Concrete Bags vs Ready-Mix Calculator

In short The choice between bagged concrete and a ready-mix delivery is usually settled by a rule of thumb, and the rule is wrong as often as it is right because it depends on prices that vary enormously. The arithmetic is worth doing, and it has one feature that surprises people. BELOW THE MINIMUM LOAD, READY-MIX COSTS THE SAME WHATEVER YOU ORDER. If the minimum is 3 m³ you pay for 3 m³ whether you take 0.3 or 2.9, so the total is flat and the cost per cubic metre falls as a hyperbola until you reach that threshold. At half a cubic metre against a 3 m³ minimum the effective price per cubic metre is six times the quoted one. That single fact is why small pours go to bags, and it is not a matter of opinion. Bags behave differently: they are a staircase, because you cannot buy part of one. One cubic metre at 0.0166 m³ a bag is 60.2 bags, so 61 — and the last one is mostly spare. That rounding is why bag costs jump at awkward volumes rather than rising smoothly. This page walks both curves in small steps and reports the first volume at which ready-mix stops being the dearer option, rather than quoting a threshold that would only be true for one set of prices. With the example figures that crossover lands at about 1.15 m³, and at 1 m³ the bags come out ahead by a useful margin. EVERY PRICE IS A FIELD AND THERE IS NO CURRENCY. Bag price, price per cubic metre, minimum load, delivery and any short-load fee all come from your supplier, in whatever money you use, so the comparison is about your job rather than an average one. AND THE NUMBER THE ARITHMETIC CANNOT SEE IS USUALLY THE DECIDING ONE. Sixty-one bags at 25 kg is over 1,500 kg to carry, tip and mix by hand, against a lorry that discharges in minutes. Above a cubic metre or so the labour tends to decide it long before the price does, and a continuous pour avoids the cold joints that batch-by-batch mixing invites.