Bags or a truck: where a concrete order comes up short
Two decisions sit inside every small concrete order: how much, and in what form. The first is arithmetic, and it fails in three predictable places. The second is a price question with an actual number behind it.
Depth is quoted in a different unit from the face
A slab is measured in metres or feet across its face and in centimetres or inches through its thickness. Nothing else on a site mixes units inside a single multiplication, and this is where the arithmetic comes apart.
A 3 × 2.5 m shed base at 10 cm is 3 × 2.5 × 0.1 = 0.75 m³. Take the 10 as metres and the same slab comes out at 75 m³. In feet: a 12 × 10 ft slab at 4 in is 40 ft³, or 1.48 yd³; read the 4 as feet and it is 480 ft³, 17.78 yd³.
Both of those are large enough to catch. The dangerous version is the quiet one — dividing centimetres by 12, or inches by 100, because that was the divisor last time. The concrete slab calculator takes the thickness in the small unit and does the division itself. Type the number as it is written on the drawing.
The allowance is a percentage of volume, added before the rounding
Ten percent is standard for a formed slab, fifteen for footings dug straight into soil. What matters is where it goes. The allowance multiplies the volume, and the bag count is rounded up from the enlarged volume — not the other way round.
For that 3 × 2.5 m base, 0.75 m³ becomes 0.825 m³. In 50 kg bags that is 33. Round the exact volume instead and add a bag or two at the merchant’s and you have 30 plus a guess. The habit is roughly right on a one-barrow pour and wrong on everything larger, because a percentage grows with the slab and “a bag or two” does not.
A bag is a yield, not a weight
Bags are sold by mass and used by volume. An 80 lb bag yields 0.6 ft³ and a 60 lb bag 0.45; a 25 kg bag yields 0.0125 m³ and a 40 kg bag 0.02. Dry mix yields in proportion to what is in it — 0.0075 ft³ per pound, 0.0005 m³ per kilogram — so any size can be worked out from its mass.
That matters because merchants across the Caucasus, Turkey and much of Europe sell cement and dry mix in 50 kg bags, a size on neither manufacturer’s list. A 50 kg bag yields 0.025 m³. The field for your own bag size exists for exactly this reason.
One thing not to do: work the volume out in one system and convert the bag count into the other. Manufacturers round their published yield per market, so the two constants are not exact converts of each other.
Where the truck charges you for being small
Below 0.4 m³ a ready-mix supplier adds a short-load surcharge, and bags genuinely win. In feet the line is 13.5 ft³, half a cubic yard — each system rounds its own figure rather than converting the other’s. The band is narrow: 16 bags of 50 kg — 0.96 t of finished concrete — or 23 bags of 80 lb.
The comparison is made against the ordered volume, allowance included. A 2 × 2 m pad at 10 cm is 0.4 m³ exactly, sitting on the line; with 10% it is 0.44 m³ and the tool starts recommending a truck. Treat that as a price hint rather than a rule. Eighteen bags is a morning’s work, while a truck arrives on its own schedule and expects the pour to finish inside its unloading window, and every supplier sets its own minimum anyway.
One pour, worked through
Shed base, 3 × 2.5 m, 10 cm thick, 10% allowance:
| Step | Figure |
|---|---|
| Face area | 7.5 m² |
| Exact volume | 0.75 m³ |
| Ordered volume | 0.825 m³ |
| 25 kg bags | 66 |
| 40 kg bags | 42 |
| 50 kg bags | 33 |
| Weight | 1.98 t |
It is over the threshold, so the tool says ready-mix — and 33 bags mixed by hand is the reason why.
The four post footings that go with it, 0.3 m across and 80 cm deep, are dug straight into soil and so take 15%: 0.26 m³, 11 bags of 50 kg. No supplier will send a truck for that, and none needs to.
The other direction: what the bags on hand will cover
The same arithmetic read backwards answers the question people ask standing in the yard with the bags already bought. Twenty bags of 50 kg, spread 10 cm thick with the same allowance, cover 4.55 m² — a square about 2.13 m on a side. The threshold in the same terms: 16 bags cover 3.64 m², a square 1.91 m on a side. If the slab you have in mind is bigger than that square, you are in truck country.
The order that works
Face area, then depth in its own unit, then the waste allowance, then — once, at the end — the bag size you can actually buy. Round up only at the last step.