deep:
Radius = 12 in ÷ 2 = 6 in = 0.5 ft Height = 36 in = 3.0 ft Volume each = π × 0.5² × 3.0 = 3.14159 × 0.25 × 3.0 = 2.356 cu ft Total (×4) = 2.356 × 4 = 9.42 cu ft = 0.35 cu yd With 10% = 0.39 cu yd 80lb bags = (9.42 × 1.10) ÷ 0.6 = 10.36 ÷ 0.6 = 18 bags
Bag Size Comparison
| Bag Weight | Volume per Bag | Bags per Cubic Yard | Approx Cost/Cu Yd |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 bags | $270–$360 |
| 50 lb | 0.375 cu ft | 72 bags | $252–$360 |
| 60 lb | 0.45 cu ft | 60 bags | $240–$360 |
| 80 lb | 0.60 cu ft | 45 bags | $225–$315 |
| Ready-mix (1 cu yd) | 27 cu ft | N/A | $125–$175 |
Concrete Strength (PSI)
Standard bagged concrete mix is typically 4,000 PSI (pounds per square inch) when properly cured. Different applications have different strength requirements:
| Application | Minimum PSI | Recommended |
|---|---|---|
| Walkways and patios | 3,000 PSI | 3,500 PSI |
| Driveways | 3,500 PSI | 4,000 PSI |
| Foundations | 3,000 PSI | 4,000 PSI |
| Structural columns/slabs | 4,000 PSI | 5,000+ PSI |
| Post holes (non-structural) | 2,500 PSI | 3,000 PSI |
Curing and Setting Times
Concrete does not simply "dry" — it undergoes a chemical reaction (hydration) that generates heat and creates strength over time. Bagged concrete mix typically sets (can be walked on) in 24–48 hours but reaches 70% of its design strength at 7 days and full strength at 28 days. Keep concrete moist for the first 3–7 days by misting or covering with plastic sheeting — this is called curing and significantly improves final strength. Do not pour in freezing temperatures without insulating blankets and accelerators.
Frequently Asked Questions
How many cubic yards of concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab at 4 inches thick requires: Volume = 10 × 10 × (4/12) = 100 × 0.333 = 33.3 cubic feet = 33.3 ÷ 27 = 1.23 cubic yards. With a 10% waste factor, order 1.36 cubic yards. In 80lb bags (each = 0.6 cu ft): 33.3 × 1.10 = 36.6 cu ft ÷ 0.6 = 61 bags.
When should I use bagged concrete vs. ready-mix?
Bagged concrete (80lb bags) is practical for small jobs: fence posts, small slabs, repair patches, and projects under about 1 cubic yard. Ready-mix concrete (delivered by truck) is cost-effective for jobs over 1 cubic yard. Ready-mix typically costs $125–$175 per cubic yard delivered; bagged mix at $5–$7 per bag works out to $225–$315 per cubic yard — significantly more expensive for large pours.
Why is a 10% waste factor recommended?
Concrete waste occurs from mixing losses, spillage, over-excavation that makes the pour area slightly larger than measured, and the reality that form dimensions are rarely perfectly uniform. Under-ordering means a second trip to buy more mix and a cold joint (a visible seam where fresh concrete meets partially set concrete) which is a structural weakness. A 10% buffer is the standard industry recommendation for small projects; some contractors use up to 15% for large pours.
How many 80lb bags of concrete are in a cubic yard?
One 80lb bag of standard concrete mix yields approximately 0.60 cubic feet. Since 1 cubic yard = 27 cubic feet: 27 ÷ 0.60 = 45 bags per cubic yard. With a 10% waste allowance, plan for 50 bags per cubic yard. Note: 60lb bags yield about 0.45 cu ft (60 bags per yard); 40lb bags yield about 0.30 cu ft (90 bags per yard).
What is the standard concrete thickness for different applications?
Recommended minimum thicknesses: walkways and patios — 4 inches; residential driveways — 4–6 inches; garage slabs — 4–6 inches (6 inches for heavy vehicles); footings — depth varies by frost line (typically 12–36 inches) and width is typically 2× the wall thickness; structural slabs with rebar — 5–8 inches. Local building codes may require greater thickness; always check before pouring.
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