Estimate the concrete or grout needed to fill the cores of concrete blocks (CMU) for a wall, in bags or cubic metres.
Hollow concrete blocks — called CMU in the US or hollow “breeze/dense concrete blocks” in the UK — have open cores. Filling those cores with concrete or grout adds strength, especially around rebar. Enter the wall face area, choose the block size, and set the fill percentage (100% for a fully grouted wall, less if only reinforced cores are filled).
| Block size | Fill per block |
|---|---|
| 8×8×16 in | ≈ 0.30 ft³ |
| 12×8×16 in | ≈ 0.50 ft³ |
| 440×215×100 mm | ≈ 0.005 m³ |
A 100 ft² wall of 8-in block holds about 112 blocks. Filling every core (100%) uses roughly 112 × 0.30 ft³ ≈ 34 ft³ ≈ 1.25 yd³ of grout. Filling only every fourth core for rebar drops that to about 0.3 yd³.
Tip: use a fluid, small-aggregate “pea-gravel” grout so it flows down the cores without leaving voids.
A standard 215mm UK hollow block takes roughly 10 litres of fill per block when fully grouted, though this varies with the exact block design.
Not always. Many walls only have cores filled where rebar is placed or where structural design requires it. Set the fill percentage to match.
Yes. Fine grout or a small-aggregate concrete flows into block cores more easily than standard mixes. The volume needed is the same.
The calculator divides the fill volume by the yield of a 25kg bag (about 0.0166 m³) and rounds up to whole bags.
Filling cores increases the wall's strength, stiffness and resistance to lateral loads, and locks reinforcing bars in place.
