A footing is a trench full of concrete, so the volume is simple arithmetic. What is not simple is how deep the trench goes — that is decided by the ground, the trees near it and the frost line, and it is the question building control will ask first.
Strip footings and pad foundations. Enter the trench as length × width, then the depth of concrete.
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This is the distinction that throws most footing sums. A traditional strip footing is typically 225–300 mm of concrete sitting at the bottom of a trench that may be a metre deep, with masonry built up from it. Enter the depth of concrete, not the depth you dug. A 20 m run of strip footing 600 mm wide with 225 mm of concrete is 2.7 m³ — about 3 m³ with the allowance.
Trench fill uses a great deal more concrete and much less labour: the trench is filled to near ground level rather than bricked up from a thin footing. On a deep trench in clay it is usually the cheaper answer overall, because bricklaying below ground is slow and awkward work. The calculator handles both — the choice is about labour and ground conditions rather than material cost alone.
Foundation depth, width and reinforcement are structural decisions that depend on ground conditions, loads and what is nearby — and they are subject to building control. Trees are the usual reason a footing gets deeper: on shrinkable clay, roots draw water out and the soil shrinks; remove the tree and it swells back. Both move foundations. Get the design from someone who has seen the ground, then come here for the quantity.
That is a design decision, not a calculation — it depends on ground conditions, the frost line, loads and nearby trees, and it is subject to building control. This calculator works out the concrete volume once the depth has been decided for you.
A strip footing is typically 225–300 mm of concrete at the bottom of a deeper trench, with masonry built up from it. Trench fill fills the trench to near ground level with concrete. Trench fill uses much more concrete and much less labour, and on a deep clay trench it is often cheaper overall.
On shrinkable clay, tree roots draw water out of the soil and it shrinks; remove the tree and the soil swells back. Both movements can damage foundations, which is why footings near trees on clay are usually taken deeper.
No. Standing water must be pumped out, and soft spots dug out and replaced with concrete rather than backfill. Do not pour onto a frozen or waterlogged trench bottom, and keep the trench open for as short a time as possible — sides deteriorate, rain fills them, and an open trench is a genuine safety risk.