Concrete Volume Calculator
Cubic Yards & Bags
Calculate cubic yards or bags of concrete for slabs, footings, columns, or steps. Includes ACI 10% overage standard and a ready-mix versus bagged cost comparison so you know which is cheaper for your pour.
Ready to calculate? Select your pour type, enter dimensions, and get your cubic yards, bag count, and cost comparison instantly.
Slab for flat pours, Footing for below-grade walls, Column/Pier for cylindrical forms
Total length of the slab or continuous footing in feet
Total width of the slab or footing in feet
Standard pedestrian/residential slab
ACI recommends 10% for slabs and footings on grade
National avg $160–$195/CY in 2025. Plus $50–$200 short-load fee if under minimum.
Enter your dimensions
to see cubic yards and bag count
Related Construction Calculators
More tools for accurate project estimation
The concrete volume formula
Concrete is bought and delivered in cubic yards (CY) in the United States. One cubic yard equals 27 cubic feet. Every concrete calculation begins by converting your project dimensions to cubic feet and then dividing by 27.
Rectangular Slab / Footing
CY = (Length ft × Width ft × Thickness in ÷ 12) ÷ 27
Round Column / Pier
CY = (π × Radius ft² × Height ft) ÷ 27
The shortcut for 4-inch slabs is the Magic Number 81: divide your total square footage by 81 to get cubic yards. This works because 4 inches is exactly 1/3 of a foot, and 3 × 27 = 81. A 10×10 foot pad needs 100 ÷ 81 = 1.24 CY.
Standard pour types and dimensions
| Application | Thickness | Notes |
|---|---|---|
| Pedestrian slab / patio | 4 inches | Standard residential grade |
| Driveway / RV pad | 6 inches | Vehicle load required |
| Structural slab (commercial) | 8–12 inches | Engineered design required |
| Strip footing | 12–24 in wide | Width varies by wall load |
| Concrete pier / column | 12–18 in diameter | Frost depth drives height |
ACI 10% overage standard
The American Concrete Institute recommends ordering a minimum 10% overage above the theoretical volume for slabs and footings on grade. This accounts for three real-world factors: form deflection under the weight of wet concrete, uneven subgrade that absorbs volume, and pour losses during placement and finishing.
For column piers and isolated footings, a 5% overage is often sufficient. For large slab pours with multiple zones, many contractors order 12–15% above theoretical to create a buffer for the final section.
Ready-mix vs bagged concrete — the break-even
The decision between ready-mix delivery and hand-mixing from bags comes down to one number: 1.5 cubic yards. Below this volume, short-load delivery fees make ready-mix more expensive than bags. Above it, the labour, time, and consistency risk of hand-mixing makes ready-mix the professional choice.
| Method | Volume Cost | Additional Cost | Total |
|---|---|---|---|
| Ready-mix ($175/CY) | $217 | $150 short-load fee | $367 |
| Bagged (56 bags × $7) | $392 | — | $392 |
For this specific example, ready-mix is actually cheaper. But if the short-load fee is $200 (some plants charge more), bagged becomes more economical. The calculator above runs this comparison in real time so you can make the right call for your pour volume and local pricing.
2025 concrete prices
Concrete prices have seen the largest year-over-year increase of any construction material category in 2024–2025, with cement-based products rising 8–10% nationally. The primary driver is energy cost — cement production is energy-intensive, and natural gas prices remain elevated. For a full walkthrough of the cubic yards formula and ordering calculations, see the concrete cubic yards guide. For specific pour types, use the dedicated calculators: footing volume, column and pier volume, CMU block fill, driveway volume, and stair volume. For a practical guide to footing depth, frost line requirements, and rebar layout, see the concrete footing calculator guide. For current ready-mix pricing, short-load fees, and the bagged vs ready-mix break-even, see the concrete cost per yard guide.
| Product | 2024–2025 Cost | Unit |
|---|---|---|
| Ready-Mix (Standard 3000 PSI) | $160–$180 | Per CY |
| Ready-Mix (High Strength 4000+ PSI) | $180–$200+ | Per CY |
| Short-Load Surcharge | $50–$200 | Per order |
| Bagged Mix (80 lb) | $6–$8 | Per bag |
| Fiber Additives | $8–$15 | Per CY |
| Finishing Labour | $2–$5 | Per sq ft |
Frequently asked questions
How do I calculate concrete in cubic yards?
For a rectangular slab: (Length ft × Width ft × Thickness in ÷ 12) ÷ 27 = cubic yards. For a 4-inch slab you can use the shortcut: divide total square footage by 81.
How much extra concrete should I order?
ACI recommends adding 10% overage for slabs and footings on grade. This accounts for form deflection, uneven subgrade, and pour losses. Ordering exactly the theoretical volume almost always results in running short.
How many 80lb bags of concrete per cubic yard?
Approximately 45 bags. One 80lb bag yields about 0.6 cubic feet, and there are 27 cubic feet in a cubic yard. 27 ÷ 0.6 = 45 bags per cubic yard.
Does this calculator work for both ready-mix and bagged concrete orders?
Yes. The calculator outputs both cubic yards (for ready-mix orders) and the equivalent 80lb bag count for small pours. A separate cost comparison panel shows which option is cheaper for your specific volume, factoring in short-load delivery fees that apply when ordering under the plant minimum. For volumes under 1 cubic yard, bagged concrete is almost always more economical once delivery fees are included.
What is the Magic Number 81 in concrete estimation?
For 4-inch slabs, dividing total square footage by 81 gives cubic yards needed. This works because 4 inches is exactly 1/3 of a foot, and 3 × 27 = 81. It is a reliable field shortcut for checking calculator results.
What is the current price of ready-mix concrete?
Ready-mix concrete averaged $160–$195 per cubic yard nationally in 2024–2025. Standard 3000 PSI mix runs $160–$180/CY. Add $50–$200 for short-load fees on orders under your plant's minimum delivery quantity.
References
American Concrete Institute. (2019). ACI 318-19: Building Code Requirements for Structural Concrete. ACI.
American Concrete Institute. (2016). ACI 301-16: Specifications for Structural Concrete. ACI.
National Association of Home Builders. (2025). Construction Cost Survey. NAHB Economics.