This concrete mix calculator turns your pour shape, dimensions, mix grade, bag size, and waste allowance into a practical concrete order. Start with on-site batch mode to see cement, sand, aggregate, water, and whole cement bags. Switch to ready-mix order when you need cubic yards and cubic meters for a supplier. The tool covers slabs, walls, columns, tubes or ring footings, strip footings, and solid stairs (summed step by step rather than as a bounding slab).
From volume to cement, sand, and aggregate
Every result here runs one chain: the pour geometry gives a wet volume, your waste percentage lifts that to an order volume, and then the tool branches. Ready-mix order mode converts the order volume to cubic yards for the truck; on-site batch mode multiplies it by the 1.54 dry-volume factor, splits the dry volume by the mix grade's cement:sand:aggregate ratio, applies each material's bulk density, and rounds cement up to whole bags. A 10-by-10-foot slab 4 thick is 33.3 of concrete; a 10 waste pad makes that 1.36 to order. At the C20 default (a 1:1.5:3 mix) that same slab batches into about 419 of cement — 10 ninety-four-pound bags — plus 698 of sand, 1,265 of aggregate, and 230 litres of water. The table and sections below are that same chain applied to real slabs, grades, and order modes.
On-site batch mode returns cement, sand, aggregate, water, and whole cement bags; ready-mix order mode returns the cubic yards and cubic meters a supplier delivers. Pick the one that matches how the concrete arrives.
The mix grade changes the cement:sand:aggregate parts and the water-cement ratio, so a stronger grade needs more cement for the same slab. C20 (1:1.5:3) is the default; C25 (1:1:2) is the common reinforced step up.
Your waste percentage is added to the wet volume before anything is converted, so the contingency follows into both the cubic-yard order and the on-site material weights. 5 to 10 is normal.
Cement is rounded up to whole bags at the bag size you pick — 94 is the US sack, 50 the global standard — so you never batch one bag short mid-pour.
Example: a 12-by-12 shed slab
- A 12-by-12-foot slab poured 4 thick has a wet volume of 48 cubic feet.
- Ready-mix order mode adds 10 waste and reports 1.96 cubic yards — round up to 2 on a quarter-yard delivery ticket.
- Switch to on-site batch at the C20 default and the same slab breaks into 15 bags of cement — about 603 kg — plus 1,005 kg of sand, 1,821 kg of aggregate, and 332 litres of water.
- Step the grade up to C25 for a reinforced slab and the cement climbs to 20 bags while the aggregate drops, because the ratio, not the volume, is what changed.
How many bags of cement per slab?
Every cell below comes straight from this calculator at a 10 waste allowance and 94-pound cement bags, so the chart and the tool always agree. Concrete-to-order is the same whatever grade you batch — the ratio only changes the material split — so the cubic-yard column is shared and the bag columns show the two most common grades side by side.
| Slab (length × width × thickness) | Concrete to order | C20 cement bags (1:1.5:3) | C25 cement bags (1:1:2) |
|---|---|---|---|
| 8 × 8 × 4 | 0.87 | 7 | 9 |
| 10 × 10 × 4 | 1.36 | 10 | 14 |
| 12 × 12 × 4 | 1.96 | 15 | 20 |
| 10 × 10 × 6 | 2.04 | 15 | 21 |
| 20 × 10 × 4 | 2.72 | 20 | 28 |
Bags scale with both size and strength: the reinforced C25 mix carries noticeably more cement than C20 for the identical slab, which is why the grade you pick matters as much as the dimensions. Change the bag size, drop the waste to 5 for a clean pour, or pick a different grade and the numbers move with you.
Which mix grade should I choose?
The presets are nominal planning ratios, strongest at the top: M7.5 lean (1:4:8) for blinding and fill, C15 general (1:2:4) for non-structural pads, C20 standard structural (1:1.5:3, the default) for most slabs and footings, C25 reinforced (1:1:2) for reinforced slabs and beams, and C30 high strength (1:0.75:1.5) for heavier structural work. The grade does more than rename the answer: on the 10-by-10-foot 4-inch slab above, C20 needs 10 bags of cement (about 419 ) while C25 needs 14 bags (about 576 ) for the very same volume, because the richer 1:1:2 ratio packs more cement per cubic yard. The old builder's rule of a 1:2:3 mix — one cement, two sand, three gravel — sits between C15 and C25; enter it under the Custom grade if that is the recipe you were handed. Choose a grade by the job, not by habit: the water-cement ratio and the parts are what control strength and durability (ACI CODE-318-19).
Batch it yourself or order ready-mix?
The mode switch is the biggest fork on the page. On-site batch is for mixing from bagged cement, sand, and aggregate — you get material weights and a whole-bag cement count. Ready-mix order is for a supplier truck — you get the volume to order in cubic yards and cubic meters and nothing else, because the plant handles the proportioning. The 10-by-10-foot 4-inch slab is 1.36 to order; suppliers sell in quarter-yard increments and often set a truck minimum around a cubic yard, so round up to 1.5 on the ticket. Multiple identical pours stack through the quantity field: four 12-inch sonotube columns 4 tall come to 0.51 cubic yards of concrete together, small enough that a short-load fee usually applies. Below a yard or so, bagged concrete is often cheaper than a truck; above it, ready-mix wins.
How much water, and why the ratio matters
Water is set by the water-cement ratio, not guessed at the mixer: the tool multiplies the grade's ratio by the cement mass. The C20 default uses a 0.55 ratio, so the 10-by-10-foot 4-inch slab's 419 of cement calls for about 230 litres of water; step up to C25 and its lower 0.50 ratio and stiffer, stronger mix, and the water lands near 288 litres even though the cement rose. A lower water-cement ratio raises strength and durability but cuts workability, which is why a rich mix feels stiff — and why adding water on site to loosen a mix quietly weakens it. The ratio is the structural control variable behind the grade (ACI CODE-318-19), so treat the water figure as a target to hit with admixtures or better aggregate grading, not a number to pad.
Before you trust this number
A few assumptions keep this an ordering estimate, not an engineered mix design. On-site quantities use the standard 1.54 dry-volume bulking factor and loose bulk densities of 1440 for cement, 1600 for sand, and 1450 for aggregate; the waste percentage is applied to the wet volume before any conversion; and the preset ratios are nominal planning mixes whose real strength still depends on materials, curing, air, admixtures, and batching accuracy. It does not design the pour — no slab thickness, footing depth, reinforcement, joints, or exposure class — and it does not dose admixtures, air entrainment, fibers, or plasticizers, nor does it add pump-line losses, truck minimums, or short-load fees. Custom ratios return material quantities only and imply no verified strength class.
The avoidable mistakes are usually unit slips: mixing feet and inches in one dimension throws the volume off badly, entering a sonotube's outer diameter when you meant the inner bore (or the reverse) mis-sizes a tube pour, calling a slab 3 thick when the spec says 4 leaves you short, and rounding the cement bag count down instead of up strands you a bag shy at the finish. Enter real dimensions, the right shape, and an honest waste factor, and the order will match the pour.