How the two differ in kind
The question gets asked as a volume question and answered badly as one, because volume is standing in for three separate things: how much material you must physically move, how fast it has to go in, and how much of a delivery's fixed cost your pour can absorb. Start with the mass, because it is the part people underestimate standing over an empty hole. At the yields printed on the sacks, a cubic yard of placed concrete is about forty-five of the largest bags — over three thousand pounds of dry mix — and a cubic metre is around seventy of the largest metric sacks, better than two tonnes. You lift all of it twice: once onto the mixer, once into the form. Forty bags, the number most people arrive at, is most of a cubic yard.
Then there is the clock, and this is the argument that actually decides most pours. Bagged concrete arrives as a sequence of batches, and each batch has to be placed and rodded into the one before it while that one is still plastic. Miss the window and you get a cold joint: a plane where fresh mix met concrete that had already begun to stiffen, with no aggregate interlock across it — weaker in bending, and a preferential path for water forever after. A ready-mix truck's entire proposition is that it collapses the sequence: a load arrives all at once, wet into wet, and where the pour takes more than one load they are timed to follow a live edge rather than joined afterwards. But the risk only exists if the pour is ONE element. Ten post footings are ten independent pours with no joints between them; a single slab of the same total volume is one, and cannot be done in instalments without a decision about where the joint goes.
Access and specification are the two constraints that override both. A loaded mixer truck is heavy and its chute only reaches so far — a back garden, a terrace with no side return, a soft or falling site, and the drum simply cannot get near the forms. Barrowing from the kerb is a second job racing the same clock; a pump solves it and is its own separate operation to arrange. Bags go anywhere a person goes, up steps and through a house. Set against that, the truck delivers something no sack can: a specified mix — strength class, slump, aggregate size, air entrainment for freeze-thaw, fibres — batched to a ticket, with the water-cement ratio taken out of the hands of whoever is holding the hose at four in the afternoon.
The factors that actually differ
| Bagged concrete, mixed on site | Ready-mix delivered by truck | |
|---|---|---|
| Cold-joint exposure | The governing risk on a continuous pour — every batch boundary is a joint until the last one goes in, and the window closes faster in heat, wind and sun. | Largely designed out at the scale most of these pours sit at: one load, discharged wet into wet. Past a single truckload it becomes a scheduling problem instead — each load placed against the live edge of the last, before that face stiffens. |
| One element or many | Made for discrete pours — post holes, pads, bollard bases — where nothing has to be continuous with anything else and you can stop between them. | Earns its keep on the monolithic element: a slab, a raft, an unbroken strip footing. |
| Reaching the pour | Wherever a barrow and a pair of legs go — through the house, down steps, along a garden path. | Needs the drum within chute reach on ground that carries a loaded truck; past that it is barrows against the clock, or a pump booked as its own operation. |
| Control of the mix | You are the batching plant. Water goes in by eye, batch after batch, and consistency is a discipline rather than a delivery. | Specified and documented — strength class, slump, admixtures, air content on a ticket you can hand to an inspector. |
| Throughput ceiling | Set by mixer cycle and hands: load, mix, barrow, place, repeat. Two mixers and three people is a different job from one person alone. | Set by how fast you can take it — the bottleneck moves from mixing to screeding and finishing the whole area at once. |
| What your body carries | The entire mass, by hand, twice — pallet to mixer, mixer to form. | Carried to the chute by the truck, then gravity. Only the barrow run, if there is one, is yours. |
| Shape of the cost | Flat per unit volume, no minimum, no delivery: ten bags cost ten bags. Labour-heavy, and cheap only where the labour is your own. | Fixed charges first — delivery, a minimum load, waiting time past the unload allowance — then a materially lower rate per cubic yard or metre. The rate per unit volume falls as the pour grows; the bagged rate never does. |
| Getting the quantity wrong | Short by four bags is a trip to the merchant. Over by four is unopened stock that keeps, provided it stays dry and off the ground. | Short is the expensive failure: a second truck will not arrive before the first pour stiffens. Over is concrete you have already bought and must discharge somewhere. |
| Stopping and weather | Abandonable. Break at a planned joint and resume next weekend; the bags are hostage only to dry storage. | Booked. Rain on the slot means pour, reschedule or pay — the plant's window is not the same thing as yours. |
Which one, and when
Choose bagged concrete, mixed on site when…
- The pour is a set of independent elements — footings, pads, post bases — with no requirement that any two be continuous.
- The truck physically cannot reach the forms and a pump is a larger intervention than the job deserves.
- The volume is small enough that a delivery's fixed charges would dominate what you are quoted for it.
- You need the option to stop: uncertain weather, a job spread over weekends, or one pair of hands doing all of it.
Choose ready-mix delivered by truck when…
- The pour is one structural element that has to be monolithic — a slab, a raft, a continuous footing.
- The mix has to be specified: a strength class, air entrainment for freeze-thaw ground, fibres, or a ticket somebody will want to see.
- The volume would need more batches than your crew can mix and place inside the setting window.
- The truck can get close and you have enough hands to screed, float and finish the whole area the moment it lands.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- So where is the crossover, in bags?
- There are two crossovers and they rarely sit in the same place. The financial one is genuinely local and this site will not invent it: bagged cost is a straight line through the origin, ready-mix is a line with an intercept, and where they cross depends entirely on your plant's delivery and minimum-load charges against your merchant's bag price. Get both quotes for your actual volume — that is one phone call and one shelf edge, and it settles the money exactly. The physical crossover is usually the tighter of the two and does not move with the market: it is the point where the batches needed exceed what your people can place before the first of them sets. Most DIYers hit the physical limit well before the financial one.
- How bad is a cold joint, really?
- It depends on what the concrete is being asked to do. In a garden path it may never be more than a visible line that a crack eventually finds. In anything carrying load or resisting water it is a defect: the aggregate does not interlock across the plane, so the section is weaker in bending and shear there, and the joint wicks. The window is temperature-driven rather than fixed — much shorter in a heatwave with wind on the pour, longer on a cool damp day — so the same bag count is a different risk in different weather. If you cannot beat it, the professional response is not to hope: put the joint where you want it, form it deliberately, and detail it as a construction joint.
- Can I do part of the job with bags and part with a truck?
- Across separate elements, freely, and it is common practice — the truck takes the slab and bags set the post bases the week before, each one costed on its own calculator. Within a single element, not casually. The boundary between the bagged part and the delivered part is a cold joint unless it is built as a proper construction joint: the face cleaned and roughened, and where the element carries real load, a shear key or dowels across it. That is a design decision made before the pour, not a rescue improvised on the day the truck runs short.
- Is bagged concrete weaker than ready-mix?
- Not by nature. Bagged mixes are manufactured to a nominal strength and a correctly batched sack can reach it. It ends up weaker in practice for two reasons that are both about the person mixing rather than the product. The first is water: every extra splash added to make a stiff batch easier to barrow trades away strength permanently, and the temptation peaks late in the day when everything is going off. The second is incomplete mixing, which shows up as pockets of unwetted powder. Measure the water into each batch instead of judging it by eye, and run the mixer longer than feels necessary. Ready-mix simply removes the decision from you — the load leaves the plant at a specified slump and water added on site is either refused or written on the ticket.
- What happens to what is left over?
- The two failure modes are not symmetrical, which is why the ordering discipline differs. Unopened bags keep if they are stored dry and clear of the ground, so over-buying bags is deferred cost rather than waste — damp ones set inside the sack and are a total loss, which makes the pallet's covering the thing to get right. A truck's surplus is yours from the moment it is batched: you have paid for it and it has to come out of the drum before it stiffens. Decide in advance where it goes — a spare form, a set of post bases, a path section — rather than deciding it in the driveway while the unload allowance runs down.