Concrete Calculator
Estimate concrete volume for slabs, strip footings and columns in metres, then convert to bagged ready-mix with a waste allowance.
Calculator
Pick the shape, enter metric dimensions, choose a waste allowance and bag size, then see volume and bags required.
Dimensions (metres)
Results
Bag yields are approximate for typical UK bagged ready-mix. Always confirm the yield on the bag and with your supplier.
How This Calculator Works
What it does
This calculator estimates the volume of concrete required for common construction projects, including slabs, strip footings and circular columns. It can also estimate the number of ready-mix bags required.
How the calculation works
The calculator first determines the volume based on the selected shape. Slab = Length × Width × Depth. Strip Footing = Length × Width × Depth. Circular Column = π × Radius² × Height. An optional waste allowance is then added to account for uneven ground, spillage and over-excavation. If bagged concrete is selected, the total volume is converted into an estimated number of bags using the selected bag yield.
Formula
- Slab Volume = Length × Width × Depth
- Footing Volume = Length × Width × Depth
- Column Volume = π × Radius² × Height
- Total Volume = Calculated Volume × (1 + Waste %)
- Bags Required = ceil(Total Volume ÷ Bag Yield)
Why it matters
Ordering too little concrete can delay a project and result in cold joints, while ordering too much increases costs and waste. Calculating the correct volume before placing an order helps improve planning and reduce unnecessary expense.
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Frequently asked questions
Straight answers about this calculator and how UK trades use the figures day to day.
Are dimensions in metres?
Yes. Convert millimetres by dividing by 1000 (for example 150 mm = 0.15 m).
How accurate is the bag count?
It is approximate for bagged ready-mix. Yield varies by product, so check the bag and confirm with your supplier.
What waste percentage should I use?
5% is a sensible default for many jobs. Use 10% for uneven digs, awkward access or higher spillage risk.
When should I order ready-mix instead of bags?
Once the total volume is above about 0.5 m³, a ready-mix delivery is often more cost-effective than carrying bagged concrete.
What about reinforcement or formwork?
This tool covers concrete volume only. Plan steel and shuttering separately.
Does a strip footing use the same formula as a slab?
Yes. Both are rectangular volumes. The difference is typical proportions, not the maths.
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