Concrete Calculator
Estimate concrete volume and bag counts.
Pour Specifications
Select shape, dimensions, and allowance
Allowance & Package Settings
How to Calculate Concrete Volume and Bags
This tool estimates concrete quantity from dimensions you enter. It is a geometry and material-quantity calculator, not a footing, slab, or structural design tool. Formwork, subgrade preparation, reinforcement, drainage, and code requirements need project-specific planning.
Concrete is typically supplied in two primary ways: delivered transitready-mix (ordered by the cubic yard or cubic meter from a batch plant) or dry pre-mixed bags (such as standard 80-lb, 60-lb, 50-lb, or metric 25-kg bags from home improvement suppliers). Our calculator computes the geometric volume for your chosen shape, accounts for embedded post displacement in fence holes, applies your chosen waste allowance, and calculates whole bag requirements using the stated product yield.
Formulas by Construction Geometry
Concrete volumes are calculated using solid geometric formulas before applying a waste allowance:
1. Rectangular Slab, Patio, or Walkway
Volume (cu ft) = Length (ft) × Width (ft) × [Thickness (in) ÷ 12]
Volume (m³) = Length (m) × Width (m) × [Thickness (cm) ÷ 100]
Dividing thickness by 12 (or 100 in metric) converts all three dimensions to consistent units. Divide cubic feet by 27 to obtain cubic yards.
2. Post Holes with Post Displacement
Gross Hole Volume = π × (Hole Radius)² × Depth × Count
Square Post Displaced = Width × Depth × Embedded Height × Count
Round Post Displaced = π × (Post Radius)² × Embedded Height × Count
Net Concrete Volume = Gross Hole Volume − Post Displaced Volume
The entered post dimensions and embedded height are subtracted from the entered hole dimensions. This volume model does not specify hole size, post size, or embedment depth; use the project plan or product requirements.
3. Round Columns, Sonotubes & Pier Footings
Volume = π × (Radius)² × Depth/Height × Count
Used for cardboard Sonotube forming tubes, structural round columns, and solid underground cylindrical pier pads where the full cylinder is cast solid without post displacement.
4. Round Slab or Circular Patio
Volume (cu ft) = π × (Diameter in ft ÷ 2)² × [Thickness (in) ÷ 12]
Used for circular fire pit pads, hot tub foundations, round flagstone bedding slabs, and circular water tank pads.
5. Solid Cast Concrete Steps
Steps Volume = Width × Rise × Run × [N(N + 1) ÷ 2]
Platform Volume = Width × Platform Depth × (N × Rise)
Where N is the number of steps. This formula calculates a solid, monolithic cast-in-place mass. It does not model hollow formed stairs, waist-slab staircases, complex multi-flight stairs, or structural rebar.
Post-Hole and Solid-Cylinder Volume
The calculator distinguishes a hole with an embedded post from a solid cylinder. Use dimensions from your project plan; these geometry examples do not recommend footing or post design:
- Hole with an Embedded Post: The geometry subtracts the entered post volume from the entered cylindrical hole volume. Bag count then depends on the yield printed for the selected mix.
- Formed Cylinders: A circular form filled with concrete; the model uses the full cylinder because no post displacement is entered.
- Solid Circular Pads: Use the full cylinder when the plan specifies a solid circular concrete volume with no embedded post subtraction.
For step-by-step guidance on hole diameters, post alignment, and gravel bases, explore our guide oncalculating concrete for fence post holes.
Choosing Dimensions for a Quantity Estimate
Enter dimensions from the project plan, product instructions, or a qualified professional. Required slab depth, footing dimensions, base material, reinforcement, and frost protection depend on site conditions, loads, materials, and locally adopted requirements. Do not use a sample dimension or the calculator result as structural guidance.
Examples of Manufacturer-Stated Bag Yields
Bag weight does not determine finished volume by itself. These are examples from specific product information; yields differ across product lines and markets. Use the yield on the product you plan to buy or enter it as a custom bag yield.
| Product example | Stated yield | Approximate bags per volume | Scope |
|---|---|---|---|
| Sakrete 5000 Plus, 80 lb | 0.60 cu ft (~17.0 L) | ~45 bags / cu yd | Product-specific example; check the current package label. |
| Sakrete 5000 Plus, 60 lb | 0.45 cu ft (~12.7 L) | ~60 bags / cu yd | Product-specific example; check the current package label. |
| Sakrete 5000 Plus, 50 lb | 0.38 cu ft (~10.8 L) | ~71 bags / cu yd | Product-specific example; check the current package label. |
| QUIKRETE Fast-Setting, 25 kg (Canada) | 0.41 cu ft (~11.6 L) | ~87 bags / m³ | Product-specific metric-market example. |
Sources: Sakrete 5000 Plus product dataand QUIKRETE Fast-Setting Concrete Mix data sheet. Check the current label because package sizes and formulations vary by market.
Use the yield for the product you buy
Comparing Pre-Mix Bags vs. Delivered Ready-Mix
One cubic yard equals 27 cubic feet (about 0.765 m³). Whether to use bagged product or delivered ready-mix depends on the mix, supplier pricing, delivery fees, site access, placement schedule, and labor. Compare local quotes and follow the selected product's instructions.
- Bagged mix: Compare the required number of bags, mixing capacity, handling, and labor with available delivery options.
- Delivered ready-mix: Ask the local supplier about its order minimum, delivery charges, access requirements, placement window, and any equipment needed at your site.
Frequently Asked Questions (FAQ)
How much extra concrete waste allowance should I add?
There is no waste percentage that fits every pour. Measurement accuracy, excavation or form variation, material placement, and supplier packaging affect the amount to order. Choose any allowance from the project plan and verify it with the supplier or a qualified professional.
How does post displacement affect my bag order?
When pouring fence post holes, the wood or steel post occupies volume inside the hole. The displaced volume depends on the hole and embedded-post dimensions you enter. Select "Post Hole with Post" to subtract the post's modeled volume from the cylindrical hole estimate.
What subbase preparation is needed before pouring concrete?
Base material and depth depend on project design, soil, loads, drainage, climate, product instructions, and local requirements. This calculator does not specify a base. If a plan gives you dimensions and a material, theGravel Calculator can estimate that entered quantity.
Related Construction Calculators & Guides
Use these tools for separate quantity-planning tasks:
- Gravel Calculator - Estimate a gravel quantity from dimensions and material inputs you provide.
- Deck Material Calculator - Estimate surface decking board quantity and layout; framing and hardware are not included.
- Fence Post Concrete Guide - Calculate hole volume and understand the geometry assumptions; it does not size a footing.