Concrete Grout Volume per Block & per 100 Blocks Reference Table
CMU blocks feature tapered internal webs and face shells that narrow slightly from top to bottom. The table below outlines true internal volume per block and the total cubic yards required per 100 blocks when fully grouted solid (including a 10% jobsite waste buffer):
| Block Nominal Size | Actual Core Volume / Block | Grout per 100 Blocks (Solid) | 80 lb Pre-Mix Bags / 100 Blocks | Blocks Filled per 1 Cu Yd |
|---|---|---|---|---|
| 6" × 8" × 16" CMU | 0.14 cu ft (0.0052 yd³) | 0.57 cu yds | ~26 bags | ~190 blocks |
| 8" × 8" × 16" CMU (Standard) | 0.22 cu ft (0.0081 yd³) | 0.89 cu yds | ~40 bags | ~122 blocks |
| 10" × 8" × 16" CMU | 0.29 cu ft (0.0107 yd³) | 1.18 cu yds | ~53 bags | ~93 blocks |
| 12" × 8" × 16" CMU | 0.36 cu ft (0.0133 yd³) | 1.46 cu yds | ~66 bags | ~75 blocks |
Critical: Why Masonry Grout Differs from Standard Concrete
A common homeowner and DIY mistake is dumping standard slab concrete (3,000–4,000 PSI ready-mix) into block cores. This frequently leads to structural wall failure due to three physical mechanisms:
- Extreme Slump (8" to 11"): Standard slab concrete has a stiff slump of 3" to 5". Masonry units are porous and absorb water rapidly upon contact. High-slump grout retains sufficient fluid mobility to completely encase vertical reinforcing bars.
- Pea Gravel Aggregates (3/8" Max): Standard concrete contains 3/4" to 1" crushed gravel that instantly lodges between rebar and block internal webs, creating severe structural voids. ASTM C476 coarse grout limits aggregate to 3/8-inch pea gravel.
- No Air Entrainment: Masonry grout should not contain air-entraining admixtures because the absorbing action of the surrounding dry masonry units naturally de-waters and densifies the core grout matrix.
Logistics: Ready-Mix Truck Delivery vs. Pre-Mix Bags
When to Order a Ready-Mix Truck
If your project requires more than 2.0 to 2.5 cubic yards (~85 to 110 bags), mixing by hand is impractical and risks cold joints. Order a commercial ready-mix truck with a masonry grout mix and coordinate a trailer-mounted grout pump to fill cores continuously.
When to Use Pre-Mix Bags
For garden walls, small retaining runs under 200 blocks, or projects requiring less than 1.5 cubic yards, pre-mixed 80 lb core-fill bags mixed on site in a portable drum mixer eliminate short-load delivery surcharges ($120–$200) from ready-mix plants.
Formulas, modular dimensions, and material yields verified by the CinderCalc Technical Team
Frequently Asked Questions About Block Core Filling
Verified answers on core volumes, pea gravel aggregates, and consolidation vibrators.
