Concrete Calculator

Enter your project dimensions to calculate how much concrete you need in cubic metres, equivalent 20kg bags, and indicative Sydney pricing.

Shape
Dimensions
Wastage allowance
Grade

Result

2.64 m³

Volume needed

286

20kg bags equiv.

$634–$686

Indicative price (N25)

A 6m × 4m × 100mm slab requires 2.40 m³ of concrete (raw volume). With 10% wastage allowance, order 2.64 m³.

Short-load surcharge applies (under 3m³): approximately $14 extra.

Check availability & pricing

How to Calculate Concrete Volume

The basic formula for concrete volume is length × width × depth, with all measurements in metres. For slabs and driveways, depth is typically 100mm (0.1m). For footings, depth is usually 300–600mm depending on soil conditions and structural requirements.

Common formulas

  • Rectangle/slab: length (m) × width (m) × depth (m)
  • Circle: π × radius² × depth (m)
  • Footing: length (m) × width (m) × depth (m)
  • Column: π × radius² × height (m) × count

Why add wastage?

Always order more than the calculated volume. Concrete is wasted through spillage, uneven formwork, and over-excavation. The standard wastage allowance is 10% for residential projects. Use 5% only with precise formwork and experienced crew. Use 15% for irregular shapes or first-time DIY projects.

Common project volumes

  • Single garage slab (3m × 6m × 100mm): ~2.0 m³
  • Double driveway (3m × 12m × 100mm): ~4.0 m³
  • Patio slab (4m × 4m × 100mm): ~1.8 m³
  • Garden path (1m × 10m × 75mm): ~0.8 m³
  • Fence post holes (10 × 300mm dia × 600mm): ~0.4 m³

For projects under 0.5m³, bagged concrete may be more practical. For anything larger, ready-mix is cheaper per m³ and much faster.

Frequently Asked Questions

How do I calculate concrete volume?
Multiply length × width × depth (all in metres). For a 4m × 3m slab at 100mm deep: 4 × 3 × 0.1 = 1.2m³. Add 10% for wastage = 1.32m³.
How many bags of concrete do I need?
One 20kg bag makes approximately 0.009m³. For 1m³ you need about 108 bags. Use our calculator for your exact number.
What wastage factor should I use?
5% for precise formwork with experienced crew. 10% is standard for most residential projects. 15% for irregular shapes, first-time DIY, or sites with difficult access.
Are these prices accurate?
Prices are indicative Sydney ranges for March 2026. Your exact price depends on suburb, supplier, delivery distance, and order volume. Check live availability for exact pricing.
What's a short-load surcharge?
Orders under 3m³ attract a surcharge of approximately $40/m³. The truck carries the same cost to deliver whether it's full or half-empty.
Which concrete grade should I use?
N20 for paths and light slabs. N25 for driveways and house slabs (most common). N32 for structural work. N40 for commercial. See our concrete grades guide for details.