How many batches will my mixer take?
Divide the concrete volume by the mixer's USABLE capacity, not its advertised drum size. The figure printed on a mixer is the gross drum volume, and you cannot fill a drum to the brim — the material has to tumble to mix at all. A usable batch is typically 55–65% of the gross figure, so a 250-litre mixer gives about 150 litres a batch. Dividing by the nominal 250 under-counts the batches by around 40%, which on site is the difference between finishing and not.
Why multiply the wet volume by 1.54?
Because dry ingredients take up more space than the finished concrete they make. The water and cement paste fill the voids between the sand and the stone, so the loose dry materials you have to buy are about 1.54 times the volume you are placing. The factor is a site convention in the range 1.52–1.57 rather than a derived constant, which is why this calculator lets you change it.
What does a mix ratio like 1:2:4 mean?
One part cement, two parts sand, four parts coarse aggregate — by loose volume, not by weight. That distinction matters: cement has a bulk density of about 1440 kg/m³ against 1600 for sand and gravel, so converting all three at the same density overstates the cement by roughly 11%. On a typical domestic slab that is several bags too many.
What is the difference between M15 and M20?
M15 is nominally 1:2:4 and intended for general non-structural work — paths, kerbs, bedding. M20 is 1:1.5:3, a richer mix with more cement, for light structural work such as slabs and small footings. The numbers refer to a target characteristic compressive strength in newtons per square millimetre at 28 days, but a nominal ratio only approximates it; a genuine M20 is a design mix verified by cube tests.
How much water should I add?
Water is set by the water/cement ratio by weight — typically 0.45 to 0.60 — and it is the one ingredient the mix ratio does not describe. It is also the one most often got wrong, because more water makes concrete much easier to place and permanently weaker, at roughly 5% of the strength for each extra 0.05 on the ratio. Hold some back, mix, and add the rest only if the mix genuinely will not place: aggregate delivered from a yard is usually already damp.
How many 50 kg bags of cement will I need?
Take the cement's share of the dry volume, multiply by 1440 kg/m³, and divide by 50. This calculator does that and rounds up, because you cannot buy part of a bag. It is worth having one spare rather than one short — cement does not keep well once opened, but stopping a pour to fetch a bag leaves a cold joint, which is a permanent weakness in the slab.
How much wastage should I allow?
Ten per cent is the usual starting point and this calculator defaults to it. It is not padding: spillage, an uneven sub-base, over-dig at the edges and material left in the drum all come out of it. The sub-base is the big one — a slab poured just 10 mm deeper than intended across its whole area needs about 10% more concrete than the drawing says, and formwork rarely sits as exactly as a drawing assumes.
Should I mix on site or order ready-mix?
Roughly speaking, under about 1 cubic metre hand mixing or a small mixer is practical. Between 1 and 2 m³ it becomes hard work — that is around forty 50 kg bags of cement and two tonnes of aggregate to move by hand. Past 2 m³, ready-mix is usually cheaper once you count the labour, and it is far more consistent, which matters for anything structural.
Why can't I just fill the mixer up?
Because a drum loaded past about two-thirds turns over without folding. Mixing depends on material falling through itself as the drum rotates, and an overfilled drum simply rotates a solid mass. The result is streaky, unevenly hydrated concrete that is weaker than the same materials properly mixed — and it strains the motor and the gearing as well.
Can I use this for structural concrete?
Use it to estimate quantities, not to specify concrete. Nominal mixes are rules of thumb for small, non-critical work. Anything that carries a load, or that would hurt someone if it failed, needs a design mix to a recognised standard — IS 456, ACI 318 or EN 206 — specified by strength class and verified by cube tests, with the aggregate grading and cement type chosen deliberately rather than by counting shovels.