Calculate how much concrete or grout you need to fill CMU (concrete masonry unit) block cores. Essential for structural walls, foundation walls, and retaining walls that require filled and reinforced cores.
Walls over 4 feet typically require engineering
Estimates only — check local building codes for fill requirements. Structural walls should be designed by a licensed engineer.
Filling all cores in a 20×8 foot CMU wall using 8×8×16 blocks? The wall has approximately 180 blocks. Filling every core takes about 13.3 cubic feet or 0.49 cubic yards of grout. That's about 23 bags of 80-lb concrete mix. For walls this size, ordering grout from a batch plant and pumping is usually faster and cheaper than mixing bags.
| Wall Size | Blocks | Fill Volume (all cores) | 80-lb Bags |
|---|---|---|---|
| 10×4 ft (40 sqft) | 45 | 3.3 cf / 0.12 cy | 6 |
| 20×4 ft (80 sqft) | 90 | 6.7 cf / 0.25 cy | 12 |
| 20×8 ft (160 sqft) | 180 | 13.3 cf / 0.49 cy | 23 |
| 30×8 ft (240 sqft) | 270 | 20.0 cf / 0.74 cy | 34 |
| 40×8 ft (320 sqft) | 360 | 26.6 cf / 0.99 cy | 45 |
Not always. Non-structural garden walls and partition walls often don't need filled cores. But load-bearing walls, foundation walls, retaining walls, and walls in seismic or hurricane zones typically require all cores to be filled per building code.
Fine grout — a flowable concrete mix with pea gravel or smaller aggregate that can flow into the cores. Standard concrete with large aggregate won't fill the cores properly. Pre-mixed bags labeled "core fill" or "fine grout" work for small jobs.
For 8×8×16 blocks with all cores filled, about 1 bag of 80-lb concrete per 8 blocks. A 20×8 foot wall (180 blocks) needs approximately 23 bags.
Not recommended. The aggregate in regular concrete is too large to flow into 1.5-inch wide block cores. Use fine grout or core fill mix specifically designed for masonry.
Typical residential: #4 or #5 rebar in filled cores spaced every 32–48 inches horizontally, plus horizontal bond beam reinforcement every 4 feet of height. Check your local code.
Typical Results
💡 Pump grout if filling more than 4 courses at a time — hand-rodding tall cores can leave voids.
Common Mistakes