Concrete Block Fill Calculator
Calculate concrete or grout needed to fill CMU block cores
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Quick Answer
A standard 8x8x16 CMU block requires about 0.033 cubic feet of fill material per block (both cores). For 100 fully-grouted blocks, you need approximately 4-5 bags of 80lb grout mix (including 10% waste). Block fill grout is recommended over regular concrete for better flow.
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Fill Volume by Block Size
| Block Size | Cu Ft / Block | Cu Ft / 100 Blocks |
|---|---|---|
| 8" x 8" x 16" (Standard) | 0.033 | 3.3 |
| 8" x 12" x 16" | 0.044 | 4.4 |
| 6" x 8" x 16" | 0.022 | 2.2 |
| 10" x 8" x 16" | 0.044 | 4.4 |
| 12" x 8" x 16" | 0.055 | 5.5 |
When to Fill Block Cores
Required Filling
- • Cores containing vertical rebar
- • At wall corners and intersections
- • Around door and window openings
- • At bond beam locations
- • Where required by structural engineer
- • In seismic zones (often all cores)
Optional Filling
- • Non-structural garden walls
- • Interior partition walls
- • Above-grade decorative walls
- • When extra thermal mass desired
- • For sound dampening
- • Fire resistance improvements
Fill Material Options
Block Fill Grout
Recommended
Specially formulated to flow into narrow cores.
- • Self-consolidating
- • Fills voids completely
- • Bonds well to rebar
- • ~2500 PSI strength
Concrete Mix
Alternative
Standard concrete can work but may have voids.
- • Use small aggregate (3/8")
- • Mix wetter than normal
- • Vibrate or rod thoroughly
- • ~3000+ PSI strength
Sand Fill
Non-Structural Only
For weight/stability without structural needs.
- • Cheapest option
- • No structural strength
- • Good for thermal mass
- • Use dry sand only
How to Fill Block Cores
- 1
Clean Out Debris
Remove any mortar droppings or debris from cores before filling.
- 2
Dampen Cores
Lightly mist block cores with water to prevent rapid moisture absorption.
- 3
Pour in Lifts
Fill no more than 4 feet (5 courses) at a time to prevent blowouts.
- 4
Consolidate
Use a vibrator or rod/puddle stick to eliminate air pockets.
- 5
Wait Before Next Lift
Allow grout to settle before adding the next lift (usually same day is fine).
Frequently Asked Questions
How much concrete does it take to fill a block?
A standard 8x8x16 CMU block requires approximately 0.033 cubic feet (about 1 quart) of fill material per block to fill both cores. For 100 blocks filled completely, you need about 3.3 cubic feet or roughly 0.12 cubic yards.
Should I use concrete or grout to fill blocks?
Block fill grout is recommended because it is specifically designed to flow into narrow cores without voids. Regular concrete can be used but may not fill all spaces completely. Grout provides better consolidation around rebar.
Do I need to fill all block cores?
Not always. Building codes typically require filling cores containing rebar and at corners, around openings, and at specified intervals (often every 4 feet). Fully grouted walls provide maximum strength but use more material.
Can I fill blocks with regular concrete?
Yes, but use a concrete mix with small (3/8") aggregate and mix it slightly wetter than normal so it flows into cores. Vibrate or rod thoroughly to eliminate voids. Block fill grout is still preferred for easier placement.
How many courses can I fill at once?
Fill no more than 4-5 courses (about 4 feet) at a time. Filling too much at once can cause excessive pressure that may blow out mortar joints. Allow each lift to settle before adding more.
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Dr. Snezana Lawrence
Mathematical Historian | PhD from Yale
Dr. Lawrence is a published mathematical historian with a PhD from Yale University. She ensures mathematical precision and accuracy in all our calculations, conversions, and academic score calculators. Her expertise spans computational mathematics and educational assessment.
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