Concrete Calculator
Calculate cubic yards, cubic feet, and 80lb, 60lb, or 50lb pre-mix bags for concrete slabs, footings, post holes, and stairs. 100% free, private browser calculation.
Slab / Patio / Walkway Dimensions
Rectangular concrete pours with length, width, and uniform thickness.
Allowance & Bag Specifications
Concrete Volume & Bags Needed
At 1.36 cu yd (over 1 cu yd), ordering ready-mix concrete delivered by transit mixer is generally faster, structurally stronger, and cheaper than hand-mixing 62 heavy bags.
Bag Count by Standard Size
| Bag Size | Yield | Bags Needed |
|---|---|---|
| 80 lb Pre-Mix BagSelected | 0.6 cu ft | 62 |
| 60 lb Pre-Mix Bag | 0.45 cu ft | 82 |
| 50 lb Fast-Setting Bag | 0.375 cu ft | 98 |
| 40 lb Compact Bag | 0.3 cu ft | 123 |
How to Calculate Concrete Volume and Bags
Whether you are pouring a backyard patio slab, setting fence posts, casting circular footings, or building outdoor steps, ordering the exact amount of concrete prevents two major construction failures: running short mid-pour (which creates weak, cold seams in curing concrete) and overpaying for excess material that goes to waste.
Concrete is ordered in either cubic yards (from ready-mix batch plants delivered by concrete transit mixer trucks) or in pre-mixed bags (Sakrete, Quikrete, or fast-setting post mix from hardware stores). Our calculator determines the exact solid volume for your chosen geometry, automatically incorporates a safety waste margin, converts the volume to cubic yards and cubic meters, and calculates bag counts across all four standard industry pre-mix sizes (80 lb, 60 lb, 50 lb, and 40 lb).
Formulas by Construction Geometry
Concrete volumes are determined by solid geometric volume formulas before applying waste factors:
1. Rectangular Slab, Patio, or Walkway
Volume (cu ft) = Length (ft) × Width (ft) × [Thickness (in) ÷ 12]
Because thickness is measured in inches while length and width are measured in feet, dividing thickness by 12 ensures consistent cubic feet dimensions before dividing by 27 to obtain cubic yards.
2. Cylindrical Post Holes, Columns, & Sonotubes
Volume (cu ft) = π × (Radius in ft)² × Depth (ft) × Number of Holes
Where Radius in feet = (Diameter in inches ÷ 2) ÷ 12. For instance, a 12-inch post hole has a 0.5 ft radius.
3. Round Slab or Circular Patio
Volume (cu ft) = π × (Diameter in ft ÷ 2)² × [Thickness (in) ÷ 12]
Used for round fire pit pads, hot tub foundations, circular flagstone bedding slabs, and round water tank platforms.
4. Solid Concrete Stairs & Steps
Step Volume = Width × Rise × Run × [N(N + 1) ÷ 2]
Platform Volume = Width × Platform Depth × (N × Rise)
Where N is the number of steps. Solid concrete stairs consist of stacked rectangular prisms plus an optional top landing platform prism.
Pre-Mix Bag Yields & Weight Variance
A common DIY mistake is assuming that an 80-pound bag of dry concrete equals an 80-pound volume of wet concrete. Concrete dry mix has high bulk density (~133 lbs/cu ft). When mixed with water, standard bagged concrete yields:
| Bag Size | Typical Yield per Bag | Bags per Cubic Yard | Common Applications |
|---|---|---|---|
| 80 lb Bag | 0.60 cu ft | ~45 bags | General contractor standard for slabs, footings, and structural pours. Most cost-effective per yard. |
| 60 lb Bag | 0.45 cu ft | ~60 bags | Homeowner favorite. Significantly easier to carry and maneuver into mixing tubs or wheelbarrows. |
| 50 lb Fast-Setting | 0.375 cu ft | ~72 bags | Rapid setting mix (20–40 min) formulated for fence posts and mailboxes without bracing. |
| 40 lb Bag | 0.30 cu ft | ~90 bags | Small masonry repairs, edge patching, post anchors, and light DIY touch-ups. |
Product Data Sheet Verification
Ready-Mix Truck vs. Bagged Concrete: When to Order Bulk
Mixing bagged concrete by hand in a wheelbarrow takes substantial physical stamina. One cubic yard of concrete weighs approximately 4,000 pounds (2 tons) and requires mixing forty-five 80-lb bags.
- Under 1 Cubic Yard (< 40–45 bags): Mixing bags on-site is practical and cost-efficient, especially if you rent an electric or gas barrel mixer. Ready-mix companies often charge a "short-load fee" (typically $75–$150 extra) for orders below their minimum delivery threshold (usually 3 to 4 yards).
- 1 to 2 Cubic Yards (45–90 bags): Borderline zone. If site access allows a ready-mix truck or concrete buggy chute, ordering delivered ready-mix saves hours of grueling manual mixing and ensures uniform slump and cure consistency.
- Over 2 Cubic Yards (> 90 bags): Almost always order ready-mix transit delivery. Hand-mixing 90+ bags takes so long that the first section starts setting up before the last section is poured, risking cold joints and premature curing cracks.
Why You Must Add 10% for Subgrade Waste
The geometric formula calculates a theoretical box with perfectly plumb formwork and a laser-flat subgrade. In real construction:
- Uneven Subgrade: Gravel base compaction has high and low spots. An extra 1/4-inch dip across a 12x12 ft slab consumes an additional 3 cubic feet (~5 extra bags).
- Form Deflection: Wood 2x4 form boards bow outward slightly under the hydraulic pressure of wet concrete.
- Spillage & Wheelbarrow Residue: Concrete clings to mixer drums, shovels, and wheelbarrow troughs.
For standard slabs poured on compacted ground, we recommend selecting 10% waste allowance. For smooth pre-built Sonotube forms, 5% waste is sufficient.
Related Construction Calculators
Planning a comprehensive outdoor build? Use our companion calculators to plan every phase of your job:
- Gravel Calculator — Estimate crushed stone and subbase tonnage for patio and slab prep.
- Deck Material Calculator — Calculate deck boards, joists, and post spacing.
- Roof Pitch Calculator — Determine roof slope, angles, and rafter lengths.