How this instrument works
Some concrete block walls need their hollow cores filled with grout or concrete — fully, for maximum strength or where rebar reinforcement needs to be embedded, or partially, such as filling only a bond beam course near the top of the wall. Either way, the volume needed depends on how many blocks are being filled, how much void space each block's cores actually hold, and what percentage of that void you're filling.
A standard 8x8x16 in CMU's two cores hold approximately 0.25 cubic feet of grout combined when fully filled — a commonly cited approximate figure, though actual core volume varies by manufacturer based on face-shell and web thickness, which is why this calculator leaves it as an editable field rather than a fixed constant. If you know your specific block's published core volume, use that figure for a more accurate estimate.
The percent-filled input lets you model partial fills, like grouting only a bond beam course (where you might fill just 100% of a smaller number of blocks) or a genuinely partial fill within every core — multiply your block count and fill percentage accordingly for your specific scope of work.
- Enter the number of blocks you're filling.
- Leave the core volume at the standard 8x8x16 in default, or enter your specific block's published core volume.
- Enter what percentage of the cores you're filling — 100% for a full grout fill, lower for a partial or bond-beam-only fill.
- Leave the bag yield at the default, or enter your grout/concrete mix's actual bag yield.
- Read the total fill volume and bags needed.
Fully grouting 198 standard CMU blocks
A mason is fully grouting 198 standard 8x8x16 in CMU blocks — in the same ballpark as what the companion concrete-block calculator would order for a similarly sized 20 x 8 ft wall. At the standard 0.25 cu ft core volume per block and a 100% fill: fill volume = 198 x 0.25 x 1.0 = 49.5 cu ft. Using standard 80 lb bags (0.60 cu ft yield): bags = ceil(49.5/0.60) = ceil(82.5) = 83 bags.
A smaller partial bond-beam fill on 100 blocks, filling only 50% of the cores, needs far less material: fill volume = 100 x 0.25 x 0.5 = 12.5 cu ft, and bags = ceil(12.5/0.60) = ceil(20.833) = 21 bags — about a quarter of the first example's bag count, reflecting both the smaller block count and the partial fill percentage.
Questions
Why isn't 0.25 cubic feet per block exact for every CMU?
Core volume depends on each manufacturer's specific face-shell and web thickness, which varies somewhat across producers even for nominally identical 8x8x16 in blocks. 0.25 cu ft is a commonly cited approximate figure for planning purposes — check your specific block's published core volume if you have it for a tighter estimate.
When would I only fill part of the cores instead of 100%?
Partial fills are common for bond beams (a reinforced horizontal course, often near the top of a wall or at floor lines) rather than filling every core in every course, or in walls where full grouting isn't structurally required. Use the fill percentage field to model whatever portion of the total core volume your project actually calls for.
What's the difference between grout and concrete for filling block cores?
Both can fill CMU cores, but masonry grout is typically formulated with finer aggregate and a more flowable consistency specifically to fill narrow cores completely around embedded rebar without voids, while standard concrete mix (like the bags used in the cement calculator) is coarser. Follow your project's engineering specification for which material is actually required.
Should I add extra bags beyond the calculated amount?
Many masons add a small buffer to cover spillage down the face of the wall and mortar-joint absorption, since some grout inevitably doesn't make it fully into the cores during pumping or pouring. This calculator gives the theoretical core volume; how much extra to buy depends on your pour method and comfort margin.