Plot No. 64 · Home & Garden
Concrete Calculator
Work out how much concrete a slab needs. It multiplies length by width by depth to get the volume in cubic metres, adds 10 percent for waste and spillage, then converts that to the number of 25 kg bags, where each bag sets to roughly 0.011 cubic metres.
The order figure already adds 10 percent for waste and spillage. For anything over roughly half a cubic metre, ready-mix delivery is usually cheaper than buying bags.
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How the concrete volume is worked out
The calculator multiplies slab length by width by depth to get the volume in cubic metres, converting the depth from centimetres to metres first. It then adds 10 percent for waste, spillage and an uneven sub-base, because almost no pour uses exactly the theoretical volume.
To turn that volume into bags it divides by the yield of a 25 kg bag of ready-mix, which is about 0.011 cubic metres once mixed, so roughly 90 bags fill a cubic metre. The bag count is rounded up to whole bags.
How to use the concrete calculator
- Enter the slab length and width in metres.
- Enter the depth in centimetres, for example 10 for a typical garden slab.
- Read off the volume with waste included and the number of 25 kg bags, or use the volume to order ready-mix.
Worked examples
Small garden slab
Inputs: 3 m by 2 m slab, 10 cm deep
Result: 0.6 cubic metres before waste, 0.66 cubic metres to order, which is about 60 bags of 25 kg ready-mix.
Same slab, deeper pour
Inputs: 3 m by 2 m slab at 15 cm deep
Result: 0.9 cubic metres rising to about 0.99 with waste, roughly 90 bags, the point where ready-mix delivery starts to beat bags on cost.
Limitations and common mistakes
Edge cases and limitations
- Bag yield varies a little by brand and how wet the mix is, so treat 0.011 cubic metres per bag as a guide.
- It models a flat rectangular slab. Footings, thickened edges and steps need to be measured and added separately.
- The 10 percent buffer suits a tidy sub-base. Add more on rough ground where over-digging is likely.
Common mistakes
- Entering the depth in metres instead of centimetres, which overstates the volume a hundredfold.
- Ordering the bare volume with no waste, then mixing one bag short mid-pour and leaving a weak cold joint.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length by width by depth in metres. A 3 m by 2 m slab at 10 cm deep is 0.6 cubic metres. Adding 10 percent waste gives 0.66 cubic metres, which is about 60 bags of 25 kg ready-mix.
How many 25 kg bags make a cubic metre?
A 25 kg bag of ready-mix sets to roughly 0.011 cubic metres once mixed, so it takes about 90 bags per cubic metre. Bagged mix suits small jobs. For anything over about half a cubic metre, ready-mix delivery is usually cheaper and faster.
Why add 10 percent for waste?
Some mix is always lost to spillage, an uneven sub-base and slight over-digging. The 10 percent buffer means you are not stopping mid-pour to mix another bag, which would leave a weak cold joint in the finished slab.
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