This calculator estimates the volume of concrete you need for a slab, path, footing or pad, and how many bags of dry mix to buy. Enter the length, width and depth, set the yield per bag printed on the packaging, and it returns both the cubic volume and a whole-bag count. It supports metric (m, mm depth, m³) and imperial (ft, in depth, ft³) units.
How it works
The tool converts the depth into the same unit as the length and width, then multiplies the three to get the volume:
volume = length × width × depth
In metric, depth in millimetres is divided by 1,000 to metres; in imperial, depth in inches is divided by 12 to feet. The bag count is the volume divided by the yield per bag, rounded up to a whole bag:
bags = ⌈ volume ÷ yield per bag ⌉
A standard 25 kg bag yields roughly 0.011 m³, which is the default.
Example
A slab 3 m × 3 m at 100 mm deep:
volume = 3 × 3 × (100 ÷ 1000) = 3 × 3 × 0.1 = 0.9 m³ bags = ⌈ 0.9 ÷ 0.011 ⌉ = ⌈ 81.8 ⌉ = 82 bags
So that slab needs 0.9 m³ of concrete, or about 82 standard 25 kg bags.
Typical slab depths by use
Different applications call for different depths, which significantly affects the concrete volume:
| Application | Typical depth | Notes |
|---|---|---|
| Footpath / garden path | 75 – 100 mm | Pedestrian traffic only |
| Driveway (cars) | 100 – 125 mm | Residential vehicles |
| Garage floor | 100 – 150 mm | With thickened edges at perimeter |
| Concrete shed base | 100 mm | With DPM underneath |
| Strip footing (domestic) | 200 – 600 mm deep | Depends on load and soil bearing |
| Patio | 75 – 100 mm | Non-vehicular |
For load-bearing applications such as foundations and structural slabs, depth and mix should be specified by an engineer — these are illustrative ranges only.
Practical tips before you order
Always add 10% for waste. Spillage, uneven ground, and over-excavation mean you will use slightly more than the calculated volume. Rounding up by 10% avoids a second trip mid-pour.
Check the bag yield on the packet. Bag sizes vary (20 kg, 25 kg, 40 kg) and the yield per bag differs between fast-set, standard, and high-strength mixes. The default 0.011 m³ per bag is for a common 25 kg general-purpose mix; adjust it if your packaging states a different yield.
Mix vs. ready-mix concrete. For volumes above roughly 0.5 m³, ordering ready-mixed concrete is usually more cost-effective and practical than mixing bags by hand. This calculator shows the bag-count equivalent regardless, which is useful as a comparison.
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