Use the slab volume formula
When length and width are in feet and thickness is in inches, cubic yards = length × width × thickness ÷ 324. The divisor combines 12 inches per foot and 27 cubic feet per cubic yard. Keep full precision until you reach the purchasing step.
| Slab size | At 4 inches | At 6 inches |
|---|---|---|
| 10 ft × 10 ft | 1.235 yd³ | 1.852 yd³ |
| 12 ft × 12 ft | 1.778 yd³ | 2.667 yd³ |
| 20 ft × 20 ft | 4.938 yd³ | 7.407 yd³ |
These displayed figures are rounded geometric volumes. They exclude an allowance, deeper edges and changes in thickness.
Convert one example into a purchase
A 10×10 slab at 4 inches contains 33.333 cubic feet. With a deliberately selected 10% allowance, the target becomes 36.667 cubic feet. If your chosen premix yields 0.60 cubic feet per bag, divide 36.667 by 0.60 and round upward: 62 bags.
Those 62 bags provide a labeled approximate yield of 37.2 cubic feet. The difference between that yield and the net slab volume includes both the chosen allowance and the final whole-bag rounding. Do not add the same 10% again after counting bags.
Check what a size-only estimate leaves out
Measure the prepared forms rather than relying only on a drawing's title. A thickened edge requires an added volume calculation, with the part already included in the slab removed to prevent double counting. Separate pads should be measured separately and summed before purchase rounding.
Use the concrete calculator for your measurements, then open the bag calculator or cost calculator. Read the current bag label and confirm the final order with your supplier. Store the original dimensions with the estimate so a later size change can be recalculated consistently.
Sources & further reading
Examples illustrate arithmetic. Use project specifications and current local supplier quotes for a purchase.