It's the most-asked question on any concrete job. The answer comes from two numbers: the volume you're filling, and what one bag actually makes once it's mixed. Get both right, add a sensible margin, and you won't be making a second trip to the store with the mix going off.
What one bag makes
| Bag | Yield | Bags per m³ |
|---|---|---|
| 20 kg concrete mix | about 0.0095 m³ | about 105 |
| 20 kg rapid-set concrete | about 0.0095 m³ | about 105 |
Yields vary a little by brand, so check the bag. These are the figures our calculators use by default.
The method
- Work out the volume in cubic metres: length × width × depth for slabs, π × radius² × depth for round holes and tubes.
- Subtract anything that takes up space, such as the post in a post hole.
- Divide by the yield of one bag.
- Add 10% and round up to a whole bag.
Quick slab table
Bags of 20 kg mix per square metre of slab, before waste:
| Thickness | Bags |
|---|---|
| 75 mm | about 7.9 per m² |
| 100 mm | about 10.5 per m² |
| 125 mm | about 13.2 per m² |
| 150 mm | about 15.8 per m² |
That adds up fast. A modest 3 × 3 m pad at 100 mm needs about 95 bags before waste, and about 105 with it. At that size, it's worth pricing ready-mix too.
Post holes
Post holes use less than you'd expect, because the post takes up part of the hole and a gravel base takes up the bottom few centimetres. A 100 mm post in a 300 mm × 600 mm hole with 100 mm of gravel needs roughly 0.03 m³: three to four 20 kg bags of rapid-set concrete.
Hole size matters more than anything else. Doubling the diameter roughly quadruples the concrete, so dig holes to the size you planned, not wider.
Piers and form tubes
A 300 mm round pier 900 mm deep holds 0.064 m³, about eight 20 kg bags with waste. A 200 mm pier of the same depth holds about 0.028 m³, three or four bags. If you are setting a dozen deck piers, that's a lot of bags, so check the ready-mix price for that job too.
Add waste
Add 10% for spills, uneven ground and the half-bag that gets wet. On anything over a cubic metre, seriously consider ready-mix: see bags vs ready-mix.
Water, mixing and curing
A 20 kg bag takes about 2–2.5 litres of water. Too much water is the most common way to weaken bagged concrete. It makes the mix easier to place but can cut its strength dramatically. Add water gradually until the mix holds its shape like thick porridge.
- Mix each bag fully. Dry pockets become weak spots.
- Don't add water to a mix that's starting to stiffen. Use it or throw it out.
- Keep the finished concrete damp for several days. Cover it in hot, dry or windy weather.
- In hot weather, pour early in the day and keep it covered and damp. In alpine areas, protect it from frost.
Storing bags
Cement starts reacting as soon as it gets damp. Keep bags off the ground on a pallet and under cover, and use them within the date on the bag. A bag that has gone lumpy has lost strength.
The slab, post-hole and pier calculators all show bag counts for every common size side by side.
Common questions
Can I pour bagged concrete directly into a post hole dry?
Fast-setting post mixes are designed for this: pour the dry mix around the post and add water as directed. Standard concrete mix should be mixed first. Dry-pouring standard mix often leaves dry pockets and a weak footing.
How much does a bag weigh once mixed?
About the same as the dry bag plus its water. Concrete weighs roughly 2,400 kg per cubic metre, so a small slab is heavy. Make sure the ground or structure under it can take the load.
Is it cheaper to mix my own from cement, sand and gravel?
Per cubic metre, yes, usually. But you pay for it in time, storage and consistency. For a handful of post holes the convenience of pre-mixed bags is worth it; for a mid-sized slab, a ready-mix delivery usually beats both.
How long before I can walk on it?
Typically 24–48 hours for foot traffic and about a week before vehicles. Concrete keeps gaining strength for around four weeks, especially if kept damp.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.