Turn a finished concrete volume into cement, sand, gravel, and water for a chosen strength, working back through an explicit dry-volume factor rather than splitting the wet volume.
Change the inputs that matter to your decision. Optional assumptions stay collapsed until you need them.
Enter the finished concrete you need to place. The dry ingredients are worked back from it.
Both defaults are visible and editable because both change the order materially.
1 : 2 : 3.5 by volume at w/c 0.68, batched from 41.58 ft³ of dry material to place 27 ft³ of concrete.
Cement, sand, and gravel are proportioned out of the dry volume, not the finished volume. Apportioning them straight from the concrete you want to place ignores that the dry ingredients bulk down as the paste fills the voids between them.
| Finished concrete | 27 ft³What you place |
|---|---|
| Dry material | 41.58 ft³Finished volume × 1.54 — dry ingredients bulk down as the paste fills the voids |
| Portland cement | 601 lb6.4 ft³ · 7 × 94 lb bags |
| Sand (fine aggregate) | 1,279 lb12.79 ft³ · Clean, sharp, well-graded concreting sand |
| Gravel (coarse aggregate) | 2,239 lb22.39 ft³ · Nominal 20 mm / ¾ in graded aggregate |
| Mixing water | 409 lb6.55 ft³ · w/c 0.68 by mass against the cement |
| Water volume | 49 galw/c 0.68 against 601 lb of cement |
Nominal volumetric mixes are working rules of thumb for site batching, and the water–cement ratios follow ACI 211.1 for non-air-entrained concrete. Anything structural, exposed to freeze-thaw, or subject to inspection needs a designed mix from a supplier, not a nominal ratio.
Practical notes about the inputs, assumptions, and result.
Because loose sand and gravel are mostly voids. When you add cement paste and water, the paste fills those voids rather than adding to the pile, so the finished concrete occupies materially less space than the ingredients did. The usual working factor is about 1.54. Splitting the ingredients straight out of the finished volume — which is easy to do and looks reasonable — short-orders every material by roughly a third.
It is the mass of water divided by the mass of cement, and it is the single strongest determinant of concrete strength. Lower is stronger and stiffer; higher is easier to place and weaker. It is a mass ratio, so applying it to the cement's volume instead gives an answer that is wrong by the density of cement against water.
Sand delivered from a yard commonly carries two to six percent free water by mass. If you add the full mixing water on top of that, the real ratio is higher than the one you specified and the concrete is weaker than intended. Entering the moisture keeps the ratio you asked for.
No. A nominal volumetric mix is a site rule of thumb for a path, a pad, or a small footing. Structural work, anything inspected, and anything with an exposure rating needs a designed mix from a supplier with the aggregate grading and admixtures accounted for.
A 94 lb bag of Portland cement is a cubic foot loose, which is why US mix proportions are so often quoted in bags. The bag count and the cement volume in cubic feet come out the same here for that reason.
Yes, and you should if you know your aggregate. Values between about 1.50 and 1.57 are all defensible depending on grading and how loosely the material is stacked. It is exposed as an editable input rather than buried, because it moves every material quantity proportionally.
Other tools in construction & diy.
Work out how much paint a room needs from its dimensions, doors, windows, and coat count, then solve for the cheapest combination of tins that actually covers it.
Size a slab, footing, round column, wall, or flight of stairs with thickness in its own unit, then compare bagging the pour against a ready-mix delivery.
Estimate tiles, boxes, and setting materials for a room from the real row-by-row layout, with grout taken from the joint geometry and no grout quoted for a floating floor.