In this section

How Many Bags of Concrete Per m³? (20kg, 25kg, 40kg)

Work out how many bags of concrete you need per cubic metre — the simple weight-over-density method, a yield table for 20kg, 25kg and 40kg bags, and a worked slab example with wastage.

The method (weight over density)

About 110 × 20 kg bags fill one cubic metre. A cubic metre of set concrete weighs about 2200 kg. Divide that by the bag weight and you get the bag count. Any bag size works the same way. There’s no code number here — concrete works the same in every country.

Here is the sum for one 20 kg bag.

20 kg ÷ 2200 kg/m³ ≈ 0.009 m³

Flip it around and you get bags per cubic metre.

2200 kg/m³ ÷ 20 kg ≈ 110 bags per m³

That’s the answer people search as bags of concrete per m3. Here’s the full set.

Bag sizeApprox yield per bagApprox bags per m³
20 kg~0.009 m³~110
25 kg~0.011 m³~88
40 kg~0.018 m³~55
11 bag ≈ 0.009 m³1 m³ ≈ 110 bags3.0 × 2.0 × 0.10 = 0.6 m³+10% = 0.66 m³0.66 ÷ 0.009 = 73.3= 74 bags
Bag yield — from one bag, to a cubic metre, to a worked slab.
  1. 1 20 kg bag

Read the bag, not this table. These numbers are typical. The stone size, the cement share and how hard you pack it all move the yield. Every bag prints how much it makes. It often reads “makes about X litres” or “X bags per m³”. If the bag says otherwise, the bag is right.

Work it out by hand

Say you’re pouring a small slab: 3.0 m long × 2.0 m wide × 0.10 m thick.

  1. Volume. 3.0 × 2.0 × 0.10 = 0.6 m³. That’s length × width × depth, all in metres.
  2. Add wastage. Spillage, digging too deep, uneven ground. Add 10%: 0.6 × 1.10 = 0.66 m³.
  3. Divide by one bag’s yield. With 20 kg bags at ~0.009 m³ each: 0.66 ÷ 0.009 = 73.3. Round up to 74 × 20 kg bags. You can’t buy most of a bag.

Want the same slab in bigger bags? 0.66 ÷ 0.018 = 36.7, so 37 × 40 kg bags. Same concrete, fewer trips.

That’s 74 bags for 0.6 m² of finished slab. Past about a cubic metre, most people stop mixing by hand. They ring the batch plant instead.

Do it in 5 seconds

The concrete bag calculator runs the same steps. Type in your slab size and bag weight. It adds your wastage and rounds up the bag count for you.

Sources

  • How much a bag makes, and mix amounts, are set by the product — the number that counts is the one printed on the concrete bag you’re using. The ~2200 kg/m³ figure and the per-bag amounts in this article are typical working values only, vary by product, so always check the current product sheet.
  • Slab volume is plain shapes: length × width × depth.

Frequently asked questions

How many 20kg bags of concrete are in a cubic metre?
About 110 bags of 20 kg pre-mix. You get that by taking the weight of set concrete — about 2200 kg for a cubic metre — and dividing by 20 kg a bag. The exact amount changes by product, so always check the bag — some mixes make a little more or less.
What volume does a 20kg bag of concrete make?
About 0.009 to 0.011 m³ (roughly 9 to 11 litres) once it's mixed and packed down. That's about the bag's own weight divided by the weight of a cubic metre of set concrete (~2200 kg). The number printed on the bag is the one to trust.
How do I work out how many bags I need for a slab?
Work out the slab volume (length × width × depth in metres), add about 10% for spillage, then divide by how much one bag makes. For a 0.6 m³ slab, +10% makes 0.66 m³; at ~0.009 m³ per 20 kg bag that's 0.66 ÷ 0.009 ≈ 74 bags. Always round up — you can't buy most of a bag.
Is it cheaper to use bags or order ready-mix concrete?
Bags win for small pours — a few post holes, a path, a small slab. Once you're past about 1 to 1.5 m³ the bag count and the hand-mixing get out of hand, and a truck of ready-mix is usually cheaper, faster and more even. Work out both prices before you decide.
How much water goes in a 20kg bag of concrete?
It depends on the product, but it's usually around 2 to 3 litres per 20 kg bag. Follow the mixing steps printed on the bag — too much water makes the finished concrete weaker.