How Much Concrete Do I Need? Slabs, Footings and Post Holes

Concrete Is Sold by Volume, Not Area

Paint is sold by how much surface it covers. Concrete is not. It is bought and priced by volume — cubic metres in the UK, cubic yards in the US — because what matters is the space you are filling, and depth is part of that space.

This is why “I need concrete for a 12 square metre patio” is not a sufficient order. A 12 m² patio at 75 mm deep and the same patio at 150 mm deep differ by nearly a full cubic metre, which is the difference between one delivery and two.

The Basic Formula

For any rectangular pour — slabs, bases, strip footings, paths — it is simply:

Volume = Length × Width × Depth

Measure all three in the same unit and your answer comes out in that unit cubed. Work in metres and you get cubic metres, which is exactly what a concrete supplier wants to hear.

The Depth Trap

Length and width are usually measured in metres. Depth is almost always quoted in millimetres. Multiply them together as they are and your answer will be out by a factor of a thousand.

Convert the depth to metres first:

75 mm → 0.075 m
100 mm → 0.1 m
150 mm → 0.15 m
200 mm → 0.2 m

The imperial version of the same trap is depth in inches against length in feet: a 4 inch slab is 4 ÷ 12 = 0.333 feet deep, not 4.

Worked Example: A Patio Slab

A 4 m × 3 m patio, 100 mm thick.

Convert the depth: 100 mm = 0.1 m
Multiply: 4 × 3 × 0.1 = 1.2 m³
Add 10% waste: 1.2 × 1.1 = 1.32 m³ to order

In imperial, the same job in a 10 ft × 10 ft bay at 4 inches works out at 33.3 cubic feet, or about 1.36 cubic yards once waste is added.

Strip Footings and Trench Fill

Footings use the identical formula — the trench is just a long, narrow box. Measure the total run of the trench rather than treating each wall separately.

A 10 m run of footing, 600 mm wide and 250 mm deep: 10 × 0.6 × 0.25 = 1.5 m³, or 1.65 m³ with waste.

Be honest about your trench width. Machine-dug trenches are rarely neat, and a 600 mm bucket that wandered to 650 mm has quietly added 8% to your order.

Round Post Holes and Columns

Circular pours need the area of a circle rather than length times width:

Volume = π × radius² × depth

Remember the radius is half the diameter — squaring the diameter by mistake quadruples your answer.

Four fence post holes, each 300 mm across and 600 mm deep. Radius = 0.15 m, so each hole is 3.14159 × 0.15² × 0.6 = 0.042 m³. Four of them is 0.17 m³, or about 0.19 m³ with waste.

Note that this ignores the volume the post itself occupies, which is the right call — that small over-estimate is useful margin.

Always Add a Waste Allowance

Add 5–10% to every calculation. This is not padding for its own sake:

Excavated ground is never perfectly flat, and the low spots take more than the maths says.
Some is lost to spillage, the mixer drum, and the barrow.
Trenches over-dig. Formwork bows outward under the weight of wet concrete.

Use 5% for a small, tightly formed pour and 10% for anything dug into open ground.

Bags or Ready-Mix?

This is the decision that catches out most DIYers, and the numbers make it obvious once you see them.

A 25 kg bag of dry mix yields roughly 0.012 m³ of wet concrete — so you need about 85 bags per cubic metre. Our modest 4 m × 3 m patio at 1.32 m³ would take around 113 bags, which is well over a tonne of material to buy, carry, and mix by hand.

As a rough guide:

Under 0.2 m³ (post holes, a small base): bags are fine.
0.2 to 0.5 m³: bags are possible with a mixer and a helper, but tiring.
Over 0.5 m³: order ready-mix. It will usually be cheaper per cubic metre as well as vastly easier.

Ready-mix is typically sold in 0.25 m³ increments, and small loads often carry a part-load charge, so it is worth checking minimum order quantities before you settle on a delivery.

Which Strength Do You Need?

UK concrete is graded by its compressive strength in newtons per square millimetre. Ordering the wrong grade is a slower, more expensive mistake than ordering the wrong volume:

C15–C20: non-structural work, blinding, filling.
C20–C25: general domestic use, footings for small walls, shed bases.
C25–C30: foundations, floor slabs, most patios.
C30–C35: driveways and anything carrying vehicle weight.

If a structural engineer or your building control officer has specified a grade, that specification wins over any rule of thumb.

Why Under-Ordering Is the Expensive Mistake

Ordering slightly too much concrete wastes some money. Ordering too little can cost you the entire pour.

Concrete begins setting as soon as it is mixed. If you run out partway through a slab and wait for a second delivery, the first section will have started to cure, and the boundary between the two becomes a cold joint — a permanent line of weakness that cracks along its length. On a structural pour that means breaking it out and starting again.

Round up, always. Spare concrete can be dropped into a post hole, a spare form, or simply barrowed off site. A cold joint through a new slab cannot be fixed.

Conclusion

Length times width times depth, with the depth converted to metres before you multiply, plus a waste allowance. Circular holes swap in πr². Anything above half a cubic metre belongs in a mixer truck rather than in bags. Put your dimensions into a Concrete Volume Calculator to get cubic metres, cubic yards, and a bag count for rectangular pours in one step — then round up before you pick up the phone.

Try our Concrete Volume Calculator

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