Concrete Mix & Volume Calculator
Enter the shape, size and number of pours, pick a mix ratio and a waste allowance. The page returns wet and dry volume, the 25 kg bags to buy, and the cement, sand, gravel and water by weight.
Concrete is ordered by volume but sold by the bag, and the bag count depends on the mix, not just the hole. Enter the shape and size, pick a mix and a waste allowance, and the page returns the wet volume, the dry volume, the 25 kg bags to buy and the cement, sand, gravel and water by weight.
What you are pouring
Measure in metres and centimetres. For several identical pours, set the quantity and the page multiplies for you.
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Material per cubic metre
| Grade | Ratio | Cement | Sand | Gravel | Water | Typical use |
|---|---|---|---|---|---|---|
| C15 | 1 : 2 : 4 | 317 kg | 704 kg | 1320 kg | 158 L | Blinding, kerbs, light footings |
| C20 | 1 : 2 : 3 | 370 kg | 821 kg | 1155 kg | 185 L | Shed floors, paths, fence posts |
| C25 | 1 : 1.5 : 3 | 403 kg | 672 kg | 1260 kg | 202 L | Driveways, garage floors |
| C30 | 1 : 1 : 2 | 554 kg | 616 kg | 1155 kg | 277 L | Structural slabs, heavy loads |
Ready reckoner for common slabs
| Size (m) | Volume | 25 kg bags | Typical job |
|---|---|---|---|
| 3 × 3 × 0.10 | 0.90 m³ | 72 | Blinding, kerbs, light footings |
| 4 × 5 × 0.10 | 2.00 m³ | 160 | Shed floors, paths, fence posts |
| 3 × 3 × 0.15 | 1.35 m³ | 108 | Shed floors, paths, fence posts |
| 4 × 5 × 0.15 | 3.00 m³ | 240 | Driveways, garage floors |
| 6 × 8 × 0.10 | 4.80 m³ | 384 | Driveways, garage floors |
| 10 × 10 × 0.10 | 10.00 m³ | 800 | Structural slabs, heavy loads |
Five rules before you order
- Volume is length × width × thickness, with thickness in metres. A 10 cm slab is 0.10 m, not 10.
- Dry volume is about 1.54 times wet volume. Cement, sand and gravel are batched dry, so the ratio applies to the dry figure.
- One 25 kg bag of all-in ballast mix yields about 0.0125 m³. That is roughly 80 bags per cubic metre.
- Add 5–10% for waste. Formwork bowing, spillage and uneven sub-base all eat into the theoretical volume.
- Water is about half the cement weight at a 0.5 ratio. Add it gradually — too much water halves the strength.
Practical notes
- Order ready-mix when the pour is over about 1.5 m³. Bag mixing that volume by hand takes hours and the first batch starts curing before the last is placed.
- Compact each layer with a vibrating poker or a stick, then tap the formwork. Air pockets are the main cause of weak corners.
- Keep concrete shaded and damp for the first 3–7 days. Curing, not the mix, decides the final strength.
- Do not pour on frozen ground or in standing water. Both stop the cement hydrating properly.
Four common mistakes
- Confusing wet and dry volume. Mixing 1:2:3 by the wet volume leaves the batch short of sand and gravel.
- Using a shovelful as a precise measure. Volume batching is approximate; weigh the cement when strength matters.
- Adding water to make it flow. Every extra litre costs about 2–3 MPa of strength.
- Forgetting the sub-base. A 10 cm slab on soft soil cracks within a season; compact 10 cm of hardcore first.
Figures are planning estimates for normal-weight concrete with damp sand and 20 mm gravel. For structural, load-bearing or exposed work, follow the project specification and the ready-mix supplier’s mix design.