Concrete Calculator — Volume, Cement Bags, Sand & Gravel
A concrete calculator estimates the volume of concrete a structure requires and translates it into materials: bags of cement, sand, and gravel. Volume depends on the shape — a rectangular slab is length × width × thickness, a column or circular foundation is π × radius² × height. Because slab thickness is usually quoted in centimeters while the other dimensions are in meters, the calculator normalizes units and adds a safety margin for spillage and uneven ground.
The material breakdown depends on the mix ratio. A common general-purpose mix is 1:2:3 (one part cement, two parts sand, three parts gravel by volume), while leaner mixes like 1:3:5 suit non-structural work. Since fresh concrete compacts as water fills the voids between aggregates, the dry materials must total roughly 1.5 times the finished volume — the standard dry-volume factor of about 1.54. For structural elements, always follow the mix and strength (e.g. 20–30 MPa) specified by an engineer.
How it works
Slab volume: V = Length × Width × Thickness. Column/circular footing: V = π × r² × h. Dry volume = V × 1.54. For a 1:2:3 mix: cement = dry volume × 1/6, sand = × 2/6, gravel = × 3/6. Cement bags = (cement volume × 1,440 kg/m³) ÷ bag weight (e.g. 50 kg).
Use cases
- Calculating cubic meters of concrete for a floor slab before ordering ready-mix
- Estimating cement bags, sand, and gravel for a small on-site mix
- Sizing the concrete volume of cylindrical columns or post footings
- Comparing the cost of ready-mix delivery versus mixing on site
- Budgeting materials for a sidewalk, driveway, or backyard patio
- Adding a safety margin so a pour is not interrupted for lack of material
Frequently asked questions
How do I calculate how much concrete I need for a slab?
Multiply length × width × thickness using the same unit. A 5 × 4 m slab that is 10 cm (0.10 m) thick needs 5 × 4 × 0.10 = 2 m³ of concrete. Add 5–10% for spillage, uneven subgrade, and small level differences, so you would order about 2.1–2.2 m³.
How many bags of cement per cubic meter of concrete?
It depends on the mix. With a common 1:2:3 mix, one cubic meter of finished concrete takes roughly 7 bags of 50 kg cement (about 350 kg), around 0.5 m³ of sand, and 0.8 m³ of gravel. Leaner mixes such as 1:3:5 use fewer bags but produce weaker concrete, so match the mix to the structural requirement.
What is the 1:2:3 concrete mix ratio?
It means one part cement, two parts sand, and three parts gravel measured by volume, plus enough water to reach a workable consistency (typically a water/cement ratio near 0.5). It is a traditional general-purpose proportion for slabs, sidewalks, and footings. For beams, columns, and any load-bearing element, use the strength class and mix specified by a structural engineer.
Why is dry volume larger than the finished concrete volume?
When cement, sand, gravel, and water are mixed, the paste fills the voids between the aggregate particles, so the mixture compacts. To end up with 1 m³ of placed concrete you need about 1.52–1.54 m³ of dry materials — the so-called dry-volume factor. Calculators apply this factor automatically; skipping it is the most common cause of under-ordering.
How do I calculate concrete for a round column or footing?
Use the cylinder formula V = π × r² × h, where r is the radius (half the diameter) and h is the height, all in meters. A column 30 cm in diameter (r = 0.15 m) and 3 m tall needs π × 0.15² × 3 ≈ 0.212 m³. Multiply by the number of identical columns and add a small margin for losses.