CMU Block Fill Calculator
Filling the hollow cores of concrete masonry units (CMUs) with grout or concrete significantly increases wall strength, fire resistance, and in many cases is required by the structural design or local building code. This calculator estimates the volume of fill concrete or masonry grout needed to fully fill the cores of standard CMU blocks. Two block sizes are supported: the standard 8x8x16 and the 12x8x16. The core fill volume per block is based on typical hollow-core dimensions. For 8x8x16 blocks, the two cores have a combined fill volume of approximately 0.026 cubic feet per block. For 12x8x16 blocks, the larger core volume is approximately 0.040 cubic feet per block. Results are given in cubic yards (for ready-mix or bulk ordering) and as equivalent 80-lb bag count (at 0.60 cu ft per 80-lb bag). For partial grouting (only cores with rebar), reduce the bag count proportionally. Always consult your structural engineer for the required grouting pattern before ordering materials.
Block fill formula
Fill volume (cu ft) = Blocks x Volume per block x (1 + Waste %)
Fill volume (cu yd) = Fill volume (cu ft) / 27
80-lb bags = Ceiling(Fill volume (cu ft) / 0.60)
Worked example (8x8x16 blocks)
200 blocks, 0.026 cu ft per block, 10% waste.
- Fill = 200 x 0.026 = 5.20 cu ft base
- With waste = 5.20 x 1.10 = 5.72 cu ft
- Cubic yards = 5.72 / 27 = 0.21 cu yd
- 80-lb bags = Ceiling(5.72 / 0.60) = 10 bags
Frequently asked questions
Why do you fill CMU block cores with concrete?
Filling the hollow cores of concrete masonry units (CMUs) with grout or concrete increases the wall's structural strength, provides fire resistance, and can improve sound transmission class (STC) ratings. Fully grouted CMU walls are required in high-seismic zones and by many structural engineers for load-bearing applications.
How much concrete does it take to fill a CMU block?
An 8x8x16 standard CMU block has two cores. Each core is approximately 6 inches x 6 inches x 7.5 inches, giving a combined core volume of about 0.026 cubic feet per block. Fill concrete (grout) is placed in cores not all at once but in lifts to prevent blowouts. The exact fill volume depends on reinforcement and whether partial or full grouting is used.
What is the difference between grout and concrete fill for CMU?
Masonry grout (ASTM C476) is specifically designed for filling CMU cores and is more fluid than standard concrete to flow around reinforcing bars. Fine grout uses fine aggregate and is used for small spaces. Coarse grout uses pea gravel and is used for larger cores. Standard concrete can be used in some applications, but grout is preferred for fully grouted walls.
Do I need to fill all cores?
Not necessarily. Many CMU walls are partially grouted, with only cores containing vertical reinforcing bars filled. Fully grouted walls have all cores filled. The structural design specifies which pattern to use. This calculator provides estimates for full core filling; reduce the quantity proportionally for partial grouting.
How many 80-lb bags of concrete does it take to fill CMU cores?
An 80-lb bag of concrete mix yields approximately 0.60 cubic feet. For 8x8x16 blocks, approximately 38 blocks can be filled per 80-lb bag (0.60 / 0.016 cu ft per block core volume). Calculations vary by actual core dimensions; this calculator shows the cubic yard total and bag equivalents.
Sources
- National Concrete Masonry Association (NCMA): TEK 9-4B: Grout for Concrete Masonry.
- ASTM C476: Standard Specification for Grout for Masonry.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. Core volumes are typical estimates. Confirm with your block supplier's product data.