Concrete and cement aren't the same thing
"Cement" gets used as a catch-all term, but it's really just one ingredient — a fine grey powder that acts as the glue. Mix it with sand, gravel or crushed stone (aggregate), and water, and the chemical reaction between cement and water is what binds everything into the solid material actually called concrete.
That distinction matters here because every result below the volume figure — bags, sand, aggregate — is cement's share of a larger mixture, not the whole thing.
Volume formulas for common pours
Every shape on this calculator boils down to multiplying an area by a thickness — the only thing that changes is how you get the area:
| Pour type | Formula |
|---|---|
| Rectangular slab | Length × Width × Thickness |
| Circular slab | π × radius² × Thickness |
| Column | π × radius² × Height |
| Footing | Length × Width × Depth |
| Wall | Length × Height × Thickness |
Worked example: a 10 × 10 ft patio slab poured 4 inches (⅓ ft) thick comes to 10 × 10 × 0.333 = 33.3 cubic feet. Divide by 27 (the number of cubic feet in a cubic yard) and you get about 1.23 cubic yards — the figure most ready-mix suppliers will actually quote you.
What the mix ratio numbers mean
A ratio like 1:2:4 lists parts by volume of cement, sand, and coarse aggregate, in that order. More cement relative to the other two ingredients makes a stronger, more expensive mix; less cement makes a leaner, cheaper mix suited to jobs that don't carry structural load:
| Ratio (cement:sand:aggregate) | Typical use |
|---|---|
| 1:3:6 | Lean mix — mass fill, blinding layers, non-structural base |
| 1:2:4 | General purpose — footings, slabs, driveways |
| 1:1.5:3 | Higher strength — beams, columns, load-bearing slabs |
| 1:2:3 | Denser structural mix, moderate cement content |
| 1:1:2 | Rich mix — water-retaining or high-strength elements |
These are commonly used starting ratios, not a substitute for an engineer's specification — any pour that carries structural load should follow the mix your project's design calls for or your local building code requires.
Turning volume into bags, sand, and aggregate
Once you know the total volume and mix ratio, splitting it into ingredients is a matter of proportions and dry densities:
- Divide the total volume across the ratio's parts — for 1:2:4 (7 total parts), cement gets 1/7 of the volume, sand gets 2/7, aggregate gets 4/7.
- Convert each ingredient's volume to weight using its dry density: roughly 94 lb per cubic foot for cement, 100 lb/cu ft for sand, and 150 lb/cu ft for coarse aggregate.
- Divide the cement weight by 94 lb (a standard bag) and round up to whole bags, since you can't buy part of a bag.
Take that same 1.23 cubic yard (33.3 cu ft) patio slab with a 1:2:4 mix: cement gets 33.3 ÷ 7 ≈ 4.76 cu ft, which at 94 lb/cu ft is about 447 lb — roughly 5 standard 94 lb bags once you round up.
Always order more than the exact number
Add 5–10% on top of the calculated volume before placing an order. Subgrade is never perfectly flat, formwork settles slightly, and some concrete is inevitably lost to spillage and overfill at the edges — all of which quietly eat into an exact-to-the-yard order.
Running short mid-pour is the scenario worth avoiding at almost any cost: concrete that's already started setting won't bond properly with a fresh batch poured against it later, creating a visible and structurally weaker cold joint. A little extra material is far cheaper than that repair.
Bagged mix vs. ready-mix trucks
For small jobs — fence post footings, a garden step, patching a small area — bagged pre-mixed concrete from a hardware store is the simplest option, and this calculator's bag count tells you how many to buy. Once a pour climbs toward a full cubic yard or more, though, a ready-mix delivery almost always works out cheaper and faster than hand-mixing dozens of bags — and gives a more consistent result across a large slab, since every batch comes from the same controlled mix.
Sources and further reading
- Portland Cement Association — Concrete Materials
- National Ready Mixed Concrete Association — About Concrete
This calculator gives a general planning estimate. For structural work, follow your project's engineered mix design and local building code rather than a generic ratio.