Concrete Calculator
Enter your slab length, width, thickness, and bag yield to see the volume of concrete you need and how many pre-mix bags to buy.
Volume and bags
Get both the cubic metres of concrete and the number of whole pre-mix bags, so you can buy exactly what the pour needs.
Yield varies
Bag yield depends on the mix and how much water you add — a 20 kg pre-mix bag is roughly 0.009 m³, but always check your bag.
How much concrete do I need?
From slab size to bags
A concrete calculator turns a slab's dimensions into the amount of concrete to buy. It works out the volume the slab occupies, then divides that by the yield of a single bag to tell you how many bags to pick up — the two figures you need at the merchant's counter.
First find the volume — the length times the width times the thickness (converted from centimetres to metres). Then divide that volume by the yield of one bag and round up, because you can only buy whole bags.
Volume = length × width × thickness | Bags = ⌈volume ÷ yield per bag⌉The thickness is entered in centimetres because that is how slabs are usually specified, so the calculator divides it by 100 to get metres before multiplying. The bag count always rounds up, since a part-used bag still has to be bought whole.
Suppose you are pouring a 3 m × 2 m garden slab at 10 cm thick using 20 kg pre-mix bags that yield 0.009 m³ each.
Convert the thickness
10 cm ÷ 100 = 0.1 m.Find the volume
3 m × 2 m × 0.1 m = 0.6 m³.Divide by bag yield
0.6 m³ ÷ 0.009 m³ = 66.7, rounded up to 67 bags.
The thickness you choose drives the answer. A typical garden path or patio sits around 10 cm, while a footing or driveable slab is firmer at more — doubling the thickness doubles the concrete. Once you pass roughly 0.5 m³, bagged pre-mix gets heavy and slow, and ready-mix delivered by the cubic metre is usually cheaper and easier than mixing dozens of bags by hand. Either way, treat the figure as a minimum: order around 10% extra to cover spillage, an uneven base, and the bit you always seem to run short on.
The formula is exact, but a few real-world factors keep it an estimate.
Bag yield and shape are approximate
The yield per bag varies with the mix and how much water you add, so use the figure printed on your bag rather than a generic default. This calculator also assumes a simple rectangular slab of even thickness — it does not account for footings, steps, curved edges, or a sloping base. For those, split the pour into rectangles and add the volumes, and order around 5–10% extra so you are not left short.