Concrete Calculator — Bags, Yards & Cost
Estimate concrete volume, bags and local material costs for slabs, footings, columns, post holes and steps.
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How we calculate concrete volumes
Start with net solid volume, combine your areas, apply your waste allowance once, then divide by the selected product’s mixed yield. Round the final bag count upward to whole bags.
From measurements to volume
(L × W × (D ÷ 12)) ÷ 27 = yd³
For a rectangular area with length and width in feet and depth in inches. One cubic yard is exactly 27 cubic feet. Metric dimensions are converted to metres before calculating.
Columns use a cylinder. Post holes subtract the full-depth post. Steps use a solid stair profile with equal risers and treads. The diagram and assumptions above change with your selected area.
Worked example: 10 × 10 ft slab
- Depth: 4 in ÷ 12 = ⅓ ft
- Net volume: 100 × ⅓ = 33.333… ft³
- Add 10% waste: 36.666… ft³
- Volume: 1.36 yd³
- 80 lb bags: ⌈36.666… ÷ 0.60⌉ = 62 bags
Intermediate numbers shown here are descriptive. The calculator retains full precision for purchase quantities.
Bags for common slab sizes
4-inch depth, 10% waste and QUIKRETE Concrete Mix No. 1101 yields. These are quantity examples, not recommended slab dimensions.
| Slab dimensions | Volume + waste | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 8 × 8 ft | 0.87 yd³ | 40 | 53 |
| 10 × 10 ft | 1.36 yd³ | 62 | 82 |
| 10 × 12 ft | 1.63 yd³ | 74 | 98 |
| 12 × 12 ft | 1.96 yd³ | 88 | 118 |
| 12 × 20 ft | 3.26 yd³ | 147 | 196 |
Before you order
Does bag weight tell me its mixed yield?
No. Use the yield for that exact product. Our standard bag options use QUIKRETE Concrete Mix No. 1101. Local metric bags and other mixes need their own data-sheet yield; use the custom product fields.
How much concrete makes one cubic yard?
One cubic yard equals 27 ft³. At 0.60 ft³ per bag, that is 45 whole 80 lb bags before any waste allowance. Smaller areas can require an additional bag after upward rounding.
What depth, reinforcement or curing time should I use?
This tool estimates quantity from dimensions you provide. It does not determine the design, required depth, reinforcement or curing schedule. Follow your project specifications, local requirements and the exact product instructions.
Are bags or ready-mix cheaper?
Enter local prices to compare material subtotals. Neither subtotal includes all project costs. Supplier minimums, delivery, access, equipment, labour and tax may change the overall choice.
Practical limits and ordering guidance
Bag estimates use approximate QUIKRETE Concrete Mix No. 1101 yields: 80 lb → 0.60 ft³, 60 lb → 0.45 ft³, and 50 lb → 0.375 ft³. Check the exact product label before ordering; other mixes may have different coverage.
This tool estimates quantities from your measurements. It does not assess structural suitability. Openings, overlaps, a gravel bed and reinforcement are not included unless explicitly entered.