Concrete calculator
Work out how much concrete you need for a slab, base or path, in cubic metres, bags or a ready-mix order.
Calculate your concrete
Enter the length, width and depth to see how much you need.
- Concrete needed
- m³
- Exact volume, before wastage
- m³
- 25kg bags of concrete mix
- 20kg bags of concrete mix
- Ready-mix to order
- m³
How the calculation works
Volume is length × width × depth, with the depth converted from millimetres to metres. A 4m × 3m slab at 100mm deep is 4 × 3 × 0.1 = 1.2 m³. Adding 10% for wastage gives 1.32 m³.
Bag counts assume each bag makes its weight divided by about2,300 kg/m³, so a 25kg bag makes roughly 0.011 m³. Yields vary between products, so check the figure printed on the bag.
The ready-mix figure is rounded up to the next 0.5 m³. Suppliers set their own minimum loads and ordering increments, so confirm with yours before you book a delivery.
Frequently asked questions
How many 25kg bags of concrete make a cubic metre?
About 92, based on the yield above. For anything over about 1 m³, ready-mix is usually less work than mixing bags by hand.
How much extra concrete should I allow?
Allow 5–10% extra to cover uneven ground, spillage and formwork that bows slightly. Running short halfway through a pour is far worse than having a little left over.
How do I work out an L-shaped or irregular area?
Split it into rectangles, calculate each one separately and add the results together.