How CMU block quantities are calculated

This calculator uses the same math commercial masonry estimators use to take off block from plan drawings. Most online CMU calculators stop at block count. This one handles mortar, grout fill, bond beams, openings, weight, and total cost – because that’s what an actual SOV submission to a GC needs.

The math is straightforward once you remember everything that has to be included. Forgetting mortar bags or grout volume is the single most common error in CMU bids.

Worked example: 50 ft long x 10 ft high interior CMU partition wall

Take a standard 50 ft long by 10 ft high interior partition wall using 8″ CMU with Type S mortar at 3/8″ joints, no grout fill, 8% waste factor.

Step 1: Calculate the wall area.

• Gross area: 50 ft x 10 ft = 500 SF
• Net area: 500 SF (no openings in this example)

Step 2: Calculate block count. Standard 8x8x16 CMU covers roughly 0.89 SF face area per block (when you account for the mortar joint). So blocks per SF is approximately:

• Blocks per SF: 1.125 blocks/SF
• Block count before waste: 500 SF x 1.125 = 563 blocks
• With 8% waste: 563 x 1.08 = 608 blocks

Step 3: Calculate mortar. Mortar usage depends on block size and joint thickness. For 8″ CMU with 3/8″ joints, typical mortar coverage is about 8.5 CF per 100 SF of wall. A 70 lb bag of Type S mortar produces roughly 1 CF of mortar.

• Mortar volume: 500 SF x 0.085 CF/SF = 42.5 CF
• Mortar bags (70 lb each): 42.5 CF ÷ 1 CF/bag = 43 bags

Step 4: Calculate grout (none in this example). Interior partition walls aren’t typically grouted. For reinforced walls, grout fills cores at the specified spacing – typically every 32″, 48″, or solid grouted.

Step 5: Total weight and cost.

• Wall weight: Standard 8″ CMU weighs ~35 lbs each. 608 blocks x 35 lbs = 21,280 lbs (10.6 tons)
• Material cost at $2.50/block + $9/bag mortar: 608 x $2.50 + 43 x $9 = $1,520 + $387 = $1,907

CMU block weight and yield reference

U.S. concrete masonry units are described by nominal dimensions. A “standard 8 inch CMU” is actually 7-5/8″ wide, 7-5/8″ tall, and 15-5/8″ long. With a 3/8″ mortar joint on top and one side, the finished block face area is exactly 8″ x 16″ = 128 square inches, or 0.89 SF.

All standard CMU sizes have the same face dimensions (7-5/8″ x 15-5/8″), so the blocks-per-SF count is the same across all widths: 1.125 blocks per SF of wall. The width only affects weight, mortar volume, and grout fill volume.

Mortar yield by block size

Mortar consumption depends on the block face perimeter exposed to mortar – meaning the bed joints (horizontal) and head joints (vertical). Wider blocks use slightly more mortar per SF because the bed joint is deeper. A standard Type S mortar bag (70 lbs) produces approximately 1 cubic foot of usable mortar.

These figures assume 3/8″ mortar joints. Wider joints (1/2″ sometimes specified) consume 25-30% more mortar.

Grout fill: when and how much

Grout fill is the concrete-like material poured into CMU cores after the wall is laid up. Grout is what makes CMU walls structural – without it, the wall is just stacked masonry held together by mortar joints. Three common patterns:

• No grout: Interior partition walls, garden walls, non-loadbearing applications. Block + mortar only.
• Partial grout (every 32″ or 48″): Most common for reinforced commercial walls. Grout is placed in cores containing vertical rebar, at the specified spacing. Cores without rebar stay hollow.
• Solid grout (every core): Heavy structural walls, retaining walls, areas needing maximum strength or fire rating. Every core is filled.

Grout volume depends on block size and fill pattern. Approximate grout fill volumes per 100 SF of wall:

Grout is most often delivered by ready-mix truck for larger pours (over 5 CY), or mixed on-site from prebagged grout for smaller jobs. The calculator output gives you the CY needed – confirm with your supplier whether to order ready-mix or prebagged.

What this calculator handles that others don’t

• Multiple block sizes: 4″, 6″, 8″, 10″, 12″ – each with correct weight and mortar yield
• Standard, lightweight, and solid blocks: Major weight differences that matter for crew planning and labor costing
• Opening deducts: Subtract door and window areas from gross wall area
• Configurable waste factor: Default 8%, adjustable for crew experience and cut complexity
• Mortar by type and joint thickness: Type M, S, N, O with 3/8″ or wider joints
• Grout fill patterns: None, every 32″/48″/24″, or solid – calculated automatically from block size and pattern
• Bond beam blocks: Counted separately based on bond beam spacing
• Wall weight calculation: Includes block weight and grout fill weight for engineering and lifting planning
• Cost estimate: Block + mortar + grout + optional mason labor

What this calculator does NOT handle (use plans)

This is a quantity estimator, not a structural design tool. It assumes you already know the wall specification from the structural drawings. If you don’t, you need an engineer, not a calculator.

• Structural design (selecting block size, grout pattern, rebar for the load)
• Vertical rebar quantities (use our Rebar Calculator for that)
• Lintel quantities for openings
• Special accessories: anchors, wall ties, control joints, weep vents, flashing
• Brick veneer over CMU (separate scope)
• Decorative or split-face block (different waste factors)
• Reinforced columns and pilasters (separate calculations)
• Scaffolding, equipment, and mobilization costs

CMU calculation FAQ