Overview
Use this calculator to turn length, width, and thickness into concrete volume in cubic metres. It then applies your selected allowance so you can account for handling loss, minor over-excavation, and small placement differences. That makes it easier to compare bagged concrete with a ready-mix order before you buy. For irregular holes or rough excavation, use the closest simple shape you can measure first, then increase the allowance if the base is uneven or the sides flare.
Use cases
- Patio and shed slabsMeasure a flat base before buying concrete for a patio, path, or shed floor.
- Strip footing runsEstimate concrete for narrow foundation strips and compare bags with ready-mix delivery.
- Fence and gate post holesCalculate fill for individual holes and total the materials for a full line of posts.
- Pad and pier basesSize concrete for small bearing pads where depth matters as much as footprint.
How it works
- 1
Enter length and width in metres.
- 2
Enter thickness in centimetres.
- 3
Set the allowance percentage for handling and placement losses.
- 4
Enter the yield of one bag in cubic metres.
- 5
Review the base volume, adjusted volume, and bag count.
Examples
Small garden slab
Input: 3.6 m x 2.4 m x 12 cm, 7% allowance, 0.018 m3 per bag
Output: Base volume: 1.0368 m3; adjusted volume: 1.1094 m3; bags: 62
A compact slab for seating, storage, or a light-duty base.
Fence footing strip
Input: 8 m x 0.25 m x 20 cm, 10% allowance, 0.02 m3 per bag
Output: Base volume: 0.4 m3; adjusted volume: 0.44 m3; bags: 22
Useful for a narrow continuous footing along a boundary line.
Deep post hole
Input: 0.45 m x 0.45 m x 0.85 m, 12% allowance, 0.02 m3 per bag
Output: Base volume: 0.1721 m3; adjusted volume: 0.1928 m3; bags: 10
A realistic hole for a heavy timber post or sign support.
FAQ
How do I estimate a round hole with this calculator?
Use the nearest rectangular footprint that matches the opening, or convert the diameter to an equivalent square base. If the sides are rough or bell-shaped, raise the allowance a little.
Does the allowance affect the bag count?
Yes. The bag count is based on the adjusted volume, so the allowance is already included in the result.
Why can a small job need so many bags?
Bag count depends on the yield per bag. If the yield is entered in the wrong unit or is too low, the total rises quickly.
Can I use it for a slab with varying thickness?
Yes, but split the slab into sections or use an average thickness. One fixed thickness only gives an approximation when the depth changes.
How should I read the ready-mix figure?
Treat it as the order quantity to discuss with the supplier. If access is tight or the formwork is uneven, keep a modest reserve.