Concrete Calculator
Estimate concrete volume and bag count for slabs, footings, columns, and steps.
How to Use the Concrete Calculator
Pick the shape of your pour — slab, footing, or column — then choose imperial (feet/inches) or metric (meters/cm) units. Enter the dimensions, select your bag size, and click Calculate. The tool returns the total volume in cubic yards, cubic meters, and cubic feet, plus the number of bags you'll need including a standard 10% waste factor that accounts for spillage, over-excavation, and form leakage.
Formulas Used
- Slab / Footing: Volume = Length × Width × Thickness
- Column (round): Volume = π × (Diameter ÷ 2)² × Height
- Bag count: (Volume × 1.10) ÷ Bag yield, rounded up to the next whole bag
- Conversions: 1 cubic yard = 27 cubic feet = 0.7646 cubic meters
Bag Yield Reference
- 40 lb bag ≈ 0.30 ft³ (≈ 0.0085 m³)
- 60 lb bag ≈ 0.45 ft³ (≈ 0.0127 m³)
- 80 lb bag ≈ 0.60 ft³ (≈ 0.0170 m³)
- 20 kg bag ≈ 0.009 m³ (≈ 0.32 ft³)
- 25 kg bag ≈ 0.011 m³ (≈ 0.39 ft³)
- 40 kg bag ≈ 0.018 m³ (≈ 0.64 ft³)
Common Use Cases
DIY homeowners use this calculator before pouring driveways, patios, sidewalks, fence-post footings, and equipment pads. Contractors use it for fast bag-count estimates during quoting and material ordering. Garden designers calculate concrete for raised-bed footings, pergola anchors, and stepping stones. The 10% waste factor matches industry standard recommendations from Quikrete, Sakrete, and Ready Mix USA.
Tips for an Accurate Estimate
Always order slightly more than the calculated minimum — running short mid-pour means a cold joint and compromised strength. For pours larger than about 1 cubic yard (27 ft³ ≈ 60 × 80 lb bags), it's almost always cheaper and faster to order a ready-mix truck instead of mixing bags by hand. For footings on uneven ground, measure the deepest point of your trench, not the shallowest.
Frequently Asked Questions
Why does the bag count include 10% waste?
Concrete spills, sticks to your mixer, gets lost to over-excavation, and seeps through form gaps. A 10% buffer is the standard recommendation from major bag manufacturers and prevents the disaster of running out mid-pour.
How big a pour should I order ready-mix for instead of bagging?
Roughly 1 cubic yard or more (about 60 × 80 lb bags). Below that, bagging is usually cheaper. Above it, ready-mix saves time, labor, and your back.
Is 4 inches enough thickness for a residential slab?
4 inches is standard for sidewalks and patios. Driveways need 5–6 inches with rebar or wire mesh. Garage and shed floors typically use 4–5 inches. Always check local codes.
How do I calculate concrete for stairs or odd shapes?
Break the shape into rectangles (treads + risers + landing) and sum the cubic yards from each. We'll add a dedicated stairs calculator in a future update.
Is anything stored?
No. Calculations run in your browser and are never sent anywhere.
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