How much concrete do I need for a 6×4 m slab?

At 100 mm deep: 6 × 4 × 0.1 = 2.4 m³. Always add 10% waste, so order 2.64 m³ or round up to 2.7 m³.

Can I use pre-mix bags instead of ready-mix?

Pre-mix bags are practical up to about 0.5 m³. Above that, ready-mix is faster, cheaper and produces a better result.

What strength concrete for a house slab?

Residential slabs typically use 25 MPa (N25). Driveways: 32 MPa. Footings: 25–32 MPa. Check your engineer's specification.

Concrete Calculator — Free Tradies Calculator

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The footing excavation is done and the pump's booked for 7am. You need the cubic metres before you can confirm the order tonight.

Concrete Calculator

Concrete

Shape

Length (m)

Width (m)

Depth (mm)

Bag size

Volume = L × W × D (+ 10% waste) Standard slab: 100mm deep. Footings typically 300mm+ deep. Always add 10% for spillage and uneven ground.

Reference: Cement Concrete & Aggregates Australia

ℹ️ Results are estimates for planning purposes. Verify with current standards and a qualified professional.

1 What this calculator does

Calculates concrete volume in cubic metres (or cubic yards in imperial) for slabs, footings, columns and paths. Enter length, width and depth for any shape to get the volume plus a recommended number of pre-mix bags as a backup estimate.

2 Formula & professional reasoning

Volume (m³) = Length (m) × Width (m) × Depth (mm) ÷ 1000

Concrete is ordered by volume. Length and width give the plan area; depth is in mm in Australian practice (converted by dividing by 1000). Add 5–10% waste for over-pour, spills and uneven excavations. In imperial mode, feet × feet × inches ÷ 12 gives cubic feet, divided by 27 for cubic yards.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Shed slab 4×3 m, 100 mm deep

Given: 4 m × 3 m × 100 mm

Working: 4 × 3 × 100 ÷ 1000 = 1.2 m³

Answer: 1.2 m³

💡 Add 10% waste → order 1.32 m³. Minimum truck load is typically 0.5 m³.

Standard

Driveway 12×3 m, 125 mm

Given: 12 × 3 × 125 ÷ 1000

Working: 12 × 3 × 125 ÷ 1000 = 4.5 m³

Answer: 4.5 m³

💡 Order 5 m³ including waste. Control joints every 3 m recommended.

Advanced

Strip footing 20 m run, 400 mm wide, 300 mm deep

Given: 20 × 0.4 × 0.3 = 2.4 m³

Working: 20 × 0.4 × 300 ÷ 1000

Answer: 2.4 m³

💡 Strip footings: run full length × cross-section area. Don't forget corner overlaps.

4 Sanity check

Residential slab

0.1–0.15 m³ per m² of floor area

100 mm slab = 0.1 m³/m²; 150 mm = 0.15 m³/m².

Minimum truck order

0.5 m³ most suppliers

Short loads carry a surcharge. Consider pre-mix bags for very small pours.

Waste allowance

Add 5–10%

10% for footings and uneven ground; 5% for clean formed slabs.

Pump vs chute

Over ~3 m from truck

Pump hire typically $400–$800. Worthwhile for complex or inaccessible pours.

5 Common errors

Error	Cause	Consequence	Fix
Depth in wrong units	Entering 100 m instead of 100 mm	Result 1000× too large	Australian concrete depth is always in millimetres — slab depth is 100 mm, not 0.1 m in this field
Ignoring waste	Ordering exact volume	Pour stops short — expensive emergency top-up	Always add 5–10% to your calculated volume when ordering
Not accounting for reinforcement	Assuming concrete fills 100% of volume	Slight over-order (acceptable)	Reo reduces volume by ~2–3% — negligible for ordering purposes
Confusing pour area with perimeter	Footings measured as perimeter only	Volume seriously underestimated	Footings: multiply the full run length × cross-section (width × depth)