About the Concrete Footing Calculator
Footings are the concrete at the very bottom of a structure. They take the load of a wall or post and spread it over a wider area of ground, so the ground is not overloaded and the structure does not settle unevenly.
Two kinds cover most building work. A strip footing is a continuous band of concrete under a load-bearing wall. A pad footing is a separate square or rectangular block under a single post or column — the kind used for decks, porches, pergolas and framed buildings.
This concrete footing calculator works out the concrete for either, with a waste allowance, the ready-mix order and the bag count. For strip footings, it can also check the footing's proportions against the common rules of thumb for the wall it carries.
How to Use the Concrete Footing Calculator
Choose feet and inches or metres and millimetres, and the footing type.
For a strip footing, enter the total length measured along the centreline of the footing, and its width and depth. Optionally add the wall thickness to check the proportions.
For pad footings, enter the number of pads and each pad's length, width and depth.
The depth here is the thickness of the concrete itself, not the depth of the trench, which is often greater.
How the Volume Is Worked Out
strip footing: length × width × depth
pad footings: number × length × width × depth
to order: × (1 + waste %)Step-by-Step Example
A strip footing 120 feet long, 16 inches wide and 8 inches deep, under an 8-inch wall.
Width: 16 in = 1.333 ft
Depth: 8 in = 0.667 ft
Volume: 120 × 1.333 × 0.667 = 106.7 ft³ = 3.95 yd³
With waste: 3.95 × 1.10 = 4.35 yd³
Order: 4.50 yd³ of ready-mixThe footing is twice the wall's width and as deep as the wall is thick — both in line with the usual rules of thumb.
Measure Along the Centreline
For a rectangular strip footing, the easy mistake is to measure each side along its outside edge and add the four lengths together. That counts each of the four corner squares twice.
The fix is to measure along the centreline of the footing — the middle of its width — all the way round. For a rectangle, that is the outside perimeter minus four times the footing width. The difference can be several per cent on a small building.
Rules of Thumb for Footing Size
Common residential practice, as reflected in the US International Residential Code and similar guidance elsewhere, uses these starting points:
Width: about twice the width of the wall it supports.
Thickness: at least as thick as the wall is wide, and not less than about 6 inches (150 mm).
Depth below ground: the bottom of the footing must sit below the frost line, the depth to which the ground freezes in winter, so frost cannot heave it. In cold climates that can be several feet.
These are starting points, not a design. The size a footing actually needs depends on the soil's bearing capacity and the load it carries, and is set by building codes, building control or a structural engineer. The calculator sizes the concrete for a footing you have already designed; it does not design one.
Pad Footings
Pad footings are sized by the load on each post and the soil beneath. A typical deck or porch pad might be 18 to 24 inches square and 8 to 12 inches thick, but larger loads and weaker soils need bigger pads. The number of pads multiplies quickly: twelve 24-inch pads, 12 inches deep, need 1.8 cubic yards before waste — often more than people expect for a small deck.
Trench Versus Formwork
How the footing is formed affects how much concrete it takes.
Trench-poured footings are cast straight into an excavated trench. They save on formwork, but trenches are rarely neat: sides crumble, widths vary and the bottom is uneven. A larger waste allowance, 10 to 15 per cent, is sensible, and measuring the trench as dug rather than as drawn gives a better figure.
Formed footings use timber or steel forms to hold the sides. They take more setting out, but the dimensions are precise, so the standard allowance of 5 to 10 per cent usually covers it.
Either way, remove loose soil and water from the bottom of the trench before pouring. Concrete poured onto loose or muddy ground loses the firm bearing the footing is there to provide.
Understanding Your Result
Concrete needed is the total including waste.
Volume gives cubic yards and feet, or cubic metres.
Ready-mix order rounds up to the supplier's quarter.
Premix bags gives the bag alternative, useful for small pad jobs.
Proportions compares a strip footing with its wall, or states the usual minimums.
Worth knowing flags thin footings and the role of building codes.
When Should You Use This Calculator?
Ordering concrete for a footing that has already been designed.
Estimating an extension, garage or garden wall.
Planning deck or porch pads, where the count adds up.
Not to decide the footing size. That depends on soil, load and local codes.
Common Mistakes
Counting corners twice. Measure the centreline.
Using trench depth as concrete depth. The trench is often deeper.
Ignoring the frost line. A shallow footing can heave in winter.
Undersizing to save concrete. Footings are cheap compared with fixing settlement.
Pouring before inspection. Many areas require the trench and reinforcement to be inspected first.