Concrete is ordered by volume
For a rectangular pour, concrete volume is length × width × thickness. The result is then increased by the allowance you choose. AmountHub shows the base volume and the final order quantity separately so you can see exactly what the allowance changes.
Worked example
A slab 5 m long, 3 m wide and 10 cm thick has a base volume of 1.50 m³. With a 10% allowance, the planning quantity becomes 1.65 m³.
Ready-mix versus bagged concrete
Ready-mix is usually quoted in cubic metres or cubic yards. Bagged concrete is sold by bag weight, but the important number for estimating is the finished yield printed on the product. The planning options let you round a ready-mix volume to the supplier’s order increment or divide the required volume by a bag’s stated yield.
Why a small allowance is useful
Excavations and formwork are rarely perfect. Slightly deeper ground, bowed forms, spills and pump or wheelbarrow losses can increase demand. A 5–10% allowance is a common planning range for simple work, but large pours should be checked with the supplier or contractor rather than automatically adding a large surplus.
Thickness drives volume quickly
| Project detail | Why it matters | Best source |
|---|---|---|
| Slab thickness | Changes the entire volume | Project specification. |
| Finished dimensions | Define plan area | Formwork / measured site. |
| Allowance | Covers small field variation | Project conditions. |
| Bag yield | Controls bag count | Manufacturer label. |
What this calculator does not design
Concrete quantity is not the same as structural design. Driveways, foundations, columns and structural slabs can require engineered thickness, reinforcement, joints, sub-base and specified concrete strength. Use the project design or professional advice for those decisions.
Before ordering concrete
- Measure formwork or excavation dimensions after preparation where possible.
- Use the specified finished thickness.
- Check supplier minimum loads and rounding increments.
- Use product yield, not bag weight alone, for bagged mixes.
- Confirm access, placement and labor before delivery.
Compare a pour with a bag order
The same volume can be purchased as ready-mix or small bags, but bag count depends on finished yield, not dry bag weight. For a 1.65 m³ planning quantity and a product yielding 10 litres per bag, the calculation is 1,650 ÷ 10 = 165 bags. Check the product label and consider delivery, mixing time and crew capacity before choosing bags for a large pour.
For a supplier selling in 0.1 m³ increments, 1.65 m³ rounds up to 1.7 m³. This is purchase rounding after the allowance; it is not a second waste percentage.
Frequently asked questions
Should I enter slab thickness in centimetres or inches?
Both are supported. Select the unit beside the thickness field. The input should be the specified finished thickness, not the excavation depth including sub-base.
Are footings included in the slab result?
No. Calculate each footing as a separate volume and add it to the slab volume. A single rectangular slab input does not capture thickened edges or piers.
Why is the bag count different from a store estimate?
Bagged mixes have different finished yields. Enter the yield printed on the exact bag; rounding each batch and allowing for losses may change the final purchase.