CMU Core-Fill Grout & Rebar Calculator
Filling hollow concrete block cores with pea-gravel grout and steel reinforcement transforms a basic dry-joint wall into a monolithic structural fortress. This guide explains how to calculate grout volume (cubic yards and 50 lb bags) and linear feet of vertical rebar for 6", 8", 10", and 12" CMUs.
■ Built-in Core-Fill Takeoff Engine
Our main calculator calculates blocks, joint mortar, ready-mix grout cubic yardage, and #4 rebar in real time. Choose 48" O.C., 32" O.C., or Solid Grouted:
1. Core-Fill Grout Volume Reference Matrix
The table below provides exact ready-mix cubic yards and 50 lb pre-mix bags required per 100 installed blocks for different wall thicknesses and reinforcement spacings:
| Block Size (W × H × L) | Grouting Pattern | Grout per 100 Blocks | 50 lb Pre-mix Bags | Rebar Spacing |
|---|---|---|---|---|
| 6 × 8 × 16 in | Solid Grouted (100%) | 0.65 yd³ | 39 bags | Every core |
| 6 × 8 × 16 in | 32" On-Center | 0.33 yd³ | 20 bags | Every 4th core |
| 8 × 8 × 16 in (Standard) | Solid Grouted (100%) | 0.96 yd³ | 58 bags | Every core |
| 8 × 8 × 16 in (Standard) | 32" On-Center (Wind/Earth) | 0.48 yd³ | 29 bags | Every 4th core |
| 8 × 8 × 16 in (Standard) | 48" On-Center (Standard) | 0.32 yd³ | 19 bags | Every 6th core |
| 12 × 8 × 16 in (Foundation) | Solid Grouted (100%) | 1.55 yd³ | 93 bags | Every core |
2. ASTM C476 Grout Specifications
Grout must comply with ASTM C476. Unlike standard 3,000 psi concrete used for footings, masonry grout has distinct fluid and aggregate characteristics:
- High Slump (8 to 11 inches): Because dry concrete masonry units absorb moisture rapidly from the wet grout, the mix must be placed at a high fluid slump so it flows around rebar without honeycombing.
- Fine vs Coarse Grout: Fine grout uses ASTM C144 masonry sand and is specified for narrow cores (< 2 inches). Coarse grout contains 3/8-inch pea gravel aggregate (ASTM C404) and is standard for 8-inch and 12-inch CMU blocks.
3. Low-Lift vs High-Lift Grouting
Contractors employ two distinct placement methods depending on project scale:
- Low-Lift Grouting (Up to 5 feet): The mason builds the wall up to 5 feet high, drops vertical rebar into the designated cells, and pours grout manually via bucket or small grout pump before continuing to the next course. Cleanout openings are not required.
- High-Lift Grouting (Over 5 feet up to 24 feet): The wall is constructed to full story height. Cleanout holes (minimum 3×3 inches) are cut out of the bottom course at every grouted cell so debris and mortar droppings can be cleaned before pumping grout. A concrete pump truck with a 2-inch reducer hose is used with mechanical consolidation vibrators.
Explore Connected Masonry Calculation Guides
- Retaining Wall Block Calculator — Buried courses, base trenches, and drainage backfill.
- Mortar Calculator for Concrete Blocks — Sizing bags of Type S/M mortar and sand ratios.
- Concrete Block Weight Guide — Empty vs grouted block weights.
- CMU Block Sizes & Dimensions — Actual vs nominal sizing reference.