How to Calculate Concrete for a Slab or Footing

Getting the numbers right for your slab or footing

Ordering ready-mix concrete involves more than picking a grade. Whether you are pouring a garden slab, a shed base, or a footing for a small extension, the first step is always the same: working out how much material you need. Get this wrong and you either end up short, with a half-finished pour, or with a surplus that hardens in the skip. This guide walks through the calculation step by step, and explains what to watch out for at each stage so your concrete mix delivery matches the job.

Measuring length, width and depth

Start by measuring the area where the concrete will go. For a rectangular slab, the dimensions are straightforward: length, width, and depth. Use a tape measure and take each measurement in metres. If your slab is, say, 3 metres long, 2 metres wide, and 0.1 metres deep (100 millimetres), write those numbers down. For footings, the same principle applies: measure the trench length, the width at the base, and the depth from the bottom of the trench to the finished ground level.

Watch out for uneven ground. If the site slopes, measure depth at several points and use the average. For footings, check the trench width at both ends: soil collapse or digging errors can make it wider than planned, which increases the volume significantly. A small error in depth, say 10 millimetres, might not matter on a small slab, but on a long footing it can add an extra cubic metre or more.

Calculating volume in cubic metres

Once you have the three dimensions, multiply them together. Length x width x depth gives the volume in cubic metres. Using the example above: 3 x 2 x 0.1 equals 0.6 cubic metres. That is the theoretical volume, the amount if the formwork or trench is perfectly square and the concrete fills every gap without waste.

For a footing, the calculation is the same but you treat the trench as a long, narrow rectangular prism. If the trench is 10 metres long, 0.3 metres wide, and 0.5 metres deep, the volume is 10 x 0.3 x 0.5 equals 1.5 cubic metres. If the footing has a wider base or a stepped profile, break the shape into separate rectangles, calculate each volume, then add them together.

Circular slabs and irregular shapes

For a circular slab, measure the radius in metres, square it, multiply by pi (3.14), then multiply by the depth. A circle with a 2-metre radius and 0.1-metre depth is 3.14 x 4 x 0.1, which equals about 1.26 cubic metres. For irregular shapes, the simplest method is to divide the area into rectangles and triangles, calculate each section, and sum the volumes. Online concrete calculators can help, but always check the inputs yourself.

Adding a wastage allowance

No pour goes exactly to plan. Spillage, uneven formwork, and leftover concrete in the mixer all eat into the total. A 5 to 10 percent wastage allowance is standard. Multiply your theoretical volume by 1.05 or 1.10, then round up to the nearest sensible figure. For the 0.6 cubic metre slab example, adding 10 percent gives 0.66 cubic metres. Most ready-mix suppliers deliver in increments of 0.1 cubic metres, so you would order 0.7 cubic metres.

On larger jobs, the wastage allowance can save a second delivery. If you are ordering a concrete mix delivery for a footing, the driver will usually pour the full load, so you want to be sure the quantity is enough. Under-ordering by even a small amount can mean a delay while a second truck is dispatched, and that second load might not arrive before the first one begins to set.

Ordering the right grade

Volume is only half the equation. The concrete grade, often called the mix, determines the strength and suitability for the job. For a domestic slab, a standard C20 or C25 mix is common. For footings supporting a structure, C30 or higher might be specified by an engineer. The site context mentions Stacey Concrete Ltd, a Derbyshire-based supplier offering grades C10, C30, C40 and more. If you are unsure which grade fits your project, ask the supplier or consult a structural engineer before placing the order.

One point to watch: the slump, or workability, of the mix. A stiff mix with low slump is harder to spread but less likely to slump in the formwork. A wetter mix flows easier but may need reinforcement to hold its shape. Discuss the application with the supplier when you call to arrange a concrete mix delivery. They can advise on the right consistency for your slab or footing.

Checking access and timing

A concrete delivery is not like a parcel. The truck needs clear access to the pour area, and the concrete starts setting within a couple of hours. Before you order, check the route for low bridges, narrow gates, or soft ground that could stop the truck reaching the spot. For small jobs, a mixer truck with a discharge boom can reach further than you think, but measure the distance from the road to the pour point.

Timing matters too. Arrange for the delivery when you have enough helpers on site to spread, level, and finish the concrete before it begins to stiffen. On a warm day, the working time shortens. On a cold day, you might need to protect the pour from frost. Discuss these conditions with the supplier when you book.

Common mistakes and how to avoid them

Beginners often underestimate the depth of a slab. A 50 millimetre slab might sound enough for a garden path, but it can crack under weight. For a slab that will take a car or a shed, 100 millimetres is a safer minimum. For footings, the depth should suit the load and the soil type; a shallow footing on clay can move.

Another frequent error is forgetting to account for the volume of reinforcement. Steel mesh or rebar displaces a small amount of concrete, but the effect is negligible for most domestic jobs. What matters more is leaving enough cover over the steel to prevent corrosion. The calculation for volume stays the same; just make sure the reinforcement is placed correctly before the pour.

Finally, double-check your measurements. A tape measure that is not straight, a depth reading taken from the wrong point, or a miscalculation on paper can all lead to an order that is too large or too small. It is worth measuring twice and calculating once.

For anyone planning a project in the East Midlands, Stacey Concrete Ltd, based at Ryder Point Works, Wirksworth, Matlock DE4 4HE, UK, can be reached on +44 1629 825188. They supply ready-mix concrete grades C10, C30, C40 and more with delivery across the region, and their team can help match the right mix to your slab or footing requirements.

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