Concrete Block Fill Calculator: Grout or Concrete to Fill CMU Cores
Work out the grout or core-fill concrete for a CMU block wall from its size or block count: 6, 8, 10 or 12 in block, solid grouted or cells every 16–48 in, bond beam courses, waste, cubic yards, bags and cost, using the NCMA grout quantity table.
How it works
Blocks = wall area × 1.125 per sq ft · Net grout = wall area × NCMA solid rate × (8 in ÷ cell spacing) + bond-beam area × solid rate · NCMA solid rates (cu ft per 100 sq ft, incl. 3% waste): 6 in 25.6, 8 in 36.1, 10 in 47.0, 12 in 58.9, divided by 1.03 for net · Order = net × (1 + waste) · Bags = ⌈order ÷ bag yield⌉ · cu yd = cu ft ÷ 27
Grouting the cells of a concrete block wall adds strength and bonds vertical rebar to the masonry, and the volume depends on the block width and how many cells you fill. The calculator uses the grout table in the masonry industry’s estimating guide (NCMA / CMHA TEK 04-02A) for two-core hollow units: a solidly grouted wall takes about 25.6 cu ft of grout per 100 sq ft with 6 in block, 36.1 with 8 in, 47.0 with 10 in and 58.9 with 12 in. Those figures include 3% waste, so the calculator divides by 1.03 for the net volume and then adds your own waste allowance. A standard 8 × 16 in block face covers 0.889 sq ft, so there are 1.125 blocks and 2.25 cells per square foot. Partly grouted walls fill only the cells that hold rebar; at 16 in spacing that is every second cell and half the grout, at 32 in every fourth cell and a quarter. Bond beam courses (U-shaped units grouted along their full length) are counted as solid-grouted courses. Bags are rounded up after adding waste; an 80 lb bag of concrete mix yields about 0.6 cu ft and an 80 lb bag of preblended grout about 0.66 cu ft. Blocks with larger or smaller cells than standard two-core units, and grout lost into mortar joints, change the real quantity.
Worked example
With the defaults, a 20 × 8 ft wall is 160 sq ft, or 180 blocks of 8 in. Solid grouted at the NCMA rate of 36.1 cu ft per 100 sq ft (0.3505 cu ft per sq ft net), it needs 160 × 0.3505 = 56.1 cu ft = 2.08 cu yd of grout, about 0.156 cu ft per core across 360 cores. With 10% waste that is 61.7 cu ft (2.28 cu yd), or ⌈61.7 ÷ 0.6⌉ = 103 bags of 80 lb mix. Grouting only every second cell (16 in o.c.) halves the net volume to 1.04 cu yd.
Sources: CMHA / NCMA TEK 04-02A, Estimating Concrete Masonry Materials (Table 3 grout volume for vertical cells, Table 4 bag yields) · Inch Calculator: Concrete block fill calculator (fill per 100 blocks and per 100 sq ft)
Frequently asked questions
How much grout does an 8 inch block wall take?
About 36 cu ft per 100 sq ft of wall when every cell is filled, according to the NCMA estimating table (including 3% waste). That is roughly 1.3 cu yd per 100 sq ft, or about 0.16 cu ft per cell. Filling a cell every 16 in halves it, every 32 in quarters it.
Do I need to fill every core?
Not usually. Many reinforced block walls are grouted only at cells with vertical rebar, commonly every 16 to 48 in, plus a bond beam at the top. Retaining walls and some below-grade walls are solid grouted. Follow the structural drawings.
How many cores does one 80 lb bag fill?
An 80 lb bag of concrete mix yields about 0.6 cu ft, enough for about 3.8 cores of 8 in block at 0.156 cu ft each, or 5 cores of 6 in block. Preblended grout bags yield slightly more, about 0.66 cu ft. Spills and joint losses reduce this in practice.
Grout or concrete for filling cores?
Use masonry grout (ASTM C476) or a high-slump core-fill mix with pea gravel so it flows into the narrow cells and around rebar. Standard concrete with large stone can bridge in the cells and leave voids.
Should I use bags or order ready-mix?
For a few cells, bags are fine. Once the job needs a cubic yard or more (45 or so 80 lb bags), ready-mix grout placed with a grout pump is usually cheaper, faster and more consistent.
Why is this lower than some other calculators?
Some calculators assume the whole inside of the block is open. Real two-core units have face shells and webs about 1 to 1.5 in thick, so an 8 in block holds about 0.31 cu ft of grout. This calculator uses the masonry industry’s measured table instead of a rough block-volume estimate.
Related calculators
Estimates only: add a margin for waste and confirm with your structural drawings, local building code and the supplier’s specifications.