How Much Grout to Fill Concrete Block Cores?
Core fill turns a hollow block wall into a reinforced structural wall, but it is easy to under-order. This guide gives you the volume per block, the yield per bag, and two fully worked examples so you can order grout or call the ready-mix plant with confidence.
A standard two-core 8x8x16 concrete block holds about 0.2 cubic feet of grout per block. An 80-pound bag of core-fill grout yields about 0.65 cubic feet, so one bag fills roughly 3 blocks. For 100 blocks, budget 20 cubic feet (0.74 cubic yards) or about 31 bags.
The math: 0.2 cubic feet per block
A standard 8x8x16 block has two hollow cores. Once the block is laid, each cell is roughly 5 to 6 inches wide, 5 inches deep, and 8 inches tall. That works out to about 0.2 cubic feet of void per block. Use this figure for any 8-inch two-core block; single-core or three-core specialty blocks differ, so check the manufacturer's data.
Core-fill grout, usually a fluid mix of Portland cement, sand, and pea gravel, comes in 80-pound bags with a typical yield of 0.65 cubic feet per bag. Divide the void by the yield: 0.2 divided by 0.65 is about 0.31 bags per block, or roughly 3 blocks per bag. That is the number to remember.
Full fill vs partial fill
Not every wall needs every core filled. The common schedules are:
Partial fill: fill only the cores that carry rebar, typically every 48 inches on center, or every fourth block. A 10 foot wall gets rebar at the 4 foot and 8 foot marks, so you fill roughly one block in four. This is the schedule for most residential foundation and garden walls.
Full fill: every core gets grout, usually with vertical rebar in every cell. Required by code for high-load walls, tall retaining walls, and seismic or high-wind zones.
Bond beam only: only the top course (and any intermediate bond beams) is filled, with horizontal rebar running through the beam channel. Common for freestanding walls and fences.
Worked example 1: 100 blocks, fully filled
Take a small structural wall that needs 100 blocks with every core filled:
100 blocks x 0.2 cubic feet = 20 cubic feet of grout.
20 cubic feet / 27 = 0.74 cubic yards.
20 / 0.65 cubic feet per bag = 30.8, so 31 eighty-pound bags.
At under one cubic yard, bagged grout is the right call. Mixing 31 bags by hand is a real workout, so a small mixer pays for itself on anything past 15 bags.
Worked example 2: partial fill on a 10x10 wall
A 10 by 10 foot wall is 100 square feet, which is about 120 blocks before waste (100 / 0.89). With rebar every 48 inches, you fill every fourth block, so 120 / 4 = 30 blocks to fill:
30 blocks x 0.2 cubic feet = 6 cubic feet = 0.22 cubic yards.
6 / 0.65 = 9.2, so 10 eighty-pound bags with a little margin.
This is a one-afternoon pour. Rod or vibrate each lift, keep the pour to lifts of about 4 feet so the grout consolidates, and recheck plumb after each lift, because wet grout is heavy and can push a wall off line.
When to call the ready-mix plant instead
Bagged grout makes sense up to about half a cubic yard. Past that, ordering grout from a ready-mix plant in a small truck is usually cheaper and much faster, but plants charge a short-load fee for anything under their minimum, often 2 to 3 yards. For a job in the 1 to 3 yard range, compare the plant's delivered price plus the short-load fee against the bag cost; the plant almost always wins on anything over a yard.
Grout vs concrete: use the right mix
Core fill should be grout, not regular concrete. Grout is more fluid (a high slump), so it flows around the rebar and fills the cells completely, and it bonds to the block. Regular concrete is stiffer and leaves voids around the bar, which defeats the purpose. If you are ordering from a plant, ask for core-fill grout, not a standard mix, and confirm the strength (usually 2,000 to 3,000 psi for residential work).
Skip the arithmetic
Planning a wall with core fill and rebar? Start with the block count. Our free calculator gives you the exact number of blocks for your wall dimensions, including waste.
Block core fill questions
How much grout do I need to fill a block?
About one third of an 80-pound bag per standard 8x8x16 block, since each block holds roughly 0.2 cubic feet of grout and a bag yields 0.65 cubic feet. In round terms, one bag fills 3 blocks. For 100 blocks, buy 31 bags.
How to make core fill grout?
Bagged core-fill grout mixes with water only, following the bag's water ratio. For site-mixed grout, a common proportion is 1 part Portland cement, 2 to 3 parts sand, and pea gravel for coarse grout, with enough water for a 9 to 11 inch slump. The mix should flow like thick soup so it consolidates around the rebar.
What is the difference between grout and concrete?
Grout is a fluid cement mixture (cement, sand, water, sometimes pea gravel) made to fill voids, while concrete is a stiffer mixture with larger aggregate made for forms and slabs. In block work, grout's high slump lets it flow into cores and around rebar without honeycombing, which regular concrete cannot do reliably.
How much concrete does it take to core fill blocks?
Budget 0.2 cubic feet per standard two-core 8x8x16 block. A 10 by 10 foot wall (about 120 blocks) needs 24 cubic feet, or 0.89 cubic yards, for a full fill, and about 0.22 cubic yards if you fill only the rebar cells every 48 inches. Use grout rather than standard concrete for the fill.
Do all block walls need core fill?
No. Garden walls, short freestanding walls, and non-load-bearing walls often need no fill at all. Core fill with rebar is required for load-bearing walls, retaining walls over about 3 to 4 feet, walls in seismic or high-wind zones, and anywhere the building code or the plans call for reinforced masonry. When in doubt, follow the plan; unfilled block has far less strength.