What This Calculator Figures Out
Hollow CMU walls get their strength from the grout poured down the cores, and you need to know how much before the crew shows up. Give this tool a block count, or a wall length and height, and it returns the fill in cubic feet, cubic yards, and 80-lb bags. A standard 8x8x16 block holds about 0.11 to 0.12 cubic feet across both cores, so the total scales directly with how many blocks you have and how many cells you actually fill.
A Worked Example
Say you have a wall of 100 standard 8-inch blocks and you're grouting every core. That's about 11 to 12 cubic feet of grout, which is roughly 20 bags of 80-lb concrete mix at 0.6 cu ft a bag, or only about 0.44 cubic yards. A wall that size is a bag job. Scale up to a full basement of 1,000 blocks and you're near 4.4 cubic yards, well past where you'd stop mixing bags and order a ready-mix grout truck instead.
Full Grout, Partial Grout, or Rebar Cells Only
The single biggest swing in your total is how many cells you fill. Retaining walls and structural walls are often grouted solid, every cell. Many code-minimum walls only need the cells carrying vertical rebar filled, typically every 32 to 48 inches, which can cut your grout by half or more. Set the fill percentage to match the drawings before you buy: 100 percent for solid, 50 percent for every other cell, 25 percent for rebar cells only.
Concrete Block Fill Calculator
Quick Presets
Block Core Fill Volume Reference
Fill needed per block and per 100 blocks by CMU size.
| Block Size | Cu Ft / Block | Cu Yd / Block | Cu Yd / 100 Blocks | 80-lb Bags / 100 Blocks |
|---|---|---|---|---|
| 8" CMU | 0.187 | 0.0069 | 0.69 | 34 |
| 10" CMU | 0.250 | 0.0093 | 0.93 | 45 |
| 12" CMU | 0.312 | 0.0116 | 1.16 | 56 |
How to Use This Calculator
- Pick How You'll Enter the Wall: Switch to Block Count if you already have a takeoff, or use Wall Dimensions to enter length and height in feet and let the tool figure the block count at about 1.125 blocks per square foot.
- Choose Your Block Size: Select 8, 10, or 12-inch CMU. Bigger blocks have bigger cores, so a 12-inch unit takes almost twice the grout of an 8-inch one.
- Set the Fill Percentage: Pick 100% for solid grouting, 50% for every other cell, or 25% for rebar cells only. Match this to the engineer's drawings or your local code.
- Calculate and Order: Read the cubic yards and bag count, add a little for waste and spillage, and decide whether to mix 80-lb bags or call for ready-mix.
How It Works
A CMU block is hollow: a standard 8x8x16 unit has two open cells that together hold about 0.11 to 0.12 cubic feet of grout. This calculator counts your blocks (or works them out from wall length and height) and multiplies by the fill volume for your block size.
The basic rule:
- A standard 8-inch CMU holds roughly 0.11 to 0.12 cu ft of fill across both cores; 10-inch blocks hold about 0.16 cu ft and 12-inch blocks about 0.20 cu ft each
- From wall dimensions, figure about 1.125 blocks per square foot of wall face, which already accounts for the 3/8-inch mortar joints
- Set the fill percentage to match your plan: 100% grouts every cell, 50% does every other cell, and 25% covers only the cells that carry vertical rebar
Order grout, not standard concrete, so the mix flows down the narrow cells and packs around the steel. On any pour taller than about 4 feet, grout in lifts and consolidate each lift with a rod or vibrator so you don't leave voids.
Tips & Considerations
- One 80-lb bag of grout mix yields about 0.6 cu ft, so figure roughly one bag for every 5 standard 8-inch blocks you fill solid.
- Grout in lifts on tall walls and rod or vibrate each lift so it packs around the rebar without leaving voids.
- Buy about 5 to 10 percent extra: cell volumes vary by block brand, and some grout is always lost to spillage and overfill.
- Confirm which cells actually need filling from the drawings before ordering, since going from rebar-only to solid can double your grout.
- Once you're past about a cubic yard, price a ready-mix grout delivery against the bag count before you haul another pallet.
Frequently Asked Questions
How much concrete does it take to fill one 8-inch block?
About 0.11 to 0.12 cubic feet per standard 8x8x16 block, counting both cores. That works out to roughly one 80-lb bag of grout mix for every 5 blocks, since an 80-lb bag yields about 0.6 cu ft. A 10-inch block takes about 0.16 cu ft and a 12-inch block about 0.20 cu ft.
Do I have to fill every core in the wall?
Not always. Load-bearing and retaining walls are usually fully grouted, but many walls only need the cells that hold vertical rebar filled, typically every 32 to 48 inches. Your local code and the engineer's drawings set the spacing, so match the fill percentage on this calculator to what those specs call for.
Should I use grout or regular concrete?
Use grout. It carries a smaller pea-gravel aggregate and more water so it flows into the cells and around rebar instead of bridging and leaving gaps. Standard concrete is too stiff for narrow cores. Fine grout suits cells under about 4 inches; coarse grout works for wider cells and bond beams.
How much grout does a 100-block wall need?
One hundred standard 8-inch blocks fully grouted take about 11 to 12 cubic feet of fill. At 0.6 cu ft per 80-lb bag that's roughly 20 bags, and it's only about 0.44 cubic yards, so a single wall usually stays a bag job rather than a ready-mix delivery.
When should I mix bags versus order ready-mix?
Mixing 80-lb bags is fine up to about half a cubic yard, or one modest wall. Past roughly a cubic yard the bag count climbs fast, so a full basement or a long retaining wall is where you call for a ready-mix grout truck. Ready-mix also gives you a consistent slump for pumping into tall cells.
How does rebar change the fill amount?
Steel bars displace only a sliver of the cell, so #4 or #5 rebar barely moves the grout total and most estimators ignore it. What rebar really changes is which cells you fill: any cell with a bar must be solidly grouted, and that determines whether you're at 25%, 50%, or 100% fill on this calculator.