Concrete volume calculator
Work out how much ready-mix or bagged concrete you need for a slab, footing or base before you order, so you don't under-order and get caught short mid-pour or over-order and pay for concrete you don't use.
Result
Concrete required
Result
Maximum length you can pour
How to use this tool
- Measure the length, width and depth of the area you're pouring.
- Enter the three dimensions below — switch the unit dropdown next to each field between metric and imperial if you measured in feet or inches.
- Leave the waste allowance at the 5% default, or adjust it if your formwork is irregular.
- Read the required volume in cubic metres.
Assumptions
- The pour area is a simple rectangle at a constant depth.
- Depth is measured as the finished concrete thickness, not including sub-base or hardcore.
- Waste allowance defaults to 5%, a typical figure for a reasonably level, well-formed slab.
Sources & reference data
Technical details: what the fields mean
- Length
length - The longest straight run of the slab or footing, in metres.
- Width
width - The width of the slab or footing, in metres.
- Depth
depth - How thick the concrete will be, in metres (100mm = 0.1m).
- Waste allowance
wastePercent - Extra volume to cover an uneven sub-base, formwork spillage and over-excavation. 5% suits a level, well-formed slab; use 10% for uneven ground.
Methodology
Volume = length × width × depth, then increased by the waste allowance percentage. This is a straightforward rectangular-prism volume calculation; it does not account for slopes, curves or stepped depths.
Worked example
A 5m × 4m patio slab, 100mm deep, default 5% waste
Inputs: Length: 5 m, Width: 4 m, Depth: 0.1 m, Waste allowance: 5%
Result: 2.10 m³ of concrete required
A 10m garden-wall strip footing, 600mm wide and 225mm deep, 5% waste
Inputs: Length: 10 m, Width: 0.6 m, Depth: 0.225 m, Waste allowance: 5%
Result: 1.42 m³ required — enough that a ready-mix delivery is usually more practical than mixing bags by hand. Footing sizes for walls should come from your designer or building control, not from this example.
Reverse: a 1 m³ ready-mix minimum order — how long a 3m-wide, 100mm path does it cover?
Inputs: Available concrete volume: 1 m³, Width: 3 m, Depth: 0.1 m, Waste allowance: 5%
Result: About 3.17m of length. Useful when a supplier's minimum load is more than you need and you want to plan a second area to use the surplus.
Practical guidance
- Ready-mix is typically ordered in whole or half cubic metres — round up to the supplier's minimum order increment.
- For volumes under roughly 1m³, bagged concrete is often more practical than ordering a ready-mix lorry.
Common mistakes
- Entering depth in millimetres instead of metres (100mm should be entered as 0.1).
- Forgetting a waste allowance and ordering exactly the calculated volume.
- Not accounting for a sloped or stepped sub-base, which makes actual depth uneven.
- Mixing units: measuring length in feet but depth in centimetres is fine here only if each field's unit selector is set to match what you measured.
FAQs
How much concrete do I need for a 5m x 4m slab at 100mm deep?
With the default 5% waste allowance, that's 2.10 cubic metres. Enter length=5, width=4, depth=0.1 above to confirm.
What waste percentage should I allow for concrete?
5% suits a level, well-formed slab. Use 10% or more if the sub-base is uneven or the formwork isn't tight.
Does this include the sub-base, reinforcement or formwork?
No — this calculates the concrete volume only. Sub-base hardcore, mesh reinforcement and formwork timber are separate costs.
I know how much concrete I have — can I work out the area I can cover?
Yes, use reverse mode below: enter your available volume plus width and depth, and it returns the maximum length you can pour.
Last reviewed: 2026-09-24
This is an estimate for a simple rectangular pour at constant depth. It is not a substitute for a structural engineer's specification on load-bearing slabs, and does not account for sloped or stepped ground.
This tool provides an indicative estimate, based on the assumptions stated above.
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