How much concrete do you need?

What This Concrete Volume Calculator Does

This calculator estimates how much concrete you need to pour a rectangular slab, returning the volume in cubic meters (m³). Enter the slab's length, width, and depth (thickness), and it multiplies them together to give the raw volume, then lets you add a waste allowance for spillage and over-excavation.

It is built for DIY homeowners pouring a shed base, patio, or driveway, as well as builders and landscapers who need a quick ordering figure before calling a ready-mix supplier. Knowing the cubic-meter total upfront helps you avoid the two most expensive mistakes: ordering a part-load top-up at a premium, or paying for concrete you cannot use before it sets.

How It Works: The Formula

Concrete volume for a slab is the area of the surface multiplied by its thickness. All three measurements must be in the same unit (meters) so the result lands in cubic meters:

V = length × width × depth (in m³)

Because some concrete is always lost to uneven sub-base, formwork gaps, and what stays stuck in the mixer or barrow, add a waste margin on top:

Order volume = V × (1 + waste %), using 5-10% for most jobs.

Worked Example

Suppose you are pouring a slab for a garden shed measuring 4 m long by 3 m wide, at a depth of 0.1 m (100 mm, a common thickness for a light foundation).

Step 1 - Raw volume: 4 × 3 × 0.1 = 1.2 m³.

Step 2 - Add 10% waste: 1.2 × 1.10 = 1.32 m³.

You would order 1.32 m³, which most suppliers round up to the nearest quarter or half cubic meter. If your depth were measured in millimetres, convert first: 100 mm ÷ 1000 = 0.1 m. Mixing units (metres for length, millimetres for depth) is the single most common error and throws the result off by a factor of 1000.

Tips and Common Mistakes

A few practical points keep your estimate accurate:

• Measure the actual excavated depth, not the planned one - dug bases are rarely perfectly level, and low spots quietly increase volume.
• Use 5% waste for small, tidy pours and 10% (or more) for large slabs, sloping ground, or hand-mixing where loss is higher.
• For thickened edges or footings around the perimeter, calculate those as separate volumes and add them on.
• Round the final figure up, never down; running short mid-pour can leave a cold joint where fresh and set concrete meet.
• 1 m³ of concrete weighs roughly 2.4 tonnes, so check access and barrow distance before the truck arrives.

Factors That Affect How Much You Need

The biggest driver of volume is slab thickness: doubling depth doubles the concrete, so confirm the right specification for your load. Footpaths and shed bases often use 75-100 mm, while driveways carrying vehicles typically need 100-150 mm or more.

Sub-base quality also matters. A well-compacted hardcore layer holds a consistent depth, whereas soft or eroded ground lets concrete sink and pushes your real usage above the calculated figure. For irregular shapes, split the area into rectangles, calculate each in cubic meters, and sum them before applying your waste percentage.