Concrete is ordered by volume, so almost every groundwork takeoff comes down to one question: how many cubic metres or cubic yards does this pour need? Get the volume wrong and you either pay for a part-used truck or stop the pour to wait for more.
The maths is simple geometry. The errors come from mixed units, ignored waste, irregular shapes, and forgetting that the ground rarely matches the drawing. This guide covers the common groundwork elements and how to quantify each one cleanly.
The base formula
Every concrete volume starts as length × width × depth. Keep all three dimensions in the same unit before multiplying.
volume = length x width x depth
Work in metres for cubic metres, or in feet for cubic feet. The most common mistake is mixing a slab area in metres with a thickness in millimetres or inches. Convert first, then multiply.
Useful conversions:
1 m = 1000 mm
1 m3 = 1000 litres
1 m3 = 1.308 cubic yards
1 cubic yard = 27 cubic feet
1 foot = 12 inches
For a slab quoted in millimetres of thickness, divide the thickness by 1000 before multiplying:
slab thickness = 150 mm = 0.15 m
Slabs and ground-bearing floors
A rectangular slab is the simplest case. Multiply plan area by thickness.
volume = area x thickness
Example, a 6 m by 4 m slab at 150 mm:
volume = 6 x 4 x 0.15
volume = 3.6 m3
For non-rectangular slabs, split the plan into rectangles and triangles, calculate each piece, and add them up. Do not try to average an L-shape into one rectangle, it will drift.
rectangle = length x width
triangle = 0.5 x base x height
Watch for thickenings. Many ground floor slabs have thickened edges or internal thickenings under load-bearing walls. These add volume and are easy to miss. Calculate the flat slab first, then add each thickening as a separate strip.
Strip footings
A strip footing is a long trench fill or reinforced strip under a wall. Treat it as length × width × depth, using the trench width and the poured depth, not the trench dig depth.
volume = total wall length x footing width x footing depth
Example, 30 m of wall on a 600 mm wide, 250 mm deep footing:
volume = 30 x 0.6 x 0.25
volume = 4.5 m3
For a building footprint, use the centreline length of the footing run rather than adding each external wall length separately. Adding outer lengths double counts the corners.
centreline length = perimeter - (4 x footing width) // rectangular footprint
For deep trench fill foundations, the depth term is the full fill depth, which can be a metre or more. Small width errors then become large volume errors, so confirm the as-dug trench width, not just the design width.
Pad footings and column bases
A pad footing is an isolated rectangular base under a column. Each pad is length × width × depth.
volume per pad = length x width x depth
total = volume per pad x number of pads
Example, 8 pads at 1.2 m by 1.2 m by 0.45 m:
volume per pad = 1.2 x 1.2 x 0.45 = 0.648 m3
total = 0.648 x 8 = 5.18 m3
If pads are different sizes, schedule them in a table so each type is counted with its own dimensions and quantity:
| Type | Size (m) | Depth (m) | No. | Volume each | Total |
|---|---|---|---|---|---|
| P1 | 1.2 x 1.2 | 0.45 | 8 | 0.648 m3 | 5.18 m3 |
| P2 | 1.5 x 1.5 | 0.50 | 4 | 1.125 m3 | 4.50 m3 |
| P3 | 0.9 x 0.9 | 0.40 | 6 | 0.324 m3 | 1.94 m3 |
Foundation walls and stem walls
A foundation wall poured above the footing is length × height × thickness.
volume = wall length x wall height x wall thickness
Example, 30 m of 200 mm wall at 1.0 m high:
volume = 30 x 1.0 x 0.2
volume = 6.0 m3
Subtract large openings only if they pass fully through the pour, such as a doorway or a service penetration block-out. Small pockets and rebates are usually not worth deducting and act as a small built-in allowance.
Columns and piers
A square or rectangular column is width × depth × height. A circular column or pier uses the area of a circle.
rectangular column = width x depth x height
circular column = 3.1416 x radius^2 x height
Example, a 300 mm diameter pier 1.2 m high:
radius = 0.15 m
volume = 3.1416 x 0.15^2 x 1.2
volume = 0.085 m3
Add a waste allowance
The calculated volume is the theoretical fill. The poured volume is always more because of spillage, over-dig, uneven subgrade, formwork deflection, and material left in the truck or pump line.
A typical allowance is 5 to 10 percent. Use the higher end for trench fill on rough ground, footings against uneven excavation, or small pours where a part bag or part load matters.
ordered volume = calculated volume x (1 + waste fraction)
Example, 4.5 m3 of footing with 10 percent waste:
ordered volume = 4.5 x 1.10 = 4.95 m3
For ready-mix, also round up to a sensible delivery quantity. Suppliers batch in fixed increments, often 0.25 m3, and a short load can stop a pour.
Convert volume to bags or ready-mix
For small pours mixed on site, convert volume to bags. Yield per bag varies by product, so check the label, but common figures are:
| Bag size | Approx. yield |
|---|---|
| 25 kg | 0.011 m3 |
| 40 lb | 0.30 cubic feet |
| 60 lb | 0.45 cubic feet |
| 80 lb | 0.60 cubic feet |
bags = volume / yield per bag
Example, 0.3 m3 from 25 kg bags at 0.011 m3 each:
bags = 0.3 / 0.011 = 28 bags (round up)
Always round bag counts up, never down. For anything above roughly 1.5 to 2 m3, ready-mix is usually cheaper and more consistent than bagged concrete.
Worked example: small extension
A small extension needs a slab plus strip footings.
slab = 5 x 3 x 0.15 = 2.25 m3
footings = 16 m x 0.6 x 0.25 = 2.40 m3
subtotal = 4.65 m3
waste at 10% = 0.47 m3
ordered = 5.12 m3, round to 5.25 m3
The footing length here is the centreline run for the footprint, not the sum of outer wall lengths.
Common mistakes to avoid
- Mixing units, such as area in metres with thickness in millimetres
- Using the trench dig depth instead of the poured concrete depth
- Adding outer wall lengths instead of the footing centreline, which double counts corners
- Forgetting slab edge thickenings and thickenings under internal walls
- Ignoring waste, then running short on the day
- Rounding bag counts down
- Treating an irregular plan as a single averaged rectangle
Quick takeoff checklist
- List every concrete element separately: slab, footings, pads, walls, columns.
- Convert all dimensions to one unit before multiplying.
- Use poured depths, not excavation depths.
- Use centreline lengths for footing runs around a footprint.
- Add thickenings and isolated bases as their own line items.
- Sum the theoretical volume.
- Add 5 to 10 percent waste.
- Round up to the supplier's delivery increment, or round bag counts up.
For a fast browser takeoff across slabs, footings, columns, and bag estimates, use the Siteometry Concrete Calculator. For structural pours, always confirm dimensions and depths against the project drawings before ordering.