Concrete Bag Calculator

How many bags of concrete for your slab? Enter the size, get the volume in cubic metres and the bag count for every common bag size.

Measure your slab in metres and enter the depth in metres or millimetres. The calculator works out the volume, adds your wastage allowance if you tick it, and counts the bags for 20 kg, 25 kg, 30 kg and 40 kg bags. Counts always round up: you cannot buy part of a bag, and running short mid-pour ruins the batch.

Length (m)

Width (m)

Depth

Depth unit

Add 10% wastage allowance (recommended)

Bag size	Yield per bag	Bags per 1 m³
20 kg	0.01 m³	100
25 kg	0.0125 m³	80
30 kg	0.015 m³	67
40 kg	0.02 m³	50

Yields are the standard figures printed on Australian bags (Bunnings 20 kg bags print 0.01 m³ per bag). Brands vary a little, so check your bag. Estimates only, not engineering advice.

How the count is worked out

Volume first: length times width times depth, all in metres, gives cubic metres. A 4 m by 3 m slab at 100 mm deep is 4 x 3 x 0.1 = 1.2 cubic metres. Then divide by what one bag yields. A 20 kg bag yields about 0.01 cubic metres, so 1.2 cubic metres takes 120 bags. A 40 kg bag yields about 0.02, so the same slab takes 60 bags. The yields scale with weight because bagged concrete is the same mix in different bag sizes: 25 kg gives 0.0125 and 30 kg gives 0.015.

With the 10% wastage allowance ticked, the same slab orders 1.32 cubic metres, or 132 bags of 20 kg. The allowance covers the usual losses: low spots in the sub-grade, formwork that bows out under the weight, and spillage in the barrow. Measure the depth at several points and use the deepest reading: a slab that is 110 mm deep in the middle and 100 mm at the edges needs the 110 mm figure.

Bags are for small jobs

Bagged concrete suits fence posts, small footings, garden slabs and patches. Past about 1 to 1.5 cubic metres, mixing 50 to 70 bags by hand is slow, the batches cure at different times, and a ready-mix truck is usually cheaper.

Match the strength grade

Bagged mixes come in different strengths: post mix is for posts, rapid set for quick small jobs, and 20 MPa and higher grades for slabs and driveways. Check the MPa rating on the bag against what your job needs before you buy.

Frequently Asked Questions

About 100. One 20 kg bag yields about 0.01 cubic metres, as printed on Bunnings Australia 20 kg bags (Easy Mix and Bastion). So 1 cubic metre takes 100 bags of 20 kg, 80 bags of 25 kg, 67 bags of 30 kg or 50 bags of 40 kg.

Multiply length by width by depth, all in metres. A slab 4 m long, 3 m wide and 0.1 m deep is 4 x 3 x 0.1 = 1.2 cubic metres. If your depth is in millimetres, divide by 1,000 first: 100 mm is 0.1 m.

Yes, 10% is the usual allowance. It covers low spots in the sub-grade, formwork bowing out, and spillage in the barrow. The calculator has a 10% wastage tick box that adds it before counting bags.

Once the job passes about 1 to 1.5 cubic metres, roughly 50 to 70 bags, a ready-mix truck is usually cheaper and faster than mixing bags by hand, and the concrete is stronger because it is mixed in one batch.

Check the bag. Standard post-mix and rapid-set bagged concrete is made for fence posts, footings and small non-structural jobs. A house slab, driveway or reinforced foundation needs the right strength grade, usually 20 MPa or higher, and an engineer or concreter should confirm it.

A little. Most AU 20 kg bags print 0.01 cubic metres, but some products yield 0.009 to 0.010. The calculator uses the standard printed figures and rounds bag counts up, so a small shortfall still leaves you covered. Always check the yield printed on your bag.