Concrete Block Fill Calculator
Work out how much grout or core fill a CMU wall needs — in cubic feet, cubic yards, and 80 lb bags — for either full grouting or partial grouting at your rebar spacing.
Core volumes are typical values — actual cores vary by manufacturer. An 80 lb bag of core-fill grout yields about 0.6 cu ft. Check your product before ordering.
Grout fill per block
| Block size | Fill per block | Blocks per cu yd | Per 100 blocks |
|---|---|---|---|
| 6" CMU | 0.17 cu ft | ~159 blocks | ~17 cu ft |
| 8" CMU | 0.24 cu ft | ~112 blocks | ~24 cu ft |
| 10" CMU | 0.32 cu ft | ~84 blocks | ~32 cu ft |
| 12" CMU | 0.41 cu ft | ~66 blocks | ~41 cu ft |
Based on two cores per block at typical core dimensions. Blocks are 1.125 per square foot of wall for standard 8×16 nominal face.
Full grout or partial grout?
This choice changes your material order by a factor of three or more, so it is worth settling before you price anything.
- Partial grouting fills only the cells containing vertical reinforcement, typically at 32 or 48 inches on centre. This is standard for most residential above-grade walls.
- Full grouting fills every cell. Specified for retaining walls, below-grade foundation walls, walls carrying significant load, and higher seismic or wind categories.
Grout, mortar, and concrete are three different things
| Material | Consistency | Used for |
|---|---|---|
| Mortar | Stiff, holds a ridge | Bedding joints between blocks |
| Grout | Fluid, 8–11 in slump | Filling cores, bonding rebar to block |
| Concrete | Stiff, coarse aggregate | Footings and slabs — not cores |
The common mistake is filling cores with standard bagged concrete. Its 3/4 inch aggregate bridges across a narrow core and leaves voids around the rebar, which defeats the purpose of grouting. Use a coarse grout, a pea-gravel mix, or a product sold specifically as core fill.
Practical notes
- Grout in lifts. Pours over about 5 feet need to be placed in lifts and consolidated, or the grout bridges and leaves voids. Follow the lift height your code allows.
- Consolidate every pour. Rod or vibrate. Grout that is not consolidated does not bond to the rebar.
- Clean-outs. High-lift grouting usually requires clean-out openings at the base of each grouted cell so debris can be removed and the pour inspected.
- Order 10% extra. Cores are never uniform, and a partially grouted wall that runs short mid-pour is a cold joint in a structural element.
Frequently asked questions
How much concrete does it take to fill an 8 inch block?
A standard 8x8x16 CMU holds roughly 0.24 cubic feet of grout across its two cores, so one cubic yard fills about 110 blocks. Core dimensions vary between manufacturers, so treat this as a planning figure and add 10%.
How many blocks does a yard of grout fill?
Approximately 160 blocks at 6 inch, 110 at 8 inch, 85 at 10 inch, and 65 at 12 inch — assuming every cell is filled.
Do I need to fill every cell in a block wall?
Usually not. Most residential walls are partially grouted, filling only the cells that contain vertical rebar — commonly every 32 or 48 inches on centre. Full grouting is specified for retaining walls, below-grade walls, and higher seismic or wind zones. Your engineer or local code determines which applies.
What is the difference between grout and mortar for block?
Mortar is the bedding material between blocks and is mixed stiff. Grout is a fluid, high-slump concrete poured into the cores to bond rebar to the block. They are not interchangeable — mortar is too stiff to consolidate in a core.
Can I use regular concrete mix to fill block cores?
Only if the aggregate is small enough. Standard bagged concrete contains stone up to 3/4 inch, which bridges in a narrow core and leaves voids. Use a coarse grout, a pea-gravel mix, or bagged core-fill grout.