Concrete Block Fill Calculator
Work out how much grout you need to fill CMU block cores — solid or at any rebar spacing — using NCMA’s published grout yield table.
Your wall
Deducted from gross wall area. Leave 0 if none.
Net count from your takeoff, before waste.
Match your structural drawings.
On top of the table’s built-in 3%.
You will need
Schematic — dots mark grouted cores, not an exact block count
- Net area
- 160 sf
- Blocks (approx)
- 180
- Grout rate
- 9.1 cf/100sf
15.3 cu ft · incl. 8% total waste
Buy 24 × 80 lb preblended grout
That’s 1,920 lb of bagged material · placed weight ≈ 2,140 lb · 0.57 yd³
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Spacing vs. solid
32 in o.c. vs solid
Weight & density
2,140 lb
Placed weight at 140 lb/cu ft
Bags, bulk, or ready-mix?
Bagged grout fits this job
The short answer
Grout volume depends far more on spacing than on wall size. Per NCMA TEK 4-2A, an 8 inch block wall grouted solid takes about 36 cubic feet per 100 sq ft, while the same wall grouted every 48 inches on center takes only about 6 cubic feet per 100 sq ft — a six-fold difference for the same wall. Always match spacing to your structural drawings, add 3–10% waste, and round up to a whole bag.
Grout volume by width and spacing
Cubic feet of grout per 100 sq ft of wall, from NCMA TEK 4-2A Table 3. Figures already include a 3% waste allowance for two-core hollow units.
| Spacing | 6 in wall | 8 in wall | 10 in wall | 12 in wall |
|---|---|---|---|---|
| Solid (8 in) | 25.6 | 36.1 | 47.0 | 58.9 |
| 16 in o.c. | 12.8 | 18.1 | 23.5 | 29.5 |
| 24 in o.c. | 8.6 | 12.1 | 15.7 | 19.7 |
| 32 in o.c. | 6.4 | 9.1 | 11.8 | 14.8 |
| 48 in o.c. | 4.3 | 6.1 | 7.9 | 9.9 |
| 96 in o.c. | 2.2 | 3.1 | 4.0 | 5.0 |
Fill product yields
| Product | Yield | Best for |
|---|---|---|
| 80 lb preblended grout | 0.66 cu ft | Standard reinforced core fill — matches spec |
| 3,000 lb bulk bag | 25 cu ft | Jobs over roughly a cubic yard, crane or forklift on site |
| 80 lb concrete mix | 0.60 cu ft | A handful of cores on small, unreinforced work |
| 60 lb concrete mix | 0.45 cu ft | Same as above, lighter bag to lift |
General bagged concrete mix is stiffer than masonry grout and doesn’t flow into narrow cores as well — treat it as a small-job substitute, not a like-for-like replacement on reinforced walls.
How to calculate block fill grout
- Find net wall area. Length × height, minus any door and window openings.
- Pick block width and spacing. Match the structural drawings — this is the single biggest factor in the result.
- Look up the rate. Cubic feet of grout per 100 sq ft, from a published table like NCMA TEK 4-2A.
- Scale and add waste. (Area ÷ 100) × rate, then add your extra waste allowance.
- Convert to bags, and round up — grout waits for no one once mixed.
Worked example
A 20 ft long, 8 ft tall wall of 8 inch CMU, grouted every 32 inches on center:
- Net area20 × 8 = 160 sq ft
- Rate (8 in, 32 in o.c.)9.1 cu ft / 100 sf
- Raw volume1.60 × 9.1 = 14.6 cu ft
- Plus 5% extra waste15.3 cu ft
- Divide by 0.66 cu ft/bag15.3 ÷ 0.66 = 23.2
- Round up24 × 80 lb grout bags
Grouted solid instead, the same wall would need about 92 bags — nearly four times as much material for the same footprint.
Solid, partial, or none?
| Pattern | Typical use | Relative grout use |
|---|---|---|
| No grout | Non-loadbearing, low interior partitions | None |
| 32–48 in o.c. | Common residential and light commercial reinforced walls | Low |
| 16–24 in o.c. | Higher seismic or wind loads, taller walls | Moderate |
| Solid | Retaining walls, foundations, high load or code-required solid grouting | Highest |
Never choose a pattern on your own for a structural wall — spacing and bar size come from the engineer’s drawings, not from a materials calculator.
Common mistakes
- Assuming grout scales with wall size aloneSpacing changes volume by 6× or more for the same wall. Always confirm the spacing before ordering.
- Using a generic “per block” figureNumbers like this vary wildly online because they mix up per-cell, per-block, and per-100-sf figures. A published table by width and spacing is far more reliable.
- Substituting stiff concrete mix on reinforced workMasonry grout is intentionally high-slump so it flows around rebar without voids. Standard bagged concrete mix isn’t a safe substitute on structural walls.
- Forgetting bond beams and lintelsTable 3 covers vertical cell grouting only. Horizontal bond beam courses need their own linear-foot estimate.
- Skipping cleanoutDebris and mortar droppings blocking a cell stop grout from reaching the bottom, leaving a void around the rebar.
- Ordering the bare minimumRound up, then keep a spare bag or two. A mid-pour shortage means a cold joint in a structural cell.
What this calculator assumes
Adjust your result if your project differs from these.
- Two-core hollow CMU, standard core geometry, per NCMA TEK 4-2A Table 3
- Grout volume rates already include a 3% waste allowance; the waste field adds more on top
- Block count approximated at 1.125 blocks per sq ft (nominal 8×16 in face, no waste added to the count itself)
- Bag yields of 0.66 cu ft (80 lb preblended grout), 25 cu ft (3,000 lb bulk), 0.60 and 0.45 cu ft (80/60 lb concrete mix)
- Placed weight uses a grout density of 140 lb/cu ft and excludes CMU self-weight
- Vertical cell grouting only — bond beams and lintel courses are not included
- Prices are whatever you enter; they are not live market rates
Common questions
How much grout do I need to fill concrete block cores?
It depends heavily on spacing. An 8 in block wall grouted solid needs about 36 cu ft per 100 sq ft, while the same wall grouted every 48 in on center needs only about 6 cu ft per 100 sq ft.
Do I need to fill every block core?
No. Most reinforced walls only grout the cells with vertical rebar, at a spacing from the structural drawings — commonly 16, 24, 32 or 48 in o.c. Solid grouting is reserved for high load, retaining, or high seismic/wind applications.
How many 80 lb bags fill 100 blocks?
For a fully grouted 8 in wall, about 100 blocks (roughly 89 sq ft) need close to 32 cu ft of grout — around 49 bags of preblended 80 lb grout at 0.66 cu ft per bag.
What’s the difference between grout and regular concrete mix for block fill?
Masonry grout is a high-slump, flowable mix designed to fully encase rebar in narrow cores. General bagged concrete mix is stiffer and coarser, and is only a reasonable substitute for a handful of cores on small, unreinforced jobs.
Should I use bagged grout, a bulk bag, or ready-mix?
Bagged grout works well under roughly a cubic yard. A 3,000 lb bulk bag or a ready-mix masonry grout delivery is usually more economical above that.
Where do these numbers come from?
The grout rate table is from NCMA TEK 4-2A, “Estimating Concrete Masonry Materials,” Table 3 — the standard reference used across the concrete masonry industry.
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Wall
| Net area | — |
| Approx. block count | — |
| Block width | — |
| Grout spacing | — |
| Waste allowance | — |
Volume
| Cubic feet (incl. waste) | — |
| Cubic yards | — |
| Placed weight | — |
Shopping list
| 80 lb grout bags | — |
| 3,000 lb bulk bags | — |
| 80 lb concrete mix | — |
| Estimated cost | — |
Notes
Estimate generated by materialcalculate.com. Grout rates from NCMA TEK 4-2A, Table 3 (two-core hollow units, 3% waste included). Confirm spacing and bar size against structural drawings before ordering.