A slab is a flat box, so its volume is length × width × thickness. The only trap is units: lengths are usually in feet and thickness in inches, and they do not multiply until the thickness is converted to feet.
Thickness (ft) = thickness (in) ÷ 12
Volume (cu ft) = length (ft) × width (ft) × thickness (ft)
Volume (cu yd) = volume (cu ft) ÷ 27
A 10 × 10 ft slab, 4 inches thick
Four inches is 0.333 ft, so the slab holds 10 × 10 × 0.333 = 1.23 yd³ (33.3 cu ft). That is the calculated volume: the amount of space the concrete has to fill, and nothing more.
Worked example 10 × 10 ft slab, 4 in thick, no allowance
| Material | Calculated | Buy |
|---|---|---|
| Concrete mix0.6 cu ft per bag, no allowance | 55.56 × 80 lb bags | 56 × 80 lb bags |
That figure assumes a perfectly flat, perfectly level sub-base and no spillage. Real jobs rarely get either, which is why the planner starts from a 10% allowance. It is an editable planning default, not a rule: manufacturer guidance such as Sakrete’s is to add about 10% to a bagged estimate and round up, while QUIKRETE notes that its bag yields are approximate.
| Allowance | Volume to fill | 80 lb bags to buy |
|---|---|---|
| None | 1.23 yd³ (33.3 cu ft) | 56 × 80 lb bags |
| 10% | 1.36 yd³ (36.7 cu ft) | 62 × 80 lb bags |
Calculated is not the same as bought
Concrete mix is sold in bags with a fixed yield, so the calculation lands between two whole numbers. With no allowance the 10 × 10 ft slab needs 55.56 × 80 lb bags, and you cannot buy the 0.56 of a bag, so the list says 56 × 80 lb bags. With the 10% allowance the list says 62 × 80 lb bags. The planner always rounds up: a hair over a whole bag becomes the next bag.
If you are ordering ready-mix instead of bags, the same cubic-yard figure is what you start from. Your supplier sets the ordering unit and any minimum.
What changes the estimate?
- The allowance you choose for uneven ground and spillage
- The bag yield printed on the mix you are buying (40, 50, 60 and 80 lb bags yield different volumes)
- Thickness that varies across the slab, which a single average thickness hides
- Form size: measure inside the forms, not the outside edges
Units
In metric, length × width × thickness in meters gives cubic meters directly, with no 27 or 12 to remember. The planner switches between the two systems and converts what you have already typed.
This guide only converts dimensions into a quantity. How thick a slab should be, what goes under it, whether it is reinforced and which mix to use are decisions for your project, your local requirements and a qualified professional.