Concrete Block Fill Calculator — CMU Core Grout Volume
Enter the number of CMU blocks, the block size and how many cores need filling to find the total grout volume and number of 80 lb bags required.
Block size
Fill pattern
%
0.767 m³ · 27.09 ft³
100%
cores filledFilled cores
100%
Unfilled cores
0%
- 1
Fill volume per block
0.258 × 1 = 0.258 ft³8-inch CMU (standard), Full fill (all cores — 100%). - 2
Total fill before waste
100 × 0.258 = 25.8 ft³ - 3
With waste allowance
25.8 × 1.05 = 27.09 ft³ - 4
Fill volume (yd³)
27.09 ÷ 27 = 1.003
How does this calculator work?
Multiply the number of blocks by the per-block void volume (0.258 ft³ for 8" CMU from NCMA data), then by the fill fraction (1.0 full, 0.5 alternate, 0.33 rebar only) and a 5% waste factor. Divide by 0.60 ft³ per 80 lb grout bag and round up. Adjust block void data for 6", 10" or 12" CMU.
Formula
How this is calculated
Hollow concrete masonry units (CMU) have two or three vertical cores running through each block. When walls require added strength, fire resistance or the encapsulation of vertical rebar, those cores are filled with grout (a fluid concrete mix). The fill volume depends on the block size and how many cores are filled.
Void volumes per block come from NCMA (National Concrete Masonry Association) published data: an 8-inch CMU holds about 0.258 ft³ of void, a 6-inch about 0.140 ft³, a 10-inch about 0.370 ft³ and a 12-inch about 0.509 ft³. These are the total void volumes assuming both cores in an 8-inch block are included. The fill pattern — full fill, alternate cores or rebar cores only — determines the fraction of that void to fill.
The grout bag yield used here is 0.60 ft³ per 80 lb bag, a typical published yield for coarse grout or high-slump concrete grout mix. Add a 5% waste factor for spillage and over-pour. The result should be ordered from a ready-mix supplier or estimated in bags for smaller jobs. This calculation assumes standard two-core blocks; three-core or custom blocks may differ.
Frequently asked questions
Structural requirements in the building code (e.g. ACI 530 in the US) specify where filled and reinforced CMU is required. Typical situations include seismic zones, retaining walls, walls supporting heavy loads, and code-specified fire ratings. Always consult a structural engineer or your local building code.
Fine or coarse grout (ASTM C476) is preferred — it has a higher slump and smaller aggregate so it flows into narrow cores without voids. Regular concrete can work for large cores (8"+ blocks) but may not consolidate as well in 6" cores.
Full fill provides the highest strength and best fire rating. Alternate-core fill is common for lightly reinforced walls (rebar every other core). Rebar-only fill consolidates just the cores containing vertical rebar — acceptable for some seismic designs — and uses the least grout.
Also known as
TG we-Calculate Editorial Team. (2026). Concrete Block Fill Calculator — CMU Core Grout Volume [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/concrete-block-fill-calculator
TG we-Calculate Editorial Team. "Concrete Block Fill Calculator — CMU Core Grout Volume." TG we-Calculate. 2026. https://we-calculate.com/calculator/concrete-block-fill-calculator.
TG we-Calculate Editorial Team, "Concrete Block Fill Calculator — CMU Core Grout Volume," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/concrete-block-fill-calculator
@misc{wecalculate_concrete_block_fill_calculator, title = {Concrete Block Fill Calculator — CMU Core Grout Volume}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/concrete-block-fill-calculator}}, year = {2026}, note = {TG we-Calculate} }
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