Grout Calculator (Masonry)
Estimate how much grout you need to fill the cores of a CMU block wall — enter your wall dimensions, block width, and grouting pattern to get cubic yards, bags, and cost.
Share of the block’s cross-section that’s hollow core. Standard two-core CMU is commonly around 40–50% — check your block supplier’s spec sheet for an exact figure.
100% for a fully grouted wall. Lower this if only cells with vertical rebar are grouted (e.g. roughly 50% for cells at 32″ o.c. on 16″-module block).
Grout pours typically run more waste than flatwork — 10% is a common starting point.
Volume = wall area × block width × core void ratio × percent of cells grouted, plus your waste allowance. Bag yields are approximate (0.6 ft³ per 80 lb bag, 0.35 ft³ per 50 lb bag) — confirm against your grout manufacturer’s stated yield before ordering. Larger pours are almost always ordered as pumped ready-mix grout rather than bagged.
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Grout Calculator (Masonry)
Short answer: grout volume = wall area × nominal block width × core void ratio × percent of cells grouted, plus a waste allowance. A typical 40 ft × 8 ft wall of 8-inch CMU, fully grouted at a 45% void ratio, needs roughly 3.7 cubic yards of grout after 10% waste.
Formula
The core void ratio and the percent of cells grouted are the two variables most specific to your project — a fully grouted wall uses 100% cells grouted, while a wall grouted only at reinforcing bar locations uses a much lower figure. Both should come from your block manufacturer’s data and structural drawings rather than a universal constant, since core geometry varies between manufacturers.
Worked example
- A wall is 40 ft long and 8 ft tall, built with 8-inch nominal CMU: wall area = 40 × 8 = 320 ft².
- Block width in feet: 8 ÷ 12 = 0.667 ft.
- Assume a 45% core void ratio and a fully grouted wall (100% cells grouted): volume = 320 × 0.667 × 0.45 × 1.0 = 96.0 ft³.
- Convert to cubic yards: 96.0 ÷ 27 = 3.56 yd³.
- Add 10% waste: 3.56 × 1.10 = 3.91 yd³, which rounds up to about 4 cubic yards to order.
Result: order roughly 4 cubic yards of pumped grout, or budget around 173 bags of 80 lb bagged grout mix if hand-mixing — which is why walls at this scale are typically pumped rather than hand-poured.
Assumptions this calculator makes
- Grout weighs approximately 150 lb per cubic foot, similar to standard-weight concrete.
- 80 lb bags yield about 0.6 ft³ each; 50 lb bags yield about 0.35 ft³ each — confirm against your product’s stated yield.
- The wall is a uniform rectangle of a single nominal block width — openings for doors and windows are not automatically subtracted.
- The core void ratio you enter accurately reflects your specific block; this figure is not standardized across manufacturers.
Common mistakes
- Using a generic void ratio without checking the block. Core geometry varies by manufacturer and block series — a figure that’s off by even 10 percentage points meaningfully changes the volume.
- Not subtracting openings. Door and window openings reduce both the wall area and the grout needed — subtract their area from your wall length × height before entering it here.
- Assuming every cell gets grouted. Many reinforced walls only grout cells with vertical steel — grouting every cell when the drawings only call for partial grouting wastes material and money.
- Underestimating waste. Grout pumping and consolidation typically lose more material than flatwork concrete placement — budgeting the same low waste percentage as a slab pour often comes up short.
FAQ
What is masonry grout, and how is it different from mortar?
Masonry grout is a flowable, high-slump mix poured into the hollow cores of concrete block or the cavity of a reinforced wall to bond rebar to the masonry and add structural strength. Mortar, by contrast, is a stiffer mix used to bed and join individual blocks or bricks as they’re laid, and the two are not interchangeable.
What is a core void ratio and where do I find it for my block?
The void ratio is the percentage of a block’s cross-sectional area that is hollow core, available to be filled with grout. It varies by manufacturer and block design, so it’s best confirmed from the block supplier’s spec sheet. This calculator defaults to 45 percent, a commonly cited midpoint for standard two-core CMU, but you should adjust it if your supplier lists a different figure.
Do I need to grout every cell in a wall?
Not always. Many reinforced masonry walls only grout the cells that contain vertical reinforcing steel, commonly spaced at 16, 24, 32, or 48 inches on center, rather than every cell. Fully grouted walls, where every cell is filled, are used where higher strength or fire ratings are required. Check your structural drawings for the specified grouting pattern.
How much extra grout should I order for waste?
Grout pours typically use more allowance than flatwork concrete because of pumping loss, minor block imperfections, and spillage during consolidation. Ten percent is a common starting allowance, though tight or difficult pours may warrant more.
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