Bagged concrete vs ready-mix: finding the crossover point
For a fence post, bagged concrete is obviously right. For a house foundation, ready-mix is obviously right. Somewhere between them is a crossover — and most DIYers put it far too high, because they price the bags and forget to price themselves.
How little concrete a bag makes
The number that surprises everyone: a 25 kg bag of ready-to-use concrete yields only about 0.009–0.012 m³ once mixed (the exact figure is printed on the bag — check yours, brands differ; our calculator uses 0.0092 m³). A single cubic metre therefore takes roughly 85–110 bags — over two tonnes of material that has to be moved from the merchant to your mixer, one bag at a time, and mixed in 25 kg batches. Our concrete calculator converts your slab or footing dimensions into both cubic metres and bag counts, with a waste allowance built in.
The true cost of each route
Bagged: the bags themselves (multiply the count by the shelf price and it stops looking cheap around the half-cubic-metre mark), possible mixer hire, and hours of heavy, repetitive labour. As volume rises the hidden cost appears: cold joints. Concrete mixed in small batches over hours cures in layers, and each layer boundary is a weakness — for a structural slab that matters.
Ready-mix: a per-m³ price that falls with volume, plus a part-load charge for small deliveries — which is exactly what makes small ready-mix orders poor value. It arrives at a consistent, specified strength in one pour, but on the truck's schedule: you pay waiting time if you're not ready, so the site, the shuttering and the wheelbarrow route must be prepared before it arrives. Volumetric (mix-on-site) trucks split the difference, mixing what you need on arrival and charging for what's used.
Where the crossover falls
- Under ~0.5 m³ (post holes, a small pad, repairs): bags win. Part-load charges make a truck absurd at this size.
- 0.5–1 m³: the grey zone — 40 to 90 bags. Compare honestly: bag cost vs ready-mix price including the part-load charge, then weigh a full day of hard mixing against an hour of barrowing. If two of you are fit, keen and free, bags can still make sense; a volumetric truck often beats both.
- Over ~1 m³: ready-mix wins on money, quality and your back — and for anything structural, a specified mix in a single pour is worth paying for even where the sums are close.
A worked comparison
A 3 m × 2.5 m shed base at 100 mm deep is 0.75 m³; with 8% waste, call it 0.81 m³. At ~0.0092 m³ per bag, that's 88 bags (2.2 tonnes) — at typical merchant prices, comfortably over £350, plus a day's mixer hire and a genuinely exhausting day. A 0.81 m³ ready-mix delivery with a part-load charge frequently lands within £50–£100 of that figure, poured in twenty minutes with no joints. The arithmetic that looked obviously pro-bag at first glance is, at this size, close to a tie — decide on labour and quality, not the bag price alone.
Bag yields, mix strengths and delivery charges vary by brand, region and supplier — always confirm with the figures on your product and your supplier's quote. For structural work (foundations, anything load-bearing), have a professional specify the mix.