Concrete Block Fill Calculator: Core Fill for CMU Walls
Filling block cores with grout or concrete is what turns a stacked wall into a structural one. Enter the wall size and block thickness for the block count and the fill volume — in cubic yards and in bags. The default is a 20 × 8 ft wall in standard 8 in block, all cells filled.
Exact fill before waste: 48.6 ft³ (1.80 yd³). In 60 lb bags with waste: 119.
Block Fill Table: Fill Volume by Block Thickness
These are the standard masonry estimating figures for blocks filled solid (every cell). A standard 8 × 8 × 16 block's cores hold about 0.27 ft³ of fill — which is why a cubic yard fills right about 100 of them.
| Block thickness | Fill per block | Blocks filled per yd³ | Fill per 100 blocks | Fill per 100 ft² of wall |
|---|---|---|---|---|
| 6 in | 0.23 ft³ | 120 | 0.83 yd³ | 0.94 yd³ |
| 8 in | 0.27 ft³ | 100 | 1.00 yd³ | 1.13 yd³ |
| 10 in | 0.34 ft³ | 80 | 1.25 yd³ | 1.41 yd³ |
| 12 in | 0.42 ft³ | 65 | 1.54 yd³ | 1.73 yd³ |
Block counts come from the face size: an 8 × 16 in block face, mortar joint included, covers 0.889 ft² of wall, so a wall takes 1.125 blocks per square foot — 180 blocks for the 160 ft² example wall, before any allowance for cuts and breakage. Add about 5–10% more blocks on walls with corners and openings.
How the Math Works
- Blocks: wall length × wall height = wall area. Multiply by 1.125 blocks per ft² and round up.
- Fill volume: blocks × the fill per block for your thickness (0.27 ft³ for standard 8 in block).
- Partial fill: if only the cells with vertical rebar are grouted, multiply by the filled fraction — cells at 24 in on center fill about a third of the wall's cores.
- Cubic yards: divide cubic feet by 27. Bags: divide by the yield printed on the bag (80 lb = 0.60 ft³) and round up.
Blocks = wall ft² × 1.125 · Fill (ft³) = blocks × fill per block × cells-filled fraction
Worked example, the default wall: 20 × 8 ft = 160 ft², × 1.125 = 180 blocks. Fill: 180 × 0.27 = 48.6 ft³, which is 48.6 ÷ 27 = 1.80 yd³. With the 10% cushion: 1.98 yd³, or 53.46 ÷ 0.60 = 90 bags of 80 lb mix.
Grout or concrete mix? Grout is the specified material for core fill — it is mixed fluid enough to flow into the cores and consolidate around rebar without leaving voids, and bagged grout mix yields about the same per bag as concrete mix, so the bag counts above hold either way. On anything past a yard or so, most masons stop bagging entirely and order grout or ready-mix by the truck — compare bag cost against a truck quote with the cost comparison on the main calculator. And note that filling is usually only part of the wall's structure: vertical and horizontal rebar sit in those filled cells per the design, and bond beam courses at the top get filled as part of the same pour.
Building the Rest of the Job?
A block wall usually sits on a concrete footing and often gets a slab poured against it. Figure both with the main calculator, and the gravel base under them with the gravel calculator.
Block Fill FAQ
How many concrete blocks will a cubic yard of grout fill?
About 100 standard 8 in blocks. Thinner and thicker blocks bracket that number: roughly 120 for 6 in block, 80 for 10 in, and 65 for 12 in. The 8 in figure works out to about 0.27 ft³ of fill per block — the two hollow cores in each block, mortar joints accounted for.
How much fill do I need for 100 blocks?
For standard 8 in block filled solid: 1.00 yd³. That is 27 ft³, or about 45 bags of 80 lb grout or concrete mix before waste — one of the handier numbers in masonry estimating, since a yard and a hundred blocks are the same order.
How many blocks are in a 20×8 ft wall?
180 blocks. The wall is 160 ft² and standard 8 × 16 in block covers 0.889 ft² per block including the mortar joint, so 160 × 1.125 = 180. Order about 5–10% extra for cuts, corners, and breakage, and fill for that wall runs 1.80 yd³ solid-grouted.
Does every cell in a block wall need to be filled?
Often not. Many structural walls are partially grouted: only the cells containing vertical rebar (commonly every 24 to 48 in), plus bond beams and cells beside openings, get filled. That can cut fill volume to a third or less of the solid-grouted figure — use the "Cells filled" selector above to match your design. Fully solid grouting shows up in high-load, high-wind, and seismic work. Which cells your wall needs is a design and code question, not a preference.
Estimates Only
Results are planning estimates based on standard industry fill figures (blocks filled per cubic yard by thickness). Actual fill varies with block manufacturer, core shape, mortar joint thickness, and how much material is lost in placement. Block counts exclude cuts and breakage — add 5–10%. Grouting and reinforcement requirements for structural walls are set by the design and local building code; consult a licensed professional for load-bearing work.