How core fill volume is worked out
Every hollow concrete block has a fixed void volume, so the whole calculation is block count × void per block × the fraction of cores you are actually filling. As a working rule of thumb, a standard 8 × 8 × 16 in block holds roughly 0.26 cubic feet of grout when both cores are filled, a 6 in block roughly 0.16, and a 12 in block roughly 0.50. Since there are 27 cubic feet in a cubic yard, that puts about 104 standard 8 in blocks in a yard of grout, about 169 six-inch blocks, and about 54 twelve-inch blocks. If you are measuring the wall rather than counting units, an 8 × 16 in nominal block covers 0.89 sq ft of face, so a wall takes about 1.125 blocks per square foot — a 40 ft × 8 ft wall is roughly 360 blocks. Partial grouting scales the result directly: filling every second core halves the volume, every fourth core quarters it.
Two things routinely make the real number come out higher than the arithmetic. Grout has to be fluid enough to flow down a narrow cell and consolidate around rebar without segregating, which is why grout for masonry is mixed much wetter than ordinary concrete and why fine pea-gravel or grout mixes are used rather than a standard bag of concrete with large aggregate. That fluidity means it finds every gap: mortar squeeze-out inside the cells, the bond beam course, lintel blocks and the knock-out webs all absorb material the block-count formula does not see. Allow around 10% over the calculated figure, and more on a wall with bond beams. The bagged-concrete number here is useful as a sanity check on small jobs, but it climbs fast — once you are past roughly a cubic yard, mixing that many bags by hand stops being realistic and ordering ready-mix grout by the yard is both cheaper and far more likely to give you a consistent, properly consolidated fill. Whatever you use, place it in lifts and consolidate it, since an unconsolidated cell leaves voids exactly where the wall was supposed to gain its strength.
Void volumes vary between block manufacturers and between two-core and three-core units, and coverage varies by mix — always confirm quantities against your actual block and follow the engineer's or building official's requirements for which cores must be grouted.