Size a slab, footing, round column, wall, or flight of stairs with thickness in its own unit, then compare bagging the pour against a ready-mix delivery.
Change the inputs that matter to your decision. Optional assumptions stay collapsed until you need them.
Thickness carries its own unit, so a slab entered in millimetres stays in millimetres.
Price both and the result names the cheaper option and the volume where they cross over.
Slab of 2.96 yd³, ordered as 3.26 yd³ with 10% for waste and over-excavation.
Add a bag price and a delivered price to see which is cheaper for this pour and where the two cross over.
| Length | 20 ft |
|---|---|
| Width | 12 ft |
| Thickness | 4 in |
| Net volume | 2.96 yd³The shape itself, with no allowance |
| Order volume | 3.26 yd³10% added, or 2.49 m³ |
| 80 lb bags | 1470.0222 yd³ per bag · 11,760 lb to move |
| 60 lb bags | 1960.0167 yd³ per bag · 11,760 lb to move |
| 50 lb bags | 2350.0139 yd³ per bag · 11,750 lb to move |
| 40 lb bags | 2940.0111 yd³ per bag · 11,760 lb to move |
| Ready-mix loads | 1 × 10 yd³Billed at the ordered volume |
Bag yields are the mixed-concrete volumes printed on standard pre-mixed bags — 0.6 ft³ for an 80 lb bag — and the same yield per unit mass is used for metric sizes. Ready-mix is normally ordered in quarter-yard or quarter-metre steps, so round the order volume up to your supplier's increment.
Practical notes about the inputs, assumptions, and result.
Because it is almost never measured in the same unit as the plan dimensions. A slab is 20 by 12 feet and four inches thick, or six by four metres and 100 millimetres thick. Sharing one unit across both is how a metric thickness ends up converted to feet and then divided by twelve as though it were still inches — which understates the concrete by a factor of twelve.
Ten percent is a common working figure for a slab on a prepared sub-base. A hole in uneven ground, an over-excavated footing, or formwork that can bow will swallow more. The allowance is applied to the ordered volume only, so the net volume of the shape stays visible beside it.
Below the point where the delivery and short-load fees stop dominating. Enter a bag price and a delivered price and the result names the cheaper option for your pour and the volume where the two cross over. The comparison is material and delivery only — it does not price the hours spent mixing.
Most suppliers bill a minimum volume regardless of how little you order, and add a short-load fee below a full truck. If your pour is under that minimum, the delivered cost here is billed at the minimum rather than at what you need.
As a solid flight: the nth step from the top sits on n risers' worth of concrete, so the volume is width × tread × riser × n(n+1)/2. That is the full solid pour. A flight built on a formed stringer or over fill uses less, and a landing has to be added as a separate slab.
The result lists all four so you can weigh fewer bags against heavier ones. An 80 lb bag yields 0.6 ft³, so a cubic yard takes 45 of them — about 3,600 lb to move and mix. That number is usually the argument for ready-mix rather than the price.
Other tools in construction & diy.
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Turn a finished concrete volume into cement, sand, gravel, and water for a chosen strength, working back through an explicit dry-volume factor rather than splitting the wet volume.