Understanding This Calculator
Standard cinder and concrete blocks are hollow, with two or three cores running through the center — most walls are built this way and never filled. But for reinforced walls, retaining walls, or anywhere added strength is required, those cores get filled solid with concrete (often around vertical rebar). This calculator gives you both the block count for the wall and, if you choose the filled-solid option, the volume of concrete needed to fill every core.
Each standard 8x8x16in block has roughly 0.075 cubic feet of hollow core space. Multiplying that by your total block count gives the fill volume — convert to cubic yards for ordering ready-mix concrete, since suppliers typically sell and price by the yard.
- 0.89 face sq ft per standard block
- 0.075 hollow core volume per block (cu ft)
- 27 cubic feet per cubic yard
| Blocks | Fill Volume (cu ft) | Fill Volume (cu yd) |
|---|---|---|
| 100 | 7.5 | 0.28 |
| 300 | 22.5 | 0.83 |
| 600 | 45 | 1.67 |
Frequently Asked Questions
Do I need to fill every core, or just some?
Many structural designs only require filling cores that contain vertical rebar, not every core in the wall. This calculator assumes 100% solid fill — if your plan calls for partial fill, scale the result down by the fraction of cores actually being filled.
Should I order extra concrete beyond the calculated volume?
Yes — core fill work involves spillage and settling as the mix works down into narrow cavities. Ordering 10-15% extra beyond the calculated volume is standard practice to avoid running short mid-pour.