How many bags of concrete do I need?
108 bags of 20 kg to the cubic metre, or 54 bags of 40 kg. Which matters less than the figure most bag calculators never show you: the weight. A modest 6 × 3 m driveway is just over six tonnes of bags, and that is the number that decides whether you are mixing or ordering.
Quick answer
108 bags of 20 kg, or 54 bags of 40 kg, to the cubic metre of mixed concrete — about 2.15 tonnes of bags either way. A 3 × 2 m shed base at 100 mm is 0.63 m³, so 68 bags of 20 kg; a 6 × 3 m driveway at 150 mm is 2.84 m³, so 305 bags weighing just over six tonnes. A 100 mm fence post in a 300 mm hole 0.6 m deep takes 5 bags of 20 kg, not the two people assume. Past about 1.5 m³, order ready-mix.
| Bag | Yield per bag | Bags per m³ | Weight per m³ |
|---|---|---|---|
| 20 kg | 0.0093 m³ | 108 | 2,160 kg |
| 25 kg | 0.0116 m³ | 87 | 2,175 kg |
| 30 kg | 0.0140 m³ | 72 | 2,160 kg |
| 40 kg | 0.0186 m³ | 54 | 2,160 kg |
| 40 lb | 0.0085 m³ | 118 | 2,141 kg |
| 60 lb | 0.0127 m³ | 79 | 2,150 kg |
| 80 lb | 0.0170 m³ | 59 | 2,141 kg |
Every row lands within 2% of 2.15 tonnes per cubic metre, which is the useful sanity check: if your bag count multiplied by its weight is not somewhere near 2.15 tonnes per cubic metre of hole, something has gone wrong in the arithmetic.
Common jobs
| Job | Volume | 20 kg bags | 25 kg bags | Weight | Ready-mix |
|---|---|---|---|---|---|
| Shed base 2 × 1.5 m, 100 mm | 0.32 m³ | 34 | 28 | 0.7 t | 0.5 m³ |
| Path 5 × 1 m, 100 mm | 0.53 m³ | 57 | 46 | 1.1 t | 1.0 m³ |
| Shed base 3 × 2 m, 100 mm | 0.63 m³ | 68 | 55 | 1.4 t | 1.0 m³ |
| Shed base 3 × 3 m, 100 mm | 0.95 m³ | 102 | 82 | 2.0 t | 1.0 m³ |
| Patio 4 × 3 m, 100 mm | 1.26 m³ | 136 | 109 | 2.7 t | 1.5 m³ |
| Driveway 6 × 3 m, 150 mm | 2.84 m³ | 305 | 245 | 6.1 t | 3.0 m³ |
| Garage base 6 × 6 m, 150 mm | 5.67 m³ | 610 | 489 | 12.2 t | 6.0 m³ |
Fence posts
| Post and hole | Depth | Concrete | 20 kg bags |
|---|---|---|---|
| 75 mm post, 200 mm hole | 0.45 m | 0.01 m³ | 2 |
| 75 mm post, 250 mm hole | 0.60 m | 0.03 m³ | 3 |
| 100 mm post, 300 mm hole | 0.60 m | 0.04 m³ | 5 |
| 100 mm post, 300 mm hole | 0.75 m | 0.05 m³ | 6 |
| 100 mm post, 350 mm hole | 0.90 m | 0.08 m³ | 9 |
| 125 mm post, 400 mm hole | 1.00 m | 0.12 m³ | 13 |
Two bags a post is the figure people carry in their heads, and it is right only for the smallest hole on that list. A 100 mm post in a 300 mm hole at the conventional depth — a third of the post's above-ground height — is five bags, and a run of eight posts is forty bags rather than sixteen. That is the single most common underestimate in fencing.
When to stop bagging and order ready-mix
The bag count is not the constraint; the weight and the clock are. Mixed concrete has to be placed, compacted and finished before it stiffens, and a large slab poured in bag-sized batches over several hours develops cold joints between pours that it will crack along later.
- Up to about 0.5 m³ — 54 bags, a little over a tonne. Hand-mixable in a barrow or a small mixer by one person in a morning.
- 0.5 to 1.5 m³ — 54 to 162 bags, up to three and a quarter tonnes. A mixer and two people. This is also where the cost of bags begins to exceed ready-mix.
- Over about 1.5 m³ — order it. The driveway above is 305 bags: carrying them, opening them, and keeping a continuous pour going is a day's work for a team before any concrete is finished. Ready-mix comes in whole and half cubic metres and arrives mixed to a specified strength.
- Any structural slab, floor or foundation — ready-mix, regardless of size, because the strength class is then certified rather than dependent on how much water went in each batch.
Bags win on small and scattered work: post holes, a few pads, a repair, anything you cannot get a lorry's chute to. That is what they are for.
Common questions
How many 20 kg bags make a cubic metre?
108, and the figure comes from the bag's own settled yield of 0.0093 m³ rather than from its weight. A bag does not yield its dry volume: the sand, aggregate and cement pack together as they are mixed and the result is less bulky than the powder was. Dividing a cubic metre by the dry volume is the usual way people end up short.
How deep should a fence post go?
The convention is a third of the post's height above ground — a 1.8 m fence on a 2.4 m post gives 0.6 m in the ground. Gate posts and anything exposed to wind go deeper. The hole wants to be about three times the post width so there is a worthwhile thickness of concrete all round, which is why a 100 mm post takes a 300 mm hole.
Do I need reinforcement?
A domestic shed base or path on well-compacted hardcore generally does not. A driveway taking vehicles, anything over about 3 m in a direction, and anything on made-up or clay ground generally does. The calculator will work out mesh or bar weight if you tick it, but whether a given slab needs it is a question about the ground under it, which no calculator can see.
Can I mix postcrete and ordinary concrete figures?
No — postcrete and other rapid-set post mixes have their own yield, usually stated as bags per post for a given hole size, and they are poured dry into water in the hole rather than mixed. The figures here are for ordinary bagged mixed concrete. Use whichever the bag in your hand tells you.
Related
- Concrete calculator — slabs, footings and post holes, with bag count, weight and the ready-mix equivalent side by side
- How to measure — getting the dimensions right before the arithmetic
- These tables as JSON — with the inputs that produced each row
Last reviewed 2026-10-17. Volumes, bag counts and weights generated from the concrete calculator on this site; a test re-checks every figure above against the live tool on each build.
Sources and confidence
Every figure used on this page, what kind of figure it is, and where it came from. Figures marked as working estimates have no single authority behind them — they are the ones to check against your own supplier.
| Figure | Kind | Source | Note |
|---|---|---|---|
| Every volume, bag count and weight in the tables | Derived | Generated from this site's concrete calculator | Produced by driving the calculator across the bag sizes, slabs and post holes listed. A test re-checks each cell against the live tool on every build. |
| Bag yields: the settled volume of mixed concrete from one bag | Published | Bag yields as printed by the major bagged-concrete manufacturers | The settled yield of mixed concrete, not the volume of dry powder. Individual products vary by a few per cent — check the bag. |
| Post hole three times the post width, buried a third of the post's height above ground | Working estimate | no primary source consulted | Fencing convention. Gate posts and exposed runs go deeper. |
| The 1.5 m³ threshold for ordering ready-mix, and the 5% waste allowance | Working estimate | no primary source consulted | A judgement about labour and cold joints rather than a published limit. Any structural slab wants ready-mix at any size, because the strength class is then certified. |
Derived Calculated from geometry. The formula is on this page. Published From a manufacturer data sheet or published standard, cited below. Working estimate Trade practice, rounded up. No single authority; the weakest category here.