What This Concrete Calculator Covers

This concrete volume calculator is designed for typical domestic and site pours. It provides:

• Volume in m³ for slabs (L × W × thickness) and footings/trenches (length × width × depth)
• Automatic unit handling (mm/in converted to metres under the hood)
• Over-order allowance (5% slab default, 10% footing default — adjustable up to 15%)
• Premix bag estimates using typical yields (e.g., 20 kg ≈ 0.01 m³, 25 kg ≈ 0.012 m³) or a custom yield
• Site-mix materials: cement bags (by 25/50 kg), plus sand & stone in m³ and tonnes using your chosen mix ratio (e.g., 1:2:3)

How to Use

1. Select Slab or Footing/Trench, then choose Metric or Imperial.
2. Enter your dimensions and thickness/depth (mm or inches are fine—converted automatically).
3. Pick an over-order allowance: 5% for flat slabs, 10% for trenches, or 15% for rough/irregular ground.
4. Choose a premix bag type (20 kg or 25 kg) or set a custom bag yield if your product datasheet differs.
5. For site-mix: select the mix ratio (e.g., 1:2:3), choose your cement bag size (25 kg or 50 kg), and (optionally) adjust the dry-volume factor.
6. Read off the m³ required, premix bag count, and the site-mix materials (cement bags, sand & stone in m³/tonnes).

How the Site Mix Is Calculated

The calculator converts the finished (wet) volume into a dry materials volume using a typical factor of 1.54 (adjustable 1.40–1.65). It then splits that dry volume by your chosen C:S:A ratio and uses typical bulk densities to estimate mass:

• Cement: 1440 kg/m³ → shown as total kg and bags at 25/50 kg
• Sand: 1600 kg/m³ → shown as m³ and tonnes
• Stone: 1500 kg/m³ → shown as m³ and tonnes

Cement bags are rounded up to whole bags. Adjust the factor and bag size to match local practice or supplier data.

Why Over-Ordering Matters

Concrete volumes are sensitive to ground irregularities, trench collapse, and wastage at edges and tools. Ordering a little extra (5–15%) prevents cold joints and short loads that risk a weak finish. The calculator’s defaults mirror common site practice: lower for level slabs, higher for trenches.

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Notes & Best Practice

• Results are guidance only—always follow local regs and your supplier’s specifications.
• Enter the wet concrete thickness/depth you want after compaction, not just excavation depth.
• Bag yields and densities vary; check your product datasheet and adjust the custom yield or dry-volume factor if needed.
• Premix bags are ready-mix concrete; cement bags shown in the site-mix are pure cement for batching with sand & stone.
• For complex pours (steps, haunching, multiple trench widths), split into sections and sum the volumes.

Whether you’re pouring a shed base, driveway bay, or strip footing, this concrete calculator helps you order the right amount—fast, practical, and site-ready.