Concrete Block Fill Calculator

For standard 2-core hollow block, filling every core takes roughly 0.17 cubic feet of grout per square foot of wall for 6-inch block, 0.26 for 8-inch, 0.34 for 10-inch and 0.42 for 12-inch block.

So a solid-grouted 8-inch wall of 320 square feet needs about 320 times 0.26, or roughly 83 cubic feet — a little over 3 cubic yards before waste. Partial grouting, where only the cells with rebar are filled, uses far less.

Multiply the net wall area (after openings) by the fill rate for your block width, then by the share of cells you are filling, and add 10 to 15 percent for grout lost into the joints and webs.

A single standard 8-inch block (nominal 8 by 8 by 16 inches) holds about 0.23 cubic feet of grout in its two cores when filled solid. Put another way, filling 8-inch block takes about 0.26 cubic feet per square foot of wall, since there are roughly 1.125 blocks per square foot.

That means around 115 to 120 solid-filled 8-inch blocks make up a cubic yard (27 cubic feet) of grout.

The exact figure varies with the block manufacturer’s core shape — some blocks have larger or smaller cores — so treat these as planning numbers and add waste for grout that squeezes into the mortar joints.

Solid grouting fills every core in the wall, giving the strongest, heaviest and most expensive result. It is used for retaining walls, high loads and below-grade walls where the engineer calls for a fully grouted wall.

Partial grouting fills only the cells that contain vertical reinforcing steel, at a set spacing — commonly 16, 24, 32 or 48 inches on center — and leaves the rest hollow. It uses much less grout and is common for ordinary residential and light commercial walls.

The spacing is set by the structural design: closer spacing and more grout for higher loads, wider spacing for lighter ones. Always follow the grouting and rebar schedule on your plans or from your engineer.

Work out the fraction of cells that get filled from the spacing. Standard block has a cell every 8 inches, so grouting at 16 inches on center fills one cell in two (half), 24 inches fills one in three, 32 inches one in four and 48 inches one in six.

Multiply the solid-fill volume for your wall by that fraction. For example, an 8-inch wall that would take 80 cubic feet solid needs only about a third of that — roughly 27 cubic feet — if the cells are grouted at 24 inches on center.

Then add any solid-grouted bond-beam courses, which run horizontally along the wall, plus a waste allowance. The grouted-cell count also tells you how many vertical rebar bars you need.

A bond beam is a horizontal course of block with a channel that holds horizontal reinforcing steel and is filled solid with grout. It ties the wall together, distributes loads and resists cracking, and is typically placed at the top of a wall, over openings and sometimes at mid-height.

Because a bond beam is grouted solid along the full length of the wall, it adds grout beyond the vertical cells — roughly the wall length times the course height (8 inches) times the fill rate for your block, for each bond-beam course.

This calculator lets you add bond-beam courses so their grout and horizontal rebar are included. The number and location of bond beams comes from your structural plans or local code.

Divide the total grout volume by the yield of one bag and round up. An 80-pound bag of core-fill grout or premixed concrete yields about 0.6 cubic foot, and a 60-pound bag about 0.45 cubic foot.

For example, 30 cubic feet of fill needs about 50 bags of 80-pound mix. Bagged product is convenient for small jobs and partial grouting, but the bag count climbs quickly on a fully grouted wall.

For larger volumes, ready-mix grout delivered and pumped is usually cheaper per yard and far faster than mixing dozens of bags by hand — most suppliers have a one-yard minimum. Compare the all-in cost for your volume.

Technically, masonry grout is the correct product for filling block cores: it is a fluid, high-slump mix of cement, sand and small aggregate designed to flow into the cells and around the rebar without leaving voids. Structural work usually specifies grout meeting the masonry grout standard.

For small, non-structural fills, many DIYers use a soupy pea-gravel concrete or a bagged core-fill mix, which works but must be made wet enough to consolidate fully around the steel. Ordinary stiff concrete with large aggregate can bridge and leave voids.

Follow your plans: engineered walls call for grout of a specified strength and slump. When in doubt, use a product labeled for core fill or masonry grout rather than standard concrete mix.

The grout itself is the main material cost, and it depends on how much of the wall is filled. A partially grouted residential wall might need only a yard or two of grout, while a solid-filled retaining wall needs far more.

Bagged core-fill mix runs a few dollars per 60 to 80 pound bag, and a fully grouted wall can take dozens of bags; ready-mix grout is often cheaper per yard for larger pours but adds delivery and pump fees. Vertical and horizontal rebar add to the material cost.

For a closer figure, enter your wall size, block width, fill pattern and local prices. Remember the estimate covers grout and rebar material only — pump rental, delivery, and labor are separate.