Strip footings and pad footings
A strip footing is a trench full of concrete under a wall. A pad footing is a block of concrete under a post or pier. Both are simple boxes for the purpose of ordering concrete: length times width times depth, in metres, gives cubic metres. The calculator takes the total run of strip footing and the count and size of pads, adds them, and adds waste.
strip (m³) = length × width × depth pads (m³) = count × length × width × depth
Waste on footings is higher than on a slab because the trench is the formwork. An excavator bucket is 300 mm wide but the trench it digs is wider where the sides crumble, and deeper where the operator went again to find firm ground. Ten percent is the minimum; on sandy ground or a hand-dug trench allow 15.
| Footing | Per lineal metre | 20 m run, +10% | 20 kg bags |
|---|---|---|---|
| 300 × 300 | 0.090 m³ | 1.98 m³ | 213 |
| 300 × 400 | 0.120 m³ | 2.64 m³ | 284 |
| 400 × 500 | 0.200 m³ | 4.40 m³ | 473 |
| 450 × 600 | 0.270 m³ | 5.94 m³ | 639 |
The bag column is there to make the point: footings are a premix job.
Where the sizes come from
Footing width and depth depend on the load coming down and on the soil holding it up. In Australia, residential footings are designed to AS 2870, which classifies sites from A (stable sand and rock) through S, M, H1 and H2 (increasingly reactive clay) to E (extremely reactive) and P (problem sites), based on a soil test. The more reactive the clay, the deeper and stiffer the footing has to be to ride out the seasonal swelling and shrinking. That is why the same brick veneer house has 300 × 400 footings in one suburb and 450 × 800 in another, and why the numbers in this calculator are only placeholders until you have the drawing.
Steel
Strip footings are reinforced with trench mesh, a flat ladder of three or four bars, usually 8 or 11 mm, welded to cross wires. It sits in the bottom of the trench on bar chairs so it ends up with 50 mm or more of concrete under it, and where the drawing calls for it, a second sheet goes near the top. Sheets are 6 m long and lap 500 mm, and they are bent or lapped around corners. Pad footings usually have a small mat of mesh or a few bars, or nothing at all for a light post. Starter bars for block walls or piers are placed in the wet concrete.
Inspection and pour
Footings for permitted work are inspected before the pour. The building surveyor or engineer checks the trench is at the right depth and on the right material, and that the steel is the right size, in the right place, on chairs. Have the trench clean, the steel tied and the level pegs in when they arrive. Then pour, vibrate or rod the concrete to get the air out, screed to the pegs and let it cure. Concrete grade is normally N20 or N25; the drawing says which.
Worked example
A garage on strip footings, 6 × 9 m, so 30 m of 300 × 400 footing. Volume 30 × 0.3 × 0.4 = 3.6 m³, or 3.96 with 10 percent, so order 4.0 m³. Trench mesh: one 3-bar sheet, 30 × 1.1 = 33 m, which is six 6 m lengths. Bar chairs about 50. Add four 600 × 600 × 600 pads for a pergola at the back: 0.86 m³ with waste, so the order becomes 4.8 m³ and still fits in one truck.
Common questions
How much concrete for a strip footing?
Length × width × depth. A 300 × 400 footing is 0.12 m³ per metre, so 20 m is 2.6 m³ with waste.
How deep should footings be?
Set by the soil class under AS 2870 and the engineer. Commonly 400 to 600 mm on stable sites, deeper in reactive clay.
What steel goes in?
Trench mesh, 3 or 4 bars, on chairs in the bottom, lapped 500 mm.
Is an inspection needed?
Yes, before the pour, on any permitted work.
Quantities only. Footing dimensions, depths, reinforcement and concrete grade are structural and come from the engineer's design for your site.