What this calculator does
Concrete volume is length times width times thickness, plus an allowance. A 4 by 3 metre slab at 100 mm thick is 1.2 cubic metres net, or 1.32 with a 10% allowance.
The allowance is not optional in practice. Ground is never perfectly level, formwork bows slightly under the weight of wet concrete, and the sub-base absorbs some. Running short mid-pour is a serious problem because the joint between old and new concrete is permanently weak, so ordering a little over is the cheap option.
The formula
The three dimensions multiply to a volume, with the thickness converted from millimetres. The allowance is added on top. Weight assumes 2.4 tonnes per cubic metre, which is typical for normal-weight concrete, and the bag count assumes a 20 kg pre-mix bag yields about 0.009 cubic metres.
| Term | Meaning |
|---|---|
| Allowance | Extra for uneven ground, formwork movement and sub-base absorption. 5 to 10% is usual. |
| Pre-mix bag | Dry mix needing only water. Economic only for small jobs. |
| Ready-mix | Delivered wet by truck, usually with a minimum order of about 1 m³. |
| Cold joint | The weak seam formed where fresh concrete meets concrete that has begun to set. |
The inputs explained
| Field | What to enter |
|---|---|
| Length (m) | Length in metres. |
| Width (m) | Width in metres. |
| Thickness (mm) | Thickness in millimetres. A domestic path is 75 to 100 mm; a driveway 100 to 150. |
| Allowance (%) | Allowance percentage. 10% is a reasonable default. |
| Price per m³ delivered ($) | Price per cubic metre delivered, for the cost estimate. |
When to use it
Ordering ready-mix
Trucks deliver by the cubic metre with a minimum order, so the volume determines the order.
Deciding bags against delivery
The bag count makes the crossover obvious: over about 0.5 m³, mixing by hand stops being sensible.
Budgeting a slab
Concrete is usually the largest single material cost in a slab.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much concrete does each thickness need?
The same slab area at a range of thicknesses.
| Thickness | Concrete required | Approximate weight | 20 kg pre-mix bags |
|---|---|---|---|
| 75 mm | 0.990 m³ | 2.38 tonnes | 107 |
| 100 mm | 1.320 m³ | 3.17 tonnes | 143 |
| 150 mm | 1.980 m³ | 4.75 tonnes | 214 |
| 200 mm | 2.640 m³ | 6.34 tonnes | 286 |
Questions
How thick should a concrete slab be?
Around 100 mm for a domestic path or shed base, 100 to 150 mm for a driveway carrying cars, and more with reinforcement for anything heavier. Local building requirements often specify a minimum, and a structural slab should be designed rather than estimated.
How many bags equal a cubic metre?
About 110 bags of 20 kg pre-mix, since each yields roughly 0.009 m³. That is over two tonnes of material, which is why mixing by hand stops being practical well below a cubic metre. Most suppliers set a ready-mix minimum around 1 m³.
Why allow extra?
Because ground is uneven, formwork bows under the weight of wet concrete, and sub-base absorbs some. Running out mid-pour creates a cold joint, a permanently weak seam that cannot be repaired properly. Ordering 10% over is far cheaper than that outcome.
How much does concrete weigh?
About 2.4 tonnes per cubic metre for normal-weight concrete. A modest 1.2 m³ slab is therefore nearly three tonnes, which matters for access, for what the sub-base must support, and for anyone planning to barrow it.
For bulk garden materials, see the mulch and soil calculator. For filling hollow blocks, see the concrete block fill calculator.