Concrete: bags or ready-mix?
27 September 2026 · 4 min read · by Rodney Lee
Concrete is the one material where the crossover between 'pop to the shed' and 'order a lorry' is dramatic. Get the volume wrong by half a cubic metre and you're either leaving a cold joint or paying for a delivery you didn't need.
Work out the volume first
Volume is length × width × depth, all in metres, giving cubic metres (m³). A pad 3 m × 2 m × 100 mm deep is 3 × 2 × 0.1 = 0.6 m³. For footings, measure the trench the same way. Always add 5–10% for uneven ground — trenches are never as neat as the drawing.
When bags make sense
A 25 kg bag of ready-mix concrete yields roughly 0.01 m³. That means about 60 bags per cubic metre — a number that gets heavy fast. Bags are the right call up to around 0.3–0.4 m³ (a small slab, some post holes); past that you're spending the day mixing by hand for a worse result.
When ready-mix makes sense
- Anything over about 0.5 m³: one lorry pour beats a day at the mixer.
- Slabs and driveways where a continuous pour avoids weak joints.
- Jobs where the strength rating matters (C20/C25) — a batched lorry is consistent; hand mixing is not.
Ready-mix suppliers often charge a part-load surcharge for small deliveries, and minimum orders apply. Ask for the delivered price per m³ including any part-load fee before comparing with bagged mix.
Prices vary significantly by region and change with cement costs — always check current prices rather than quoting a forum post from three years ago.