CMU core fill & grout estimator

Block Fill Calculator Core Fill & Grout Volume

Estimate how much grout or concrete it takes to fill the cores of a concrete block wall — cubic yards, 80-lb pre-mix bags, and vertical rebar columns, from 8×8×16 CMU up to 12-inch block.

Default sample is ready

Input

20 x 8 ft

Grout

1.98 cu yd

80-lb bags

Assumes 8-inch CMU with every cell filled and 10% waste. This is a volume planning estimate, not an engineered grout or rebar design.

Edit the estimate Count blocks instead

Estimate Block Core Fill

Use this for grout volume inside CMU cores. To count the blocks themselves, use the standard concrete block calculator instead.

Unit:

Wall Dimensions

Wall Length (ft)

Wall Height (ft)

Block & Fill Settings

Block Size

Core Fill Scope

Bag size:

Waste Allowance

10%

Use 5-10% for hand pours; increase for pump pours, spillage, and uneven cores.

Cost Estimation

Enable to enter your local pre-mix bag price.

How This Block Fill Calculator Works

Core fill is a volume problem, not a block-count problem. The calculator first estimates how many blocks are in the wall face — each nominal 8×16 inch block covers 0.889 square feet — then converts that count to grout volume using the industry blocks-per-cubic-yard benchmarks: 120 blocks for 6-inch CMU, 100 for 8-inch, 80 for 10-inch, and 65 for 12-inch when every cell is filled.

Cores sit every 8 inches along the wall, so a 16-inch block has two cells. If you only grout the vertical rebar cores, the volume scales by the spacing: cores at 16 inches on center fill half the cells, 24-inch spacing fills a third, and 48-inch spacing fills a sixth. The waste slider covers spillage, uneven webs, and mixing loss.

Use this for grouting CMU cores — rebar columns, fully grouted walls, or safety fills. It does not estimate solid poured walls, slabs, footings, bond beams, or lintels, and the rebar count is a placeholder for planning, not a reinforcement design. For 4-inch CMU, see the FAQ — those cores are rarely grouted.

Wall area & blocks

blocks = ceil(wall area ÷ 0.889 sq ft)

Each nominal 8×16 inch CMU face covers 0.889 square feet, including mortar joints.

Cores filled

fraction = 8 ÷ spacing (in.)

Fill scope	Share of cells
Every cell	1
16" o.c.	1/2
24" o.c.	1/3
32" o.c.	1/4
48" o.c.	1/6

Grout volume

cu yd = blocks ÷ blocks-per-cu-yd × fraction × waste

Block size	Blocks per cu yd
6 in CMU	120
8 in CMU	100
10 in CMU	80
12 in CMU	65

Pre-mix bags

bags = ceil(grout cu ft ÷ bag yield)

An 80-lb bag yields 0.60 cubic feet of mixed concrete; a 60-lb bag yields 0.45 cubic feet. For pours over a few yards, order ready-mix instead.

Benchmarks are industry blocks-per-cubic-yard tables used by masonry suppliers and pre-mix manufacturers, based on nominal CMU core volumes. Bag yields follow standard 80-lb and 60-lb pre-mix specs. Last reviewed June 2026.

Example Core Fill Estimate

The prefilled sample estimates a 20 x 8 ft wall in 8 in CMU (8×8×16) with fill all cells and a 10% waste allowance.

Quick sanity check: 100 standard 8-inch blocks fully filled take about 1 cubic yard of grout — the same benchmark masons use in the field. This sample is 180 blocks, so just under 2 cubic yards before waste.

Grout volume

1.98 cu yd

53.46 cu ft incl. waste

80-lb bags

or ready-mix by the yard

Blocks in wall

180

Vertical rebar

30 bars

planning estimate

Core Fill Planning Checklist

All cells or rebar cores only

Most reinforced walls only grout the cores with vertical rebar. Check the structural drawings for spacing before ordering — it changes the volume by up to 6x.

Rules of thumb

"100 blocks per cubic yard" and "one bag per two cores" are conservative field checks. This calculator runs the geometric math; use the rules of thumb to sanity-check the result.

Grout vs concrete vs site mix

Masonry grout is the code-standard fill. Pre-mix concrete with pea gravel works for DIY cores, and Portland-and-sand mixes suit non-structural fills. Keep the mix fluid enough to consolidate.

Lift heights & pump loss

Pour in lifts and stop 1.5 to 3 inches below the top of the final course if another pour follows. For pump pours, order roughly half a yard extra for line loss.

Frequently Asked Questions

How much concrete do I need to fill an 8×8×16 block?

About 0.25 to 0.3 cubic feet of mixed grout or concrete per block. The industry benchmark is 100 standard 8-inch blocks per cubic yard when every cell is filled. One 80-lb bag of pre-mix yields about 0.60 cubic feet, which fills roughly two block cores.

How do I calculate CMU core fill / grout volume?

Estimate the block count first: wall area divided by 0.889 square feet per block. Then divide the block count by the blocks-per-cubic-yard benchmark for your block thickness (120 for 6-inch, 100 for 8-inch, 80 for 10-inch, 65 for 12-inch), multiply by the fraction of cells you are filling, and add a waste allowance.

Should I fill every cell or only the cores with rebar?

Most reinforced block walls only grout the cells that contain vertical rebar, typically spaced 16 to 48 inches on center per the structural drawings. Filling every cell is common for below-grade walls, high-load walls, and security or sound walls. Always follow your engineer’s spacing, not a rule of thumb.

How many 80-lb bags of pre-mix fill a concrete block?

One 80-lb bag of pre-mix concrete yields about 0.60 cubic feet. A standard 8-inch block has roughly 0.27 cubic feet of open core space, so one bag fills about two blocks. Masons often quote "one bag per two cores" as a conservative field rule. For larger pours, order ready-mix by the cubic yard instead.

Can I use grout, pre-mix concrete, or Portland and sand to fill block cores?

Yes. Masonry grout is the code-standard choice because it is fluid enough to consolidate around rebar. Pre-mix concrete with small aggregate (pea gravel) is widely used for DIY core filling, and a Portland-and-sand mix also works for non-structural fills. Whatever you mix, keep it wet enough to flow to the bottom of the cores without voids.

Is a cinder block fill calculator the same as a concrete block fill calculator?

Yes. "Cinder block" is an informal name for the same concrete masonry unit (CMU). The nominal sizes and core volumes are identical, so the same fill math applies — this calculator covers both.

Do I need to fill the cores of a 4-inch CMU?

Almost never. The cores of a 4-inch block are too small for practical grouting, and 4-inch CMU is normally used for non-structural partition or veneer work that does not require reinforcement. That is why this calculator starts at 6-inch block. If your wall needs grouted rebar, the design will typically call for 8-inch or thicker block.

Estimating the rest of the project? Count the blocks themselves with the concrete block calculator , or plan a landscape wall with the retaining wall calculator . Laying the blocks with mortar? Estimate bags and a site mix with the block mortar calculator .