Concrete Volume Calculator

Last updated: 2026-05-18

How much concrete do you need?

Whether you're pouring a garden path, a shed base, or a driveway, this calculator tells you exactly how much concrete to order or buy in bags — including a waste allowance so you don't run short mid-pour.

Formula

For a rectangular slab:

V_{\text{net}} = L \times W \times T

V_{\text{with waste}} = L \times W \times T \times \left(1 + \frac{\text{waste \%}}{100}\right)

Bag counts are then:

N_{40\text{kg}} = \left\lceil \frac{V_{\text{with waste}}}{0.020} \right\rceil

N_{25\text{kg}} = \left\lceil \frac{V_{\text{with waste}}}{0.0125} \right\rceil

A standard 40 kg premix bag yields approximately 20 litres (0.020 m³); a 25 kg bag yields about 12.5 litres (0.0125 m³). Always round up — you never want to run short.

Choosing the Waste Factor

Scenario	Waste factor
Simple, level slab	5–8%
Typical residential pour	10% (default)
Uneven ground, complex shape	15%
Multiple small pours	15–20%

The 10% default handles spillage, over-mixing, slight subgrade irregularities, and the small amount left in the drum.

Bags vs. Ready-Mix

Use bags when: the volume is under roughly 0.5 m³ (about 25 × 40 kg bags). Bags are convenient, widely available at hardware stores, and leave no minimum-order waste.

Use ready-mix when: the volume exceeds 0.5–1.0 m³. A concrete truck delivers mixed concrete directly to your pour, saving labour and ensuring a consistent mix. Most suppliers have a minimum order of 0.2–0.5 m³ and charge a short-load fee below that. When ordering from a North American ready-mix supplier, switch the volume unit to cubic yards (yd³).

Practical Tips

Subgrade preparation matters more than extra concrete. A well-compacted, level base prevents cracking far better than a thicker slab.
Reinforcement: for slabs thicker than ~75 mm, add steel mesh or rebar. The calculator covers volume only.
Temperature: avoid pouring in temperatures below 5°C or above 32°C without special admixtures.

Frequently Asked Questions (FAQ)

How do I calculate how much concrete I need?

For a rectangular slab: volume = length × width × thickness. Then add a waste factor (typically 10%) to account for spillage, uneven subgrade, and mixing losses: volume with waste = net volume × (1 + waste%). For example, a 3 m × 2 m slab 100 mm thick needs 3 × 2 × 0.10 = 0.60 m³ net, or 0.66 m³ with 10% waste. Always round up to whole bags — it is better to have a little left over than to run short mid-pour.

How many bags of concrete do I need for a slab?

A standard 40 kg bag of premixed concrete yields approximately 20 litres (0.020 m³) when mixed; a 25 kg bag yields about 12.5 litres (0.0125 m³). Divide your total volume (with waste) by the bag yield and round up. For 0.66 m³: 40 kg bags needed = ⌈0.66 / 0.020⌉ = ⌈33⌉ = 33 bags. Exact yield varies by manufacturer — check your bag for the precise figure. These values are standard estimates.

What is a waste factor for concrete?

The waste factor accounts for concrete lost to spillage, over-mixing, uneven subgrade (the ground is rarely perfectly level), and the small amount left in the mixer drum. A 10% waste factor is the standard rule of thumb for most residential slabs. Use 15% for sites with uneven ground, complex shapes, or multiple small pours. Use 5% for very large, simple pours where waste is minimised by a ready-mix truck.

When should I use ready-mix concrete instead of bags?

Bagged premix is practical for small pours up to about 0.5–1.0 m³ (roughly 25–50 bags).

For larger volumes, ready-mix concrete delivered by truck is more economical and consistent — most suppliers have a minimum order of 0.2–0.5 m³ and charge a short-load surcharge below their minimum. Ready-mix is also better for structural applications (footings, foundations) where batch-to-batch consistency matters. Switch the volume unit to cubic yards (yd³) when ordering from US ready-mix suppliers.

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