How Much Concrete to Fill Cinder Blocks

"How much concrete does it take to fill cinder blocks?" is one of the most-searched masonry questions, and it is also one of the most-botched. The honest answer for a standard 8 in hollow unit is about a third of a cubic foot of grout per block, but the figures published around the web scatter across a 20-fold range, from silly-low to roughly correct. Before you buy anything, run your wall through our concrete block fill calculator: it returns the volume, the cubic yards, and a bag count, and it shows the source next to every number so you can check the arithmetic yourself.

The short answer

A standard 8 × 8 × 16 in hollow concrete block has two cells. Filled solid, those cells hold roughly 0.32 cubic feet of grout per block. Put another way, one cubic yard of grout (27 ft³) fills about 84 hollow 8 in blocks. Six-inch units hold less; ten- and twelve-inch units hold considerably more, because the cell cavity grows with wall thickness.

That is the number you order to. It already accounts for the reality that grout does not stop neatly at the face of the block. It slumps into the mortar joints and overfills slightly, which is exactly what you want for a sound, void-free fill.

Why the published figures disagree so wildly

Search around and you will find per-block figures published elsewhere that are wrong by 15 to 27 times. The most common failure is a units error, someone divides by 1,728 twice, or confuses cubic inches with cubic feet, and publishes something near 0.014 ft³ per block. A wall estimated that way would be short on grout by a factor of more than twenty. Others quietly copy a single wrong figure from each other, so the same mistake propagates across dozens of pages that never show where the number came from.

Here is the useful range, with the two figures this site actually stands behind:

Why the two sourced numbers land so close

The two credible figures, geometry and the industry table, sit within a couple of percent of each other, and that is not a coincidence.

The geometric figure is built from ASTM C90 minimum thicknesses: a 1.25 in face shell and a 0.75 in web on an 8 in unit. Minimum thicknesses mean maximum cavity, so this calculation produces the largest cell volume the standard permits. It is a true lower-bound-of-material, upper-bound-of-cavity number, computed from the block's actual dimensions minus the solid shells and webs.

The CMHA Table 2 figure starts from real filled walls and adds a field-waste and overfill allowance. Because the geometric number is already near the top of the plausible cavity range, adding a modest waste allowance lands the table only slightly higher. The two methods approach the same answer from opposite directions, pure geometry from below, field experience from above, which is why you should trust a number in this band and distrust anything far outside it.

One practical caveat: real block is usually made thicker than the C90 minimum. Producers add material for durability and handling, so the actual cavity in the unit on your pallet is often a little smaller than either figure suggests. That means a wall built from typical block frequently needs less grout than the table predicts, never more. Ordering to the CMHA number gives you a safe margin without the gross over-buying the wrong internet figures would cause.

Grout, not concrete

The words get used interchangeably on the jobsite, but they are not the same material. Grout for filling masonry cells is a high-slump mix, an 8 to 11 in slump under ASTM C476 and TMS 602, specifically so it flows down the cells and consolidates around the reinforcing steel without leaving voids. Bagged concrete mix is far stiffer; poured into a narrow cell it bridges across the cavity and leaves air pockets that defeat the entire point of filling. If you are filling cells, buy a product sold as masonry grout or mix your concrete wet enough to meet a grout slump. The bag counts this site produces assume a grout-consistency mix.

Solid fill versus partial fill

The per-block number above assumes you are filling every cell. Most reinforced walls are not filled solid. They are grouted only at the cells that contain rebar, on a spacing your engineer specifies. That single decision changes the order more than any other input. At 8 in wall thickness, solid grouting runs 36.1 ft³ per 100 ft² of wall, while grouting at 48 in on center runs just 6.1 ft³, a 5.9× difference. If your drawings call for reinforcement at a spacing, do not price the wall as solid. See partial grouting and why spacing changes your order for the full breakdown.

Do not forget the bond beams

Horizontal grouted courses, bond beams, are filled solid regardless of your vertical cell spacing, and they are not included in the CMHA cell-grout table. You add their volume on top. A single bond-beam course in a 20 ft wall is only a few cubic feet, but it is real material and it is easy to leave off a takeoff. The calculator breaks bond beam volume out on its own line so you can see it separately.

How to order

Take your wall area, pick your wall thickness and grout spacing, add any bond-beam courses, and read the cubic-yard figure. If you are buying bags, the calculator converts to a bag count for your chosen product and bag size. If you are ordering ready-mix grout, order to the cubic-yard number and round up to the supplier's minimum. Either way, the figure you trust should show its source. That is the whole point.

For related planning, see should you fill cinder blocks at all, the concrete & grout bag yield chart, and the full list of masonry calculators.