Concrete Mix Calculator — Bags & Raw Materials by Ratio

What a 1:2:3 mix actually means

The ratio is by volume, not by weight: one part cement, two parts sand, three parts gravel — a nominal mix for general work, commonly quoted at around 3,000–3,500 psi at 28 days, and the fallback when no engineered design is specified. Water is not part of the ratio; it is added to a water-to-cement ratio, typically about 0.5 by weight.

The unobvious step is that dry materials take up more space than the concrete they make. Sand fills the voids between the gravel and cement paste fills the gaps around the sand, so you need roughly 1.54 times the finished volume as dry material. The calculator applies that factor, splits the dry volume by the 1:2:3 parts, and reports cement as bags (a 94-lb bag of Portland cement is about 1 cubic foot).

The quantities, per cubic yard

Run 1 cubic yard through the calculator and it returns:

That is the whole arithmetic: 27 cu ft × 1.54 = 41.58 cu ft of dry material per cubic yard, divided 1:2:3. Cement lands at 6.93 cu ft, which is 7 bags; sand at 13.86 cu ft; gravel at 20.79 cu ft. Water follows the cement, not the total volume — about half the cement weight, roughly 39 gallons for 7 bags.

Worked example 1 — one cubic yard

Enter 1: 7 × 94-lb bags of cement, about 0.51 cubic yards of sand, 0.77 cubic yards of gravel and 39 gallons of water. Sand and gravel are the two quantities people under-order, because at the merchant they are bought by volume or by the tonne and rarely match the bag of cement you counted yourself.

Worked example 2 — half a cubic yard

Enter 0.5: 4 bags of cement, 0.26 cubic yards of sand, 0.39 cubic yards of gravel and 20 gallons of water. Half a yard is the classic wheelbarrow-and-mixer job — small enough to batch and place before it stiffens, large enough that a helper earns their keep. Note the cement rounds up: 0.5 × 6.93 = 3.47 bags, so you buy 4.

Worked example 3 — three cubic yards

Enter 3: 21 bags of cement, 1.54 cubic yards of sand, 2.31 cubic yards of gravel and 117 gallons of water. At this size the bag count stops being the story — 21 bags is nearly a tonne of cement — and three yards is a pour that wants a mixer and several people. It is also where ready-mix usually wins on both price and quality.

The one-bag batch, and why it matters

Nobody measures out 14 cubic feet of sand. On site you mix one bag at a time and count repetitions, so the recipe that matters is the single-batch version: 1 bag of cement, 2 cubic feet of sand, 3 cubic feet of gravel, about 5.5 gallons of water. That is the same 1:2:3 ratio, and it yields a little under 4 cubic feet of concrete — which is why it takes about seven of those batches to fill a cubic yard.

Where a site mix goes wrong

• Too much water. Water decides strength. The cement needs only about 0.22–0.25 of its weight to hydrate, but a workable mix carries more; extra gallons weaken the concrete and cannot be undone by adding cement.
• Shovel-batching. A “part” by shovel varies by 30% or more from person to person and between wet and dry sand, so the nominal ratio drifts. Use one bucket, filled the same way every time.
• Wet sand and gravel. Bulk sand carries surface moisture that adds water you did not count and bulks the volume you did. If the sand is visibly damp, cut back the added water.
• Treating it as engineered concrete. A 1:2:3 site mix is a general-purpose, non-structural mix. Footings under beams and anything an inspector will look at should follow a specified mix design, not a bench ratio.

Quantities at a glance

Every material scales linearly with volume, so one table covers the common pour sizes. Each row is the calculator output for that volume:

Order a little over: sand and gravel are sold by volume or by the tonne and are cheap relative to a wasted pour, so 5 to 10% extra is normal. Cement bags keep, so a spare bag is never wasted.

Water, workability and the strength you get

Water is the ingredient people treat casually and the one that decides the result. The water-to-cement ratio is the weight of water divided by the weight of cement: at 0.5, a 94-lb bag takes about 47 lb, or 5.5 gallons. Cut the ratio toward 0.4 and the concrete gets stronger but stiffer to place and finish; push it toward 0.6 and it flows easily but sets weaker. Adding water to loosen a stiff mix is the most common way a good recipe makes poor concrete, and the strength lost that way cannot be recovered later by adding cement.

Aim for a mix that holds its shape without slumping into a puddle, roughly a 4- to 6-inch slump for most residential work, rather than judging by eye how wet it looks. If the mix is too stiff, a plasticising admixture does the job that extra water would otherwise do.

Batching and mixing on site

Fix the batch size before you start and keep it fixed. The easiest unit is one bag of cement with 2 cubic feet of sand and 3 cubic feet of gravel, which makes a little under 4 cubic feet; count batches and you always know how much concrete you have made. Mix the dry sand, gravel and cement together first, add about 80% of the water, mix to an even colour, then add the rest a little at a time until the consistency is right. Place the concrete within about 30 minutes of mixing: beyond that it stiffens, and trying to revive it with water only dilutes the cement.

Sources and standards

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