Concrete Volume Calculator

The concrete volume calculator tells you how much concrete you need for a rectangular slab, pad, path or footing, and how many bags that works out to. It uses the simple volume formula, where volume equals length multiplied by width multiplied by depth, then divides that volume by the yield of each bag to find how many bags to buy. You enter four inputs: the slab length and width in metres, the depth or thickness in metres, and the set volume each bag yields in cubic metres. The calculator returns the total volume in cubic metres and the number of bags, rounded up because you cannot buy part of a bag. Builders, landscapers, fencers and DIYers around New Zealand use it when pouring a shed floor, setting posts, laying a patio or forming steps, so they order the right amount and avoid a half-finished pour. A typical 20 kilogram premix bag yields roughly 0.01 to 0.012 cubic metres, so check the figure printed on the bag you plan to use. A few tips save money and hassle. First, add about 5 to 10 percent for spillage, uneven ground and over-excavation, since running short mid-pour is far worse than a little waste. Second, measure depth carefully, because a slab that is even 10 millimetres thicker than planned adds up across a large area. Third, for anything beyond a few bags it is usually cheaper and easier to order ready-mix delivered by the cubic metre rather than mixing dozens of bags by hand.

Length (m)

Width (m)

Depth (m)

Bag yield (m3 per bag)

1.20 m3

Concrete volume

Bags needed (rounded up)100 bags

Volume = length x width x depth. Bags = volume / bag yield, rounded up.

How it works

The volume is the length times the width times the depth, giving cubic metres. The number of bags is that volume divided by the yield of one bag, then rounded up to the next whole bag because you cannot buy a fraction of one.

Worked example

For a slab 4 m long, 3 m wide and 0.1 m deep, the volume is 4 times 3 times 0.1, which is 1.20 cubic metres. Dividing by a bag yield of 0.012 gives exactly 100 bags.

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Frequently asked questions

How many bags are in a cubic metre?

It depends on the bag yield, which is printed on the bag. A 20 kilogram premix bag typically yields about 0.01 to 0.012 cubic metres, so a cubic metre takes roughly 83 to 100 bags. Always check your specific product.

Should I order extra concrete?

Yes. It is wise to add about 5 to 10 percent on top of the calculated volume to cover spillage, uneven ground and slightly over-dug formwork. Running short during a pour is far more costly than a small amount of waste.

When should I use ready-mix instead of bags?

For anything more than a few bags, ready-mix delivered by the cubic metre is usually cheaper, faster and gives a more consistent result. Bags suit small jobs like setting a few posts or patching. Large slabs are best poured from a truck.