Footing Concrete Calculator
The footing concrete volume is the simple prism volume of each footing multiplied by how many you are pouring: net volume = length × width × depth × number of footings.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the footing length, width and depth (thickness) in metres — for a trench strip footing, length is the run and width is the trench width.
- Set the number of identical footings if you are pouring several the same (leave at 1 for a single footing); add a waste / over-order allowance (5% is typical for footings).
- Optionally enter a bag yield (m³ per bag, e.g. 0.01 for a 20 kg pre-mix bag) to get an approximate bag count for small hand-mixed pours.
How it works
The footing concrete volume is the simple prism volume of each footing multiplied by how many you are pouring: net volume = length × width × depth × number of footings. The waste allowance then scales this up: total = net × (1 + waste% / 100). All lengths are taken in metres so the result comes out directly in cubic metres (m³), the unit ready-mix concrete is ordered in.
Bags are estimated as ceil(total volume / yield per bag), where a standard 20 kg pre-mix bag yields roughly 0.01 m³. Bag counts are rounded up because you cannot buy part of a bag. This is a quantity/take-off estimate only: it does not size the footing structurally — bearing capacity, reinforcement and footing dimensions must come from a geotechnical report and a structural engineer under the relevant standard (AS 2870 / AS 3600, Eurocode or ACI 318).
Worked example
A 10 m strip footing, 400 mm wide × 500 mm deep, 5% waste. For one strip footing 10 m long × 0.4 m wide × 0.5 m deep, the net volume is 10 × 0.4 × 0.5 = 2 m³. Adding a 5% over-order allowance gives 2 × 1.05 = 2.1 m³ to order. If you were hand-mixing from 20 kg bags at 0.01 m³ each, that is ceil(2.1 / 0.01) = 210 bags — which is why footings this size are almost always ordered as ready-mix rather than bagged.
Common mistakes
- Mixing units — entering the width or depth in millimetres (e.g. 400 or 500) instead of metres (0.4, 0.5). Every dimension here is in metres; a 400 mm wide footing is 0.4, not 400.
- Forgetting a waste / over-order allowance. Footing trenches over-dig, the base is rarely perfectly level, and concrete is lost to pump priming and spillage — order 5–10% extra rather than the exact net volume.
- Treating the bag count as realistic for a large pour. Hundreds of 20 kg bags means you should be ordering ready-mix; the bag figure is only sensible for very small hand-mixed footings.
Frequently asked questions
Does this size the footing or check bearing capacity?
No. It only works out the concrete quantity for a footing whose dimensions you already have. The width, depth and reinforcement must be designed by a structural engineer from a geotechnical report against the relevant standard (AS 2870 / AS 3600, Eurocode 7/2 or ACI 318). This tool is a take-off estimate, not a design.
How much waste allowance should I add?
For footings, 5–10% is common. Trench footings over-dig and the base is uneven, so the poured volume usually exceeds the nominal box volume. Ready-mix is sold in fixed truck loads, so rounding up and ordering slightly over avoids a costly second delivery for a short pour.
How many 20 kg bags are in a cubic metre of concrete?
Roughly 100 bags, since a 20 kg pre-mix bag yields about 0.01 m³. Check your product's stated yield — it varies by mix. For anything more than a few tenths of a cubic metre, ready-mix is almost always cheaper and easier than bagging.
Can I use it for pad footings as well as strip footings?
Yes. For a rectangular pad, enter its plan length and width and its depth; for multiple identical pads, set the number of footings. For a run of strip footing, length is the run and width is the trench width. Any prism-shaped footing works — stepped or tapered footings should be split and added up.
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