How many bags of concrete or mortar you need
Measure the pour and get the number of bags, or the cement, sand, gravel and water you need if you mix it yourself.
Waste covers an uneven base, what stays in the mixer and the bags that split open. Below 10% you risk running out halfway through, and a pour finished the next day leaves a cold joint.
The yield in litres of mixed material per bag is printed on the bag and changes from product to product, so copy it from there: the value proposed here is only the typical one. The mixer size is used to count the batches, leave it at zero if you are not using one. Nothing leaves your device.
Volume first, materials second
Every figure starts from the volume you have to fill, and the usual mistake is mixing units: slab thickness is written in centimetres and has to be divided by a hundred before multiplying by the square metres. A 4 × 3 metre raft 10 cm thick is 12 m² × 0.10 m, that is 1.2 m³, not 12. Round posts go by the area of a circle, π × radius², so four columns 30 cm across and 2.5 m tall come to 0.71 m³ rather than twice that. The waste margin is real work, not padding: the ground is never flat, some mix always stays in the drum and the odd bag splits, so 10% is the sensible minimum and a rough excavation deserves 15%.
How many bags make a cubic metre
A 25 kg bag of ready mix yields roughly 12 litres of mixed material, because the powder plus the water take up far less room than the loose ingredients they replace. A cubic metre is a thousand litres, so you need about 80 bags, two tonnes to buy, carry and open one at a time. That is the number worth looking at before deciding whether to do it yourself. The exact yield is printed on the bag and is not the same for every product, which is why it is an editable field here instead of a fixed constant: fine mortars yield more, gravel concretes yield less.
Site mix proportions, and why they do not add up
One cubic metre of general purpose concrete takes roughly 300 kg of cement, 690 kg of sand, 1250 kg of gravel and 150 litres of water, the classic 1 : 2 : 4 by volume, while the richer 1 : 2 : 3 pushes the cement to 350 kg. Add up the loose materials and you get about one and a half cubic metres, which is not an arithmetic slip: the sand slots into the voids between the stones and the cement paste fills the voids between the grains of sand. Order a cubic metre of sand and one of gravel expecting two cubic metres of concrete and you end up with barely one. The water shown is a water to cement ratio of 0.5, which is also the line not to cross, since a sloppy mix spreads more easily and sets weaker.
When bags stop making sense
Past one cubic metre a ready mix truck almost always beats the equivalent 80 bags, arrives correctly batched and saves you a day at the mixer. The real constraints are elsewhere: whether the truck can get close enough to discharge, and whether you can place the whole load before it starts to set. The opposite limit matters too. Below 5 cm of thickness gravel concrete cannot be used at all, because the stones do not fit in the layer, and the job moves to a sand and cement screed or a fine mortar.
What this tool does not cover
The proportions are traditional small site mixes, suited to paths, shed slabs, kerbs, fence post footings and blinding. They are not a mix design: they ignore strength class (C25/30 and the like), exposure class, admixtures, the real grading of the aggregate and curing, and they say nothing about reinforcement. Anything structural, a retaining wall or any pour that carries a load, is sized by an engineer and this calculation is no substitute for that. Lime mortars, used for renders and traditional masonry, are also outside this table. Everything is metric: in the United States bags are 60 lb (27 kg) and 80 lb (36 kg) with the yield printed in cubic feet, so convert first, since 0.6 cu ft is 17 litres and a cubic yard is 0.765 m³.