Published fill volumes by block size
These are typical solid-fill volumes from masonry industry fill tables for standard concrete masonry units; face-shell-bedded (partially filled) walls need proportionally less grout.
| Block size | Grout per 100 blocks (all cores filled) |
|---|---|
| 8×8×16 in CMU | ≈ 0.83 yd³ |
| 12×8×16 in CMU | ≈ 1.23 yd³ |
- These figures assume all cores are filled solid; if only reinforced cores are being grouted (partial fill), actual grout volume will be significantly less — consult project specifications.
- Fill volumes vary somewhat by manufacturer and exact block geometry; published industry averages are used here as a planning estimate.
What is CMU core fill, and why does it use published tables?
Concrete masonry units (CMU, or 'concrete blocks') are hollow, with vertical cores that can be filled solid with grout for added strength, to embed reinforcing bar, or to meet a structural or fire-rating requirement. The volume of grout needed per block is not simply the block's outer dimensions, since much of the block is solid webbing and face shell — it depends on the block's actual core geometry.
Because core geometry varies by manufacturer and block type, the masonry industry publishes standard fill-volume tables (grout volume per 100 blocks) for common block sizes, and this calculator applies those published figures rather than estimating core volume from the block's outer dimensions alone.
How to use this block fill calculator
- Enter the total number of blocks whose cores need to be filled.
- Select the block size — 8×8×16 inches or 12×8×16 inches.
- Read the grout volume needed in cubic yards and cubic meters, plus the number of 80 lb grout bags to order.
The formula behind the block fill estimate
Grout volume equals the number of blocks divided by 100, multiplied by the published fill volume per 100 blocks for the selected block size (about 0.83 cubic yards per 100 for an 8×8×16 in block, and about 1.23 cubic yards per 100 for a 12×8×16 in block, based on standard masonry industry fill tables for solid-filled cores). Bag count converts this volume to 80 lb bags using a published yield of about 0.6 cubic feet per bag.
Worked example: filling 100 standard 8×8×16 in blocks needs about 0.83 cubic yards of grout (about 0.63 m³). At a yield of 0.6 ft³ per 80 lb bag, that is (0.83 × 27) ÷ 0.6 ≈ 37.4 bags, rounded up to 38 bags.
Common mistakes
- Assuming grout volume scales with the block's full outer dimensions rather than using published core-fill tables, which overstates the quantity needed.
- Ordering for a full solid fill when the specification only calls for partial (cell) grouting of reinforced cores.
- Mixing up grout (used to fill CMU cores) with mortar (used between block courses) — they serve different purposes and are estimated separately.
- Not accounting for grout lift height limits and consolidation requirements in taller pours, which is a placement/code issue beyond this volume estimate.
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How much grout do I need to fill concrete block cores?
For standard 8×8×16 in CMU with all cores filled, published fill tables give about 0.83 cubic yards of grout per 100 blocks; larger 12×8×16 in blocks need about 1.23 cubic yards per 100 blocks.
How many 80 lb bags of grout fill 100 concrete blocks?
At a typical yield of about 0.6 cubic feet per 80 lb bag, filling 100 standard 8×8×16 in blocks (≈0.83 yd³, or about 22.4 ft³) needs roughly 37-38 bags.
What is the difference between grout and mortar in block work?
Mortar bonds and beds the block courses together; grout is poured into the hollow cores of the blocks (often around reinforcing bar) to fill them solid for strength — they use different mixes and are estimated separately.
Do I need to fill every core, or just some?
That depends on the structural specification — some walls require solid grouting of every core, while others only require grouting cells containing reinforcing bar; check the project drawings before ordering a full-fill quantity.
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- National Concrete Masonry Association (NCMA) — TEK notes on grouting concrete masonry, including standard fill-volume tables by block size.
- American Concrete Institute / The Masonry Society (ACI 530 / TMS 402) — Building Code Requirements for Masonry Structures: grouting provisions.
- QUIKRETE technical data sheets — published grout bag yield (cubic feet per bag) for core-fill estimating.