Concrete calculator
Work out exactly how much concrete to order — in cubic metres and bags — before you book the truck or head to the yard.
Use the calculator ↓Calculate concrete volume
Bag count is approximate (a 20kg bag yields ~0.0091 m³). For anything beyond a small pour, order pre-mixed by the m³ and confirm with your supplier.
How builders use this
Order the right amount
Under-order and the pour stops; over-order and you pay for waste. Get the m³ right before you book the truck.
Price the materials
A firm volume turns a rough guess into a real materials line on the quote you give a customers.
Bags or truck
For a small job the bag count tells you whether to mix on site or call for pre-mix.
How to work out concrete volume
Concrete is ordered by volume in cubic metres. The volume of a rectangular pour is simply length times width times depth, with the depth converted to metres — a 100mm slab is 0.1m thick. Multiply the three together and you have the cubic metres for one element; multiply by the number of identical elements for the total.
Always add a waste allowance. Uneven ground, spillage, over-excavation and pump losses mean the real amount placed is a little more than the theoretical volume — around 5–10% is typical. This calculator adds your chosen allowance and also estimates the number of 20kg bags, on the basis that one bag yields roughly 0.0091m³.
Bag estimates are a guide only and vary by product; for anything beyond a small pour it is cheaper and easier to order pre-mixed by the cubic metre and confirm the quantity with your supplier.
Frequently asked questions
How many 20kg bags are in a cubic metre?
Roughly 108–110 bags, since a 20kg bag of pre-mix yields about 0.0091m³. It varies by product, so treat the bag figure as a guide and check the yield printed on the bag.
What waste allowance should I add?
For most slabs and footings, 5–10% covers spillage, over-excavation and uneven subgrade. Difficult access or pumping can justify a little more.
How do I convert slab thickness to metres?
Divide millimetres by 1,000 — a 100mm slab is 0.1m, a 125mm slab is 0.125m. Enter the thickness in millimetres here and the calculator handles the conversion.
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This calculator gives an indicative concrete volume for general information only. Actual quantities depend on site conditions, formwork and product. Confirm requirements with your supplier or engineer.