My concrete quantity estimate for a patio project was just far enough off that I had to make a second order, which taught me to take the calculation more seriously rather than rounding loosely.
Concrete is one of the few construction materials where getting the quantity wrong has immediate, potentially irreversible consequences. Order too little and you're scrambling while the slab starts to set. Ready-mix lorries don't wait — they tip, they go, and you deal with the consequences. Here's how to calculate it properly before anyone picks up a phone.
The Core Formula
Volume (m³) = Length × Width × Depth (all in metres). Our concrete calculator handles this instantly for slabs, paths and post holes. Combine with our area calculator for base dimensions.
Example: Patio Slab
4m × 3.5m × 100mm deep: convert depth to metres (0.1m). Volume = 4 × 3.5 × 0.1 = 1.4 m³. Add 10% wastage: 1.54 m³. Order 1.6 m³.
Recommended Depths
- Pedestrian path: 75-100mm
- Patio: 100mm
- Driveway (cars): 150mm minimum
- Garage floor: 150mm
- Fence post footing: 600mm deep, diameter ≈ 3× post width
- House extension foundations: 300mm+ — always consult a structural engineer
Ready-Mix vs Mixing Yourself
Ready-mix: essential above 0.3 m³. Minimum order typically 0.5-1 m³. Costs £80-£150/m³ plus delivery. Ensure good lorry access before booking. Self-mix: practical for small pours. Standard ratio: 1 cement : 2 sand : 3 aggregate. One 25kg cement bag ≈ 0.01 m³ finished concrete.
Sub-Base: Do Not Skip
A compacted hardcore sub-base (100-150mm crushed stone) is essential under any driveway or patio in UK conditions — particularly on clay soil. Without it, ground movement will crack the slab within a few winters. Calculate sub-base volume separately using the same formula.
Have Everything Ready Before the Lorry Arrives
Ready-mix begins setting within 2 hours of batching. Formwork installed, sub-base compacted, reinforcement placed, helpers present — all before ordering. Concrete will not wait while you rummage for a screed board.
Further reading: British Precast provides technical guidance on domestic concrete specification. Visit British Precast for concrete guidance.
Choosing the Right Concrete Mix
The ratio you use changes the strength and workability of your concrete significantly. A standard 1:2:3 mix (cement, sand, aggregate) is suitable for most household applications including paths, patios, and shed bases. For foundations bearing structural loads, a stronger mix such as 1:1.5:3 is more appropriate. Ready-mix concrete arrives pre-proportioned and is usually specified by strength grade — C20 or C25 for most domestic uses, C30 or above for driveways and anything subject to vehicle loading.
The Calculation Step by Step
Volume = length x width x depth, where depth is in the same units. A common mistake is mixing units — measuring length in metres and depth in centimetres without converting. For a 3m x 4m patio at 100mm depth: 3 x 4 x 0.1 = 1.2 cubic metres. Add 10% for waste, consolidation, and any uneven sub-base: 1.2 x 1.1 = 1.32 cubic metres. For ready-mix, round up to the nearest 0.25 cubic metres as that is typically the smallest order increment.
When to Use Ready-Mix vs Mixing on Site
For anything over approximately 0.5 cubic metres, ready-mix is almost always more practical. Mixing that volume by hand is exhausting and difficult to achieve consistently. Ready-mix also sets uniformly across the whole pour rather than in sections, which matters for strength and appearance. For small jobs under 0.3 cubic metres — fence post collars, small repair patches, isolated post bases — bagged dry mix is entirely adequate and avoids the minimum order requirements of ready-mix suppliers.
Planning Your Pour
Concrete needs to be placed and levelled before it begins to set, which starts within 45-90 minutes of mixing or delivery depending on temperature and mix design. Have your formwork in place, your sub-base compacted, and any reinforcement positioned before the concrete arrives or before you begin mixing. Work in sections if the area is large. If you order ready-mix, have more people than you think you need — waiting while concrete sets because you are short of labour is an expensive mistake.