Concrete Block Fill / CMU Core Fill Calculator
Enter the number of CMU blocks, block size, and grouting percentage to calculate fill volume and how many bags of concrete or grout you need for core filling.
Estimate only — not professional advice. Always verify results independently before purchasing materials or beginning work. Terms of Use
What is Concrete Block Fill / CMU Core Fill?
A concrete block fill calculator estimates the volume of concrete or grout needed to fill the hollow cores of concrete masonry units (CMUs). Core filling is required for structural reinforcement, meeting building codes in seismic zones, and improving wall strength, fire resistance, and sound insulation. This calculator supports both 8-inch and 12-inch standard two-core blocks with adjustable fill percentages for partially or fully grouted walls. Standard CMU blocks have two hollow cores separated by a center web. An 8-inch standard block (nominally 8x8x16 inches, actual 7-5/8 x 7-5/8 x 15-5/8 inches) has two cores with a combined void volume of approximately 0.26 cubic feet per block, or about 0.13 cubic feet per individual core. A 12-inch standard block (nominally 12x8x16 inches) has larger cores with a combined void volume of approximately 0.40 cubic feet per block, or about 0.20 cubic feet per core. Fully grouted walls have every core filled with concrete or grout. This is required in Seismic Design Categories D, E, and F per the International Building Code. Partial grouting fills only the cores containing rebar — typically every 48 inches (every third cell) for vertical reinforcement. Common partial fill percentages include 33 percent (every third cell, standard rebar spacing), 50 percent (every other cell), and 25 percent (every fourth cell for light reinforcement). The choice between concrete fill and grout depends on core size and application. Grout is a fluid cementitious mixture designed to flow around rebar and fill voids completely. It uses smaller aggregate than standard concrete and has a higher water-to-cement ratio for workability. An 80-pound bag of premixed grout yields approximately 0.5 cubic feet. Standard concrete mix (80-pound bag yielding approximately 0.6 cubic feet) can be used for larger cores but may not flow as well around rebar in 8-inch blocks. For walls with rebar, grout is strongly preferred because it consolidates around the reinforcing steel without voids. Voids around rebar compromise the structural bond and can trap moisture, leading to corrosion. For non-reinforced fills (thermal mass or sound insulation), either concrete or grout will work, and concrete is typically more economical. Always add 10 to 15 percent to the calculated volume to account for waste, spillage, and mortar joint absorption. CMU walls absorb some moisture from fresh grout through the block face, which can reduce the effective fill volume.
How to Calculate
- Count the total number of CMU blocks in your wall
- Select the block type: 8-inch standard or 12-inch standard
- Enter the percentage of cores to fill (100% for fully grouted, 33% for typical rebar-only fills)
- Review the total cores, cores to fill, and fill volume in cubic feet
- Note both the concrete bag and grout bag estimates to compare options
- Add 10-15% to the bag count for waste and absorption
Formula
Total Cores = Number of Blocks x 2 Cores to Fill = ceiling(Total Cores x Fill Percentage / 100) Fill Volume (cu ft) = Cores to Fill x Volume per Core Volume per Core: - 8" block: 0.13 cu ft per core (0.26 cu ft per block / 2 cores) - 12" block: 0.20 cu ft per core (0.40 cu ft per block / 2 cores) Concrete Bags (80 lb) = ceiling(Fill Volume / 0.6 cu ft per bag) Grout Bags (80 lb) = ceiling(Fill Volume / 0.5 cu ft per bag)
Example Calculation
100 blocks of 8-inch standard CMU, 100% core fill: Total Cores = 100 x 2 = 200 cores Cores to Fill = ceiling(200 x 100 / 100) = 200 cores Fill Volume = 200 x 0.13 = 26.00 cu ft Concrete Bags (80 lb) = ceiling(26.00 / 0.6) = ceiling(43.33) = 44 bags Grout Bags (80 lb) = ceiling(26.00 / 0.5) = ceiling(52.00) = 52 bags
Frequently Asked Questions
What is the difference between grout and concrete for block fill?
Grout is specifically formulated for masonry core fill with smaller aggregate (3/8-inch maximum) and higher water content for better flow. It fills around rebar completely without vibration. Standard concrete has larger aggregate (up to 3/4-inch) and is stiffer, making it harder to fill narrow cores. For reinforced walls, always use grout. For non-reinforced thermal fill, concrete mix is acceptable and more economical.
Do I need to fill every core in a block wall?
Not always. Building codes in low-seismic areas (SDC A and B) allow partially grouted walls where only the cells containing rebar are filled. In high-seismic areas (SDC D, E, F), the IBC requires fully grouted walls with every core filled. Check your local building code for specific requirements based on your seismic design category.
How many blocks can I fill with one bag of grout?
One 80-pound bag of grout yields about 0.5 cubic feet. For 8-inch blocks with 0.13 cu ft per core, one bag fills about 3.8 cores or roughly 2 full blocks (both cores). For 12-inch blocks with 0.20 cu ft per core, one bag fills 2.5 cores or about 1.25 blocks. These are theoretical yields; actual coverage is 10-15% less due to absorption and waste.
Should I use fine grout or coarse grout?
Fine grout (no aggregate larger than 3/8-inch) is required when the minimum clear dimension of the grout space is less than 2 inches, which applies to most 8-inch CMU blocks. Coarse grout (aggregate up to 3/4-inch) can be used when the grout space is at least 3 inches in its smallest dimension, common in 12-inch blocks. Fine grout flows better in tight spaces.
Do I need to vibrate or consolidate the grout?
Yes. Per TMS 602 (Masonry Standards Joint Committee), grout must be consolidated by mechanical vibration or puddling. Pour grout in lifts of no more than 5 feet 4 inches (one story) and reconsolidate by vibrating within 3 minutes of placement. Low-lift grouting (under 5 feet) can be puddled with a rod. High-lift grouting (over 5 feet) requires mechanical vibration.
How do I estimate fill for partially grouted walls with rebar?
For standard vertical rebar every 48 inches (every third cell in 16-inch modular blocks), approximately 33% of cores are filled. For rebar every 32 inches (every other cell), use 50%. Enter these percentages in the calculator. Always fill the cells at corners, jambs (door and window sides), and at the top course bond beam regardless of the general fill percentage.