Concrete Calculator — How to Estimate Concrete Volume
Estimate concrete volume for a slab, column, or cylinder from length, width, and thickness (or diameter and height). Example: a 5 m × 4 m × 0.1 m slab → 2.0 m³. Optional bag count uses 0.015 m³ (or 0.45 ft³) per bag — that bag size is an assumption shown on the page. Runs in your browser; numbers stay on your device and are not uploaded to a server.
How it works
Choose a shape and unit system, then enter the dimensions shown for that shape. The tool multiplies length × width × thickness for slabs and rectangular columns, or uses π × (diameter/2)² × height for cylinders. Optionally divide volume by the documented bag size and round up. All math runs in your browser tab; your numbers are not uploaded to a server.
Formula and assumptions
Volumes use closed geometric solids. Keep these rules in mind:
- Slab: V = length × width × thickness.
- Rectangular column: V = length × width × height.
- Cylinder: V = π × (diameter ÷ 2)² × height. Bag estimate = ceil(V ÷ bag volume) with bag volume 0.015 m³ or 0.45 ft³.
Example
Example (slab, metres): length 5 m, width 4 m, thickness 0.1 m → V = 5 × 4 × 0.1 = 2.0 m³. With the 0.015 m³ bag assumption, bags ≈ ceil(2.0 ÷ 0.015) = 134.
When to use it
- Rough-order a patio or garage slab before calling a ready-mix plant.
- Size bagged concrete for fence posts or small cylindrical footings.
- Cross-check a contractor quote against simple L×W×T geometry.
Frequently asked questions
What formula does the slab mode use?
Slab volume is length × width × thickness in the units you select. The default sample 5 × 4 × 0.1 m equals 2.0 m³.
How are bag counts calculated?
When you enable bag estimate, bags = ceil(volume ÷ bag volume). Metric mode uses 0.015 m³ per bag; feet mode uses 0.45 ft³ per bag. That is a planning assumption, not a branded bag label.
Does cylinder mode need radius or diameter?
Enter the outside diameter. The calculator converts to radius as diameter ÷ 2 inside V = π r² h.
Are my numbers uploaded?
No. Dimensions are calculated in your browser on your device and are not uploaded to a server.
Related tools
Questions or feedback
Something unclear, broken, or missing? Draft a message below — we read every note about these tools.