Which cores get filled
Structural walls fill every core containing rebar — typically verticals every 4 feet and all corner and opening cores. A standard 8×8×16 block has two cores; filling one block takes about 0.25 cubic feet of grout or concrete. The calculator multiplies your block count by that volume and converts to bags and yards.
Grout consistency matters
Core fill is poured, not placed — it must flow to the bottom of every cavity without honeycombing. Fine grout (small aggregate) pumps easily; if you're using bagged concrete as fill, expect to add water to a soupy consistency and rod every core. Fill in lifts of about 4 feet so the grout doesn't blow out the block faces.
The core-fill formula
Core fill is a volume problem, not an area one. A standard 8 × 8 × 16 concrete masonry unit has two cores, and both together take about 0.25 cubic feet of grout or concrete per block in a normal running-bond wall. That gives the estimating rule:
Fill volume (cubic feet) = number of grouted blocks × 0.25
From there, the two conversions that matter on site:
- Cubic feet to cubic yards: divide by 27 — the units a ready-mix truck is ordered in.
- Cubic feet to bags: an 80-lb bag of concrete mix yields about 0.6 cubic feet, so bags = fill volume ÷ 0.6, rounded up.
| Grout volume | Blocks fully grouted | 80-lb bags |
|---|---|---|
| 1 cu ft | 4 blocks | 2 bags |
| 1 cu yd | 108 blocks | 45 bags |
Read the right-hand column as a cross-check rather than a target: batching whole bags is usually cheaper than paying a short-load charge on a part load of ready-mix.
Fully grouted vs partially grouted
It is rare to fill every core, and the difference in volume is large. A wall is fully grouted when every cell is filled, and partially grouted when only the cells containing reinforcement are filled. In a typical reinforced wall that means:
- Every vertical-bar core — commonly on a grid of 32 or 48 inches horizontally, which is every fourth or sixth cell in a 16-inch CMU.
- Every core at a corner, a wall intersection, and both sides of every opening.
- Every core inside a bond beam, lintel or other horizontal reinforcement.
On a long straight run that works out to roughly one grouted core in four — about 25% of the block count. The calculator assumes full grouting by default, so for a partially grouted wall enter the equivalent number of fully grouted blocks you will actually fill — counting one filled core in a two-core block as half a block — not the total block count. Getting this wrong is the most common reason a grout order runs short near the top of a wall.
Three worked examples
Example 1 — 200 blocks, every core filled
Volume = 200 × 0.25 = 50 cubic feet. In yards, 50 ÷ 27 = 1.85 cubic yards. In bags, 50 ÷ 0.6 = 83.3, so 84 × 80-lb bags — the same figure the calculator returns for 200 blocks.
Example 2 — a 40 ft × 10 ft wall, partially grouted
A 40 ft × 10 ft wall is 400 sq ft. At 1.125 blocks per square foot — the same figure the concrete block calculator uses — that is 450 blocks, plus 5% waste = 473 blocks. If roughly one core in four is grouted, the fill count is 473 × 0.25 ≈ 118 blocks. Volume = 118 × 0.25 = 29.5 cubic feet = 29.5 ÷ 27 = 1.09 cubic yards ≈ 50 bags. The wall is twice the area of Example 1, yet the grout is barely more than half — because most of it is left hollow.
Example 3 — 1,000 blocks, half the cores filled
Fill count = 1,000 × 0.5 = 500 blocks. Volume = 500 × 0.25 = 125 cubic feet = 125 ÷ 27 = 4.63 cubic yards = 125 ÷ 0.6 = 208.3, so 209 bags. At this size the ready-mix truck wins on price; bags only make sense for the small top-of-wall pours a truck cannot reach.
Grout or concrete? And how high to pour
Core fill is not placed like a slab — it is poured and has to flow to the bottom of every cell without bridging. Two practical rules keep it sound:
- Use fine grout, not stiff concrete. A high-slump grout with small aggregate travels down a narrow core; a coarse, low-slump concrete mix bridges over the webs and leaves voids you cannot see. Filling from bags, mix to a pourable consistency and rod every core.
- Pour in lifts of about 4 feet. Filling a full-height core in one pass builds enough pressure to blow the block faces off the wall. Lift by lift also lets each pour settle before the next goes in.
Two more site habits save a callback: block off the bottom of open cores so grout does not run out onto the footing, and never grout a wall in a hard freeze — fresh grout has no strength until it cures.
Where these figures come from
The 0.25 cu ft per block allowance is a field average for two-core 8-inch units. Published grout volumes for these units — see the National Concrete Masonry Association — vary with core geometry and the waste allowance included, and can run higher than this figure, so round a grout order up rather than down. The block itself is specified by ASTM C90 (loadbearing concrete masonry units), and the grout poured into it by ASTM C476 (grout for masonry). Where a project specifies a different core geometry or grout percentage, follow the engineered drawings rather than this rule of thumb.
Tools for filling block cores
Core fill is poured, not placed, so it has to reach the bottom of every cell without honeycombing. These are the items that make that work on a reinforced wall.
- Mortar mixing tub and hoe — Bagged grout or concrete has to be mixed to a pourable slump. A tub and hoe let you batch to the consistency the cores need without a mixer.
- Grout bag or pouring bucket — A heavy-duty bucket or grout bag is how the mix reaches the top of the wall and down into each narrow core.
- Concrete vibrator — Grout poured in lifts has to be rodded to close voids and settle it around the rebar. A vibrator does in seconds what tapping the block only partly manages.
- Rebar and tie wire — The cores that get filled are the ones carrying vertical reinforcement, the steel the grout is there to bond to.
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Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- ASTM International — C94 for ready-mixed concrete, C150 for cement and C143 for slump testing.
- American Concrete Institute — ACI 318 code requirements and ACI 302 slab construction guidance.
- Portland Cement Association — mix design, yield and curing data published by the US cement industry.
- National Concrete Masonry Association — grout tables and TEK notes for concrete masonry core fill.
- ASTM C90 — standard specification for loadbearing concrete masonry units (block geometry and face shells).
- ASTM C476 — standard specification for grout for masonry, the material poured into the cores.
Figures, formulas and reference tables last verified: 25 September 2026.