How to calculate how much concrete you need
Concrete is ordered by volume in cubic metres (m3). Measure length, width and depth in metres, multiply them together, then add an allowance for over-dig and waste before you order.
The basic sum
Volume in m3 = length (m) x width (m) x depth (m). Convert millimetres to metres first: 150 mm is 0.15 m, 450 mm is 0.45 m.
For an awkward shape, split it into rectangles, work out each one and add them up. For a round pad or pier, use 3.14 x radius x radius x depth, with the radius in metres.
Slab
A slab 4 m x 3 m and 100 mm thick:
- 4 x 3 x 0.1 = 1.2 m3
- with 10 percent extra: 1.2 x 1.10 = 1.32 m3
The thickness of the slab is the concrete only. Hardcore, sand blinding and insulation under it are separate.
Strip footings and trench fill
Foundation width and depth come from the design, the engineer and building control. The sizes below are only there to show the arithmetic, so use your own.
A strip footing is a layer of concrete in the bottom of the trench, with masonry built up from it. For 14 m of trench, 0.6 m wide with 0.25 m of concrete:
- 14 x 0.6 x 0.25 = 2.1 m3
Trench fill is concrete placed up to near ground level, so the depth is much greater. For 14 m of trench, 0.45 m wide and 1.0 m deep:
- 14 x 0.45 x 1.0 = 6.3 m3
Measure the centre line of the trench around corners and across internal walls so that you do not count the corners twice.
Allow for over-dig and waste
Trenches are rarely dug to the exact size drawn. If the trench above is dug 0.5 m wide, not 0.45 m, the volume becomes 14 x 0.5 x 1.0 = 7.0 m3, which is 0.7 m3 more. Soft spots, ground that crumbles and a deeper dig to reach firm ground all add more.
On top of that, some concrete is lost in spillage, uneven levels and what stays in the truck or pump. Adding 5 to 10 percent is a common rule of thumb. It is a judgement call: use more where the ground is poor or the dig is rough, and less for a neat shuttered slab. Measuring the open trench before you order is better than guessing.
Ordering ready-mix
Ready-mix is sold by the cubic metre, and suppliers set a minimum load. Check the minimum with your supplier, because a small order may be charged as a short load. Tell them the use (footing, slab), the strength or mix your design asks for, the site access, whether a pump is needed and when you will be ready.
Concrete is heavy, at roughly 2.4 tonnes per m3, so check that the access and ground can take the truck. Have the formwork, the labour and the finishing tools ready before the truck arrives.
Frost
Fresh concrete that freezes can be damaged and lose strength. Check the forecast before you order, not on the morning. If frost is likely, ask your supplier or engineer what protection and mix suit the job, and be ready to cover the concrete or move the pour.