How Much Concrete Do I Need? How to Calculate m³ for Slabs, Footings and Foundations

Get the volume wrong on a concrete pour and it costs you either way. Order too little and the site stands waiting for another truck while the first pour starts to stiffen. Order too much and you are paying for concrete that goes back to the plant. The maths is simple, but the measuring is where most jobs go wrong. Here is how to work out exactly how much concrete you need, in cubic metres, for the pours that come up most often.

The Basic Formula: Length x Width x Depth

Ready-mix concrete is ordered in cubic metres (m³). To get the volume of any rectangular pour, multiply the three dimensions together, with every measurement in metres:

Length (m) x Width (m) x Depth (m) = Volume (m³)

The most common mistake is mixing units. Depths are usually given in millimetres, so convert them first: 100mm is 0.1m, 150mm is 0.15m, and 225mm is 0.225m. Put 150 into the sum instead of 0.15 and your answer will be a thousand times too big.

Worked Examples

Concrete slab

A slab 6m long, 4m wide and 150mm thick:

6 x 4 x 0.15 = 3.6m³

Strip or trench fill foundation

Work out the total length of trench first, then multiply by the trench width and the depth of concrete. A 20m run of trench, 0.6m wide, filled to a depth of 1m:

20 x 0.6 x 1 = 12m³

Where trenches meet at corners, measure the length along one side only, or you will count the corner twice.

Pad footings

Calculate one pad, then multiply by the number of pads. Eight pads, each 1m x 1m and 0.8m deep:

1 x 1 x 0.8 = 0.8m³ per pad, x 8 pads = 6.4m³

Round post holes and columns

For anything circular, use pi (3.14) x radius x radius x depth. The radius is half the diameter. Twenty post holes, each 300mm across and 600mm deep:

3.14 x 0.15 x 0.15 x 0.6 = 0.042m³ per hole, x 20 holes = 0.85m³

Measure the Ground, Not the Drawing

Drawings give you the design dimensions. The excavation you actually have is rarely the same. Trenches get over-dug, sides collapse and sub-bases are never perfectly level, and every extra few centimetres of depth across a large area adds up fast.

Before you order, measure the depth at several points along each trench or across each slab and use the average. On a slab, an extra 20mm of depth across 100m² is another 2m³ of concrete, which is a good part of a truck.

How Much Extra Should You Order?

Once you have the measured volume, add an allowance on top. Many contractors add somewhere between 5 and 10% to cover uneven ground, spillage and over-dig. If the concrete is being pumped, allow for the concrete that stays in the pump and the lines at the end of the pour as well.

On bigger pours there is a better way to handle the uncertainty than ordering a large margin up front. Order the bulk of the volume, then measure what is left to fill before you call off the final truck. That last truck is the balancing load, and it means you only order what the pour actually needs.

How Many Truck Loads Is That?

Ready-mix drum trucks commonly carry 6m³ or 8m³, and some suppliers run smaller mini-mix vehicles for tight sites and small volumes. Divide your total volume by the truck size to see how many loads to expect. A 30m³ pour on 8m³ trucks is three full loads and a fourth part load.

That number also tells you how long the day will run, which matters once trucks start arriving. If you are planning a large pour, read our guide on avoiding waiting time charges before you agree the delivery schedule.

Mixing It Yourself Instead?

For very small jobs some builders mix on site from bags or loose materials rather than ordering ready-mix. In that case you need tonnes of aggregate and bags of cement rather than cubic metres of concrete. Our aggregate calculator guide covers that side of it.

Common Questions

How many m³ are in a concrete truck?

Most ready-mix drum trucks carry 6m³ or 8m³. Mini-mix vehicles carry smaller loads for restricted access sites. Ask your supplier which trucks they will send, because it changes how many loads you need.

How much does 1m³ of concrete weigh?

Normal concrete weighs roughly 2.4 tonnes per cubic metre. That matters if you are working out loads on a suspended floor or barrowing it any distance.

How thick should my slab or foundation be?

That comes from the drawings, the structural engineer or Building Control, not from a rule of thumb. Use the specified dimensions, then measure what has actually been dug.

What happens if I run short mid-pour?

You wait for another truck, and the concrete already placed keeps setting in the meantime. Measuring properly and holding back a balancing load is how you avoid it.

Need Concrete for a Commercial Job?

WasteHub arranges ready-mix concrete for commercial projects, alongside the skips and grab lorries on the same job, so your site has one contact for the waste going out and the concrete coming in. Send us your volume, the date and the site address, call 0204 638 2307 or get a quote.