Last reviewed: 5 October 2026

Unit conversion

All calculations use metres internally. Millimetres are divided by 1,000. The resulting volume is shown in cubic metres (m³), where 1m³ equals 1,000 litres.

Geometric formulas

  • Rectangular slab or footing: length × width × depth
  • Circular slab or round column: π × radius² × depth or height
  • Square column: width × width × height
  • Post hole: cylindrical hole volume minus the volume occupied by the post

Repeated identical items multiply the single-item volume by the count. Different sizes should be calculated separately and added.

Multi-pour project totals

The project planner calculates each rectangular or circular item independently, multiplies repeated identical items by their quantity, and sums those measured volumes. It then applies one user-selected allowance to the combined volume. The planner does not add a hidden allowance to individual rows.

Post-hole rows reuse the post-hole calculator's subtraction of post volume. “Add to project” transfers measurements in metres, the shape and quantity before allowance. The first imported pour sets the allowance for a new project; adding to an existing draft keeps that draft's allowance. The summary lists measured volumes separately from the project allowance, so nothing is counted twice.

Allowance

The selected allowance is applied as volume × (1 + allowance ÷ 100). Ten per cent is the interface default, not a mandatory construction rule. Set it to zero when no allowance is wanted, or choose a value appropriate to the accuracy of the measurements and excavation.

20kg bag conversion and rounding

ConcreteCalc multiplies the volume including the selected allowance by 108 and rounds up to a whole bag. The 108 value comes from Boral's published guidance that 108 20kg Concrete Mix bags equate to one cubic metre. Check the yield printed by the product you are buying because formulations differ.

Primary yield source: Boral Concrete Mix product data sheet.

Indicative bag pricing

Calculator cost ranges use AUD $8.50–$14.50 per 20kg bag. This range was reviewed on 24 August 2026 against concrete-mix products listed by Bunnings Australia. Location, product, stock and promotions can change the price; the displayed result is not a quote.

Retail reference: Bunnings Australia concrete range. The bags-versus-ready-mix tool instead accepts a shelf price supplied by the visitor.

Ready-mix pricing

ConcreteCalc does not publish a national ready-mix price estimate. Delivery distance, strength grade, load size, waiting time, access, pumps and surcharges can materially change the total. The comparison tool accepts the supplier's quoted per-cubic-metre price and quoted fees.

What is outside the calculation

No calculator selects thickness, dimensions, reinforcement, strength, drainage, curing, ground preparation or approval requirements. It also cannot measure excavation irregularity. Those decisions and measurements must come from the project documentation, supplier or an appropriately qualified professional.

Corrections

To report a reproducible issue, send the calculator URL, inputs, displayed result and the result you expected through Contact.