For an irregular outline, calculate separate pieces that do not overlap, add their measured volumes and apply one selected allowance to the total. The outline below can be calculated as two rectangles or as a large rectangle minus a cut-out. Both methods should give the same answer.
Hypothetical example: every dimension and the 10% allowance below is chosen to demonstrate arithmetic. They do not specify a suitable slab thickness, footing design or construction method.
Split the outline at a shared edge
Record the main rectangle as 4 m × 3 m and the extension as 2 m × 2 m. A common mistake is to enter a full 4 m × 5 m rectangle and then add the extension again. The full rectangle already includes that space and also includes the empty cut-out.
Convert thickness before multiplying
For this exercise, 100 mm ÷ 1,000 = 0.1 m. Area is measured in square metres; multiply it by thickness in metres to obtain cubic metres. Do not enter “100” into a field labelled metres.
| Item | Working | Volume |
|---|---|---|
| Rectangle A | 4 × 3 × 0.1 | 1.200 m³ |
| Rectangle B | 2 × 2 × 0.1 | 0.400 m³ |
| Measured total | 1.2 + 0.4 | 1.600 m³ |
| Selected 10% allowance | 1.6 × 0.10 | 0.160 m³ |
| Planning quantity | 1.6 + 0.16 | 1.760 m³ |
Check by subtraction: the enclosing 4 m × 5 m rectangle has an area of 20 m². Remove the empty 2 m × 2 m corner: 20 − 4 = 16 m². At the same 0.1 m thickness, 16 × 0.1 = 1.6 m³ before allowance.
At ConcreteCalc's cited Boral reference of 108 × 20 kg bags per cubic metre, the final bag count is round up(1.76 × 108) = 191 bags. That conversion applies to the referenced product, not to every bag with a 20 kg label.
Open these pours in a new planner tab
The example opens in a new tab so it does not replace an existing planner draft. Change each dimension to match your own measured job. The planner shows each base volume separately from the allowance and can copy or print the complete summary.
Treat different depths as separate volumes
Adding areas first only works here because both pieces use the same thickness. If the extension in this arithmetic exercise were 0.15 m thick instead, calculate 4 × 3 × 0.1 + 2 × 2 × 0.15 = 1.8 m³ before allowance. Applying the same selected 10% gives 1.98 m³. This comparison demonstrates the effect of an input; it does not recommend changing a design.
For a beam below a slab, avoid entering both the full slab and the beam's full depth if that counts their shared volume twice. Establish whether the design's beam depth includes the slab thickness, then count only non-overlapping volumes. Ask the designer to clarify ambiguous measurements rather than guessing.
Take a checked quantity to the supplier
The calculator totals geometry; it cannot determine the actual shape of an excavation or decide your structural requirements. Confirm dimensions from the relevant plans or qualified professional. A planning allowance is editable and should reflect your measurements and circumstances.
Carry the measured total, the separate allowance and the planning quantity into your quote enquiry. Confirm the product specification and the supplier's ordering increments, minimum load and fees. If the supplier rounds a quantity, record that separately so the same allowance is not applied again.
Sources and checking
The diagram and arithmetic are original worked examples by ConcreteCalc. Reproduce the two rectangles in the planner to check the result. Volume formulas, conversion and rounding are documented in our methodology. The bag conversion is from Boral's Concrete Mix product data sheet, checked 31 October 2026.
Next: compare bag yield with a ready-mix quote, or report an issue with this example.