Work out how much concrete you need. This free concrete / cement calculator finds volume for a slab, footing or column, with premix bags and a 1:2:4 cement-sand-gravel breakdown.
Pick the shape and enter the dimensions. Thickness/diameter are in cm (or inches in feet mode). The result includes premixed bags and a 1:2:4 cement-sand-gravel breakdown if mixing your own.
Volume = the shape’s dimensions, plus a waste allowance. Premix is estimated at about 0.011 m³ per 20 kg bag. The 1:2:4 mix is a common general-purpose ratio (~1 part cement, 2 sand, 4 gravel); check your mix design for structural work.
How the Concrete / Cement Calculator Works
This tool estimates the volume of concrete needed for a slab, footing, or column pour, then converts that volume into premixed 20 kg bags or a 1:2:4 cement-sand-gravel breakdown for mixing your own batch.
Formula: For a slab or footing, Volume = Length × Width × Thickness. For a column, Volume = π × (Diameter ÷ 2)^2 × Height. Total concrete = Volume × Number of pours × (1 + Waste% ÷ 100). Premix bags = Total ÷ 0.011 m³ per 20 kg bag, rounded up. Cement (1:2:4 mix) ≈ Total × 320 kg, with Sand ≈ Total × 0.5 m³ and Gravel ≈ Total × 0.8 m³.
- Pour shape — whether you’re pouring a slab, footing, or column (changes which dimension fields apply)
- Length & Width — the slab or footing’s horizontal dimensions, in metres or feet
- Thickness / Depth — slab thickness or footing depth, in cm or inches
- Diameter & Height — used instead of length/width/thickness when the shape is a column
- Number of pours — how many identical pours to multiply the volume by
- Waste % — extra margin for spillage, over-excavation, or subgrade unevenness (10% default)
Example Scenarios
| Length (m) | Width (m) | Thickness (cm) | Waste % | Concrete Volume (m³) |
|---|---|---|---|---|
| 4 | 3 | 10 | 10% | 1.32 |
| 5 | 4 | 15 | 10% | 3.30 |
| 2 | 2 | 10 | 5% | 0.42 |
| 6 | 3 | 20 | 15% | 4.14 |
| 3 | 3 | 10 | 0% | 0.90 |
Concrete / Cement Calculator FAQ
How many 20 kg premix bags do I need per cubic metre of concrete?
Roughly 91 bags per m³, since each 20 kg bag yields about 0.011 m³ of mixed concrete. The calculator divides your total volume (including waste) by 0.011 and rounds up so you don’t come up short mid-pour.Should I use premix bags or mix my own 1:2:4 cement-sand-gravel batch?
Premix bags are simpler and more consistent for small pours like posts or patches. Mixing your own 1:2:4 batch (roughly 1 part cement, 2 sand, 4 gravel) is cheaper for larger volumes but needs accurate batching — check a mix design for structural work.How much extra concrete should I order for waste?
10% is a reasonable default for a straightforward slab or footing on prepared ground. Increase it toward 15-20% for uneven subgrade, hand-mixing losses, or if the excavation isn’t perfectly square.How do I calculate concrete for a round column or post footing?
Switch the shape to Column and enter the diameter and height. The calculator uses the cylinder formula (π × radius² × height) instead of length × width × thickness used for slabs and footings.Is a 1:2:4 mix ratio strong enough for a driveway or load-bearing slab?
1:2:4 is a common general-purpose ratio suited to paths, patios, and non-structural slabs. For driveways, foundations, or anything load-bearing, follow an engineer-specified mix design rather than a general ratio.Do I enter thickness in centimetres or inches?
It depends on your selected unit system. Choosing Metres sets thickness and diameter in centimetres; choosing Feet switches those same fields to inches automatically.Related Calculators
If you’re pricing a poured slab by raw volume rather than a bag-and-mix breakdown, the concrete calculator may be a better fit, while the concrete block calculator handles block-count masonry work instead of poured concrete. For the aggregate side of a DIY mix, the gravel calculator can help estimate a separate sub-base or drainage layer.