Concrete Mix Ratio Calculator — Cement, Sand, Gravel

A concrete mix ratio lists the volumes of cement, sand and coarse aggregate in that order, so 1:2:4 is one part cement, two parts sand and four parts gravel. The parts are by volume, which is why a bucket works as the measuring unit on a small job. This mix design calculator also works from a strength grade, because the grade is what a job actually specifies and the ratio is only a way of reaching it. M20 means 20 MPa, about 2,900 psi, on a 28-day cube, and C16/20 is the European grade that sits closest to it.

Dry materials take up more room than the concrete they make. Loose sand and gravel are full of air gaps, and once cement paste and water fill them the pile shrinks, so you need about 1.54 times the wet volume in dry material. A cubic yard of concrete is 27 cubic feet, so you load 41.6 cubic feet of dry cement, sand and gravel. At 1:2:4 the seven parts split that into 5.94 cubic feet of cement, 11.88 of sand and 23.76 of aggregate. Cement at 94 lb per cubic foot comes to 558 lb, which is six bags. To size the pour in the first place, use a concrete volume calculator and enter the result here.

Strength depends on water as much as on the ratio. Cement needs only about a quarter of its weight in water to hydrate, and the rest exists to make the mix workable. Surplus water leaves tiny voids as it evaporates, so each 0.1 added to the water-cement ratio costs roughly 13% of strength in this calculator's estimate. At 0.50 a 94 lb bag takes 47 lb of water, about 5.6 gallons. Damp sand already holds water, so hold back part of the allowance and add the rest until the mix just slumps. The strength figure is an Abrams-style rule of thumb for comparing mixes and is not a test result.

Pick the leanest mix that meets the strength you need, because every extra bag of cement adds cost and shrinkage. Mass fill, blinding and bedding need little more than 1:4:8. Footings on firm soil and floor bases are comfortable at 1:3:6 or 1:2:4. Slabs, paths and patios usually land at 1:2:4, and driveways, garage floors and steps do better at 1:2:3 or 1:1.5:3, where the extra cement resists wear and freeze-thaw. Posts, precast parts and anything heavily loaded move up to 1:1:2.

Nominal ratios stop being reliable above M30. A strength of M35 or M40 depends on the grading and cleanliness of your sand and aggregate, and it needs a designed mix with test cubes. The same applies to any structural member, such as a beam, column or suspended slab, where an engineer must specify the mix. For ordinary sand gravel cement work on the ground, the table ratios are a sound starting point.

Batch size is set by your mixer. The recipe above converts the totals into whole buckets for one drum, so every batch carries the same proportions. Treat the drum rating as the wet concrete it turns out, and fill to about two thirds if your mixer is rated for dry loading. Add about three quarters of the water first, then the aggregate, cement and sand, and bring the mix to a workable slump with the rest.

For small pours, bagged premix saves buying and storing three separate materials. The cost block above compares both routes, and a concrete bag calculator counts the premix bags for any shape. Past roughly a cubic yard, ready-mix wins on time and consistency. If a supplier quotes aggregate by the tonne or you need to switch between yards, metres, pounds and kilograms, a unit converter settles it.