Concrete volume:
how much to order for a slab, footing or column

The question every pour starts with

Before a pour, someone has to say how much concrete to order. Too little and the slab is poured in two sessions with a cold joint through the middle. Too much and you pay for material that ends up on the roadside. The estimate itself is simple geometry, and the risk is almost always in the units and the assumptions.

For a flat slab, the volume of concrete is the volume of the box it fills: length times width times thickness. Footings, plinth beams and columns are boxes or cylinders too, so the same idea carries across a whole site if you keep each piece separate and add them at the end.

Volume formula and unit rules

Slab thickness is usually quoted in millimetres or inches, while the plan dimensions are in metres or feet. Convert the thickness first: 150 mm is 0.15 m, and 4 inches is 4 ÷ 12 = 0.333 ft. A depth entered as 150 in a metre calculation inflates the answer a thousand times.

The result comes out in cubic metres, or cubic feet in imperial work. Ready-mix suppliers in metric countries sell by the cubic metre, and in the US by the cubic yard, which is 27 cubic feet.

Worked example: a 6 m × 4 m slab

In imperial units the same method works. A 12 ft × 10 ft patio at 4 inches thick is 12 × 10 × (4 ÷ 12) = 40 cubic feet, which is 40 ÷ 27 = 1.48 cubic yards before waste. Converting the metric answer for comparison, 3.6 m³ is about 127 cubic feet or 4.7 cubic yards.

Adding a waste and over-excavation allowance

The theoretical volume assumes perfect forms and a perfectly level base. In practice the subgrade dips, forms bow outwards a little and some mix stays in the pump or on the barrow. Contractors usually add somewhere between 5 and 10 percent, and more on rough ground or very small pours where spillage is a larger share.

Round the final figure up to whatever the supplier's minimum load or increment is. If the plant sells in half-cubic-metre steps, a 3.96 m³ requirement becomes a 4.0 m³ order. If you are mixing on site, the allowance also covers batch-to-batch variation.

Breaking a complex job into simple shapes

Few buildings are a single rectangle. Split the job into pieces and handle each one with its own formula, then add the volumes.

For beams cast monolithically with a slab, count the rib below the slab thickness only, otherwise the overlap is counted twice. Openings for stairs or ducts are deducted if they are large; small ones are often ignored for safety margin.

Add the pieces and then apply one waste allowance to the total. Applying it separately to each piece compounds the rounding and is harder to audit later.

Ordering, delivery and placing

Once you have a number, a few practical details decide whether the pour goes smoothly. Ready-mix is batched for a time slot, and the supplier will usually give you a window for discharge after which the mix starts to stiffen. Have the formwork checked, the reinforcement tied and the labour in place before the truck arrives, not after.

If the job needs more than one truck, ask for them to be spaced so the placed concrete does not start to set before the next load arrives. For a small pour, such as a 3.6 m³ slab, a single delivery is normal, and the supplier may charge a small-load fee below a minimum quantity.

After placing, the surface needs curing: kept damp or covered for several days so it gains strength without cracking. That detail is not part of the volume maths, but it decides how well the slab you paid for will last.

From cubic metres to cement bags

A rough materials list follows from the mix ratio. Dry volume is about 1.54 times the wet volume, because the loose materials compact when water is added. For an M20 mix of 1 : 1.5 : 3 (cement : sand : coarse aggregate), the parts add to 5.5.

Sand and aggregate quantities follow in the same proportion, as the note shows. Always confirm the mix design with your engineer or the specification for the structural element, because grade and admixtures change the cement content.

Pitfalls that leave you short

• Mixing units: millimetres in one box, metres in another.
• Counting the slab only and forgetting thickened edges or beams that go below it.
• Ignoring compaction of the sub-base, which can take more concrete when the surface settles.
• Rounding down. Always round the order up, not down.
• Forgetting that pumped mixes need a little extra to prime the line.

A calculator keeps the conversions straight and lets you try different thicknesses quickly, but the numbers only mean something if the measurements on site were taken carefully.