The method (weight over density)
There’s no code number here — concrete works the same in every country. The whole thing comes down to one idea: a bag of pre-mix makes about its own weight divided by the weight of a cubic metre of set concrete.
A cubic metre of set concrete weighs about 2200 kg. So a 20 kg bag makes about:
20 kg ÷ 2200 kg/m³ ≈ 0.009 m³
Flip that around and you get bags per cubic metre:
2200 kg/m³ ÷ 20 kg ≈ 110 bags per m³
That’s the answer most people want for bags of concrete per m3: about
110 × 20 kg bags to fill one cubic metre. Here’s the full set.
| Bag size | Approx yield per bag | Approx bags per m³ |
|---|---|---|
| 20 kg | ~0.009 m³ | ~110 |
| 25 kg | ~0.011 m³ | ~88 |
| 40 kg | ~0.018 m³ | ~55 |
- 1 20 kg bag
Read the bag, not this table. These numbers are typical, but they change with the product — the size of the stone, how much cement is in it, and how hard you pack it all move the number. Every bag prints how much it makes (often as “makes about X litres” or “X bags per m³”). If the bag says something different to the table above, 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.
- Volume. 3.0 × 2.0 × 0.10 = 0.6 m³. (Length × width × depth, all in metres — that’s all a slab volume is.)
- Add wastage. Spillage, digging too deep, uneven ground. Add 10%: 0.6 × 1.10 = 0.66 m³.
- Divide by how much one bag makes. Using 20 kg bags at ~0.009 m³ each: 0.66 ÷ 0.009 = 73.3 → 74 × 20 kg bags (always round up — you can’t buy most of a bag).
Want the same slab in bigger bags? 0.66 ÷ 0.018 = 36.7 → 37 × 40 kg bags. Same concrete, fewer trips.
That’s 74 bags for 0.6 m² of finished slab. Past about a cubic metre, this is where most people stop mixing by hand and ring the batch plant instead.
Do it in 5 seconds
The concrete bag calculator does exactly the steps above — type in your slab
size and bag weight, and it adds your wastage and rounds up the bag count for you.
Related
- Up to the pillar: Concrete Calculations: The Complete On-Site Guide
- Get the volume first: How to Calculate Concrete Volume for a Slab
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.