Concrete Bag Calculator
Work out how many bags of premix concrete you need — using the yield printed on your bag, not a guess.
Your result
Enter your dimensions above to see how much concrete you need.
Bag counts depend entirely on the yield of the product you buy. This calculator does not specify slab thickness, reinforcement or concrete strength — check those with your supplier, builder or engineer.
Why bag counts vary so much
Almost every wrong bag estimate comes from the same mistake: assuming that weight tells you volume. It does not. What matters is yield — the volume of wet concrete one bag produces once mixed — and that depends on the mix design, not just the weight of the bag.
Two 20 kg products from different manufacturers can yield noticeably different volumes. Rapid-set and high-strength mixes can differ again. The yield is printed on the bag, usually in litres, and that number is the only reliable input. Enter it in the field above and the count will be right for the product actually sitting on your trolley.
Rough guide to bags per cubic metre
Useful for sanity-checking a result, not for ordering:
| Bag size | Typical yield | Bags per m³ |
|---|---|---|
| 20 kg | ~9 L | ~110 |
| 25 kg | ~11.3 L | ~88 |
| 30 kg | ~13.5 L | ~74 |
| 40 kg | ~18 L | ~56 |
These are typical figures used as a starting point. Always use the yield on your own product.
When bags stop making sense
Bags are the obvious choice for post holes, small pads, a few pavers' worth of footing, or anywhere a truck cannot reach. They stop being sensible faster than most people expect.
At roughly 110 bags per cubic metre, a 3 m × 3 m shed base at 100 mm is about 0.9 m³ — near enough to a hundred bags before any allowance. That is a tonne and a half of material to carry, and hours of mixing during which the first section is already setting while you are still mixing the last. The result is often a slab with cold joints through it.
As a rule of thumb, somewhere around 0.4 to 0.5 m³ is where a ready-mix delivery becomes both cheaper and dramatically less work — even accounting for minimum-load charges. Below that, bags usually win.
How many bags do I need for common jobs?
A single fence post hole
A 300 mm diameter hole 600 mm deep is about 0.042 m³ — roughly 5 or 6 × 20 kg bags per hole once you allow for waste. Calculate post holes exactly.
A clothesline or letterbox footing
A 400 × 400 × 400 mm pad is 0.064 m³, about 8 × 20 kg bags with an allowance.
A small equipment pad — 1 m × 1 m × 100 mm
0.1 m³, about 13 × 20 kg bags with a 10% allowance.
Mixing tips that affect your count
- Do not add extra water. It makes mixing easier and the finished concrete significantly weaker. Follow the water quantity on the bag.
- Mix full bags where you can. Splitting bags makes the water ratio guesswork.
- Buy one or two spare. Unopened bags can usually be returned; a half-finished pour cannot be rescued cheaply.
- Keep them dry. Bags that have taken on moisture will have partially set and will not yield or perform as stated.