Grout volume is core volume times the number of blocks
Only the hollow cores get filled, not the solid webs, so a block's fill volume is its overall size multiplied by its void fraction. Standard CMU is roughly 50% void. Multiply one block's core volume by the number of blocks you are filling, then add 5–10% for waste and spillage.
Fill volume for a standard 8-inch CMU
| Quantity | Grout needed |
|---|---|
| Per 8×8×16" block (2 cores) | ~0.017 yd³ (0.46 ft³) |
| Blocks per cubic yard | ~60 blocks |
| 100 blocks, fully grouted | ~1.7 yd³ |
| 80 lb bags per cubic yard | ~45 bags |
Void fraction varies by block: lightweight and partition units run higher (60–70%), heavy load-bearing units lower (35–45%). Check the manufacturer's spec or measure a sample if you can.
Ordering the right amount
- Add waste. 5–10% covers spillage, tools and imperfect measurement; skimping leaves cores unfilled.
- Use the real void percentage. Guessing 50% for every block is the biggest source of error. It swings the total by a lot.
- Bags vs ready-mix. Below about half a cubic yard, bagged mix is practical; above that, ready-mix grout is cheaper and faster.
- Consolidate. Rod or vibrate the grout so it fills every cavity; trapped voids weaken the wall.
Common questions
How much grout does it take to fill a concrete block?
A standard 8x8x16 inch CMU holds roughly 0.017 cubic yards of grout in its two cores when fully filled. That works out to about 60 blocks per cubic yard of grout, before adding waste.
How many bags of concrete fill a block wall?
An 80 lb bag of mix yields about 0.6 cubic feet. One cubic yard is 27 cubic feet, so it takes roughly 45 bags to make a cubic yard. Bagged mix only makes sense for small jobs; larger fills use ready-mix grout.
Do I have to fill every core?
No. Many walls are only partially grouted, filling the cores that hold reinforcing bar plus the bond beams. Count just the cells you actually fill, not the whole wall, or you will badly overorder.
