4-Inch CMU Dimensions: 4x8x16 Partition Specs (2026)

Architectural 3D cutaway wall assembly featuring 4-inch CMU masonry partition blocks and veneer backup

Nominal Size

4" x 8" x 16"

Slim partition grid

Actual Size

3⅝" x 7⅝" x 15⅝"

92 x 194 x 397 mm

Fire Rating

1 – 2 Hours

IBC Chapter 7 rated

Avg Unit Weight

22 – 26 lbs

Lightweight partition

Detailed ASTM C90 Cross-Section Specifications for 4-Inch CMU

Standard 2-Core Stretcher

Component / Measurement	Standard Dimension (Imperial)	Metric Equivalent	ASTM C90 Standard Requirement
Unit Thickness (Width)	3-5/8 in	92 mm	± 1/8 in manufacturing variance
Unit Height	7-5/8 in	194 mm	Standard 8-inch modular course height
Unit Length	15-5/8 in	397 mm	Standard 16-inch center-to-center spacing
Face Shell Thickness	3/4 in	19 mm	Min 0.75 in per ASTM C90 Table 1
Internal Web Thickness	3/4 in	19 mm	Min 0.75 in cross web divider
Core Cell Dimensions	1-3/4 in × 5 in	44 × 127 mm	Too narrow for standard rebar centralizing
Net Cross-Sectional Area	~32.5 sq in	210 cm²	~56% solid volume fraction

IBC Chapter 7 Fire-Resistance Ratings for 4-Inch CMU

Equivalent Solid Thickness Method

Aggregate Type (ASTM C90)	Hollow Unfilled Rating	Filled with Perlite/Vermiculite	Solid 4-Inch Concrete Unit	Code Application
Siliceous Gravel Aggregate	1.0 Hour	1.5 Hours	2.0 Hours	Standard 1-hr commercial tenant demising partitions
Calcareous Gravel / Limestone	1.0 Hour	1.5 Hours	2.0 Hours	Institutional firewalls, school storage corridors
Expanded Shale, Slate or Clay	1.5 Hours	2.0 Hours	3.0 Hours	High-performance 2-hr fire separation assemblies
Expanded Slag or Pumice	1.75 Hours	2.0 Hours	3.0 Hours	Stairwell enclosures and elevator shaft fire barriers

* Per IBC Table 722.3.2. Filling hollow cores with loose-fill silicone-treated perlite or vermiculite increases equivalent solid thickness to 100%, boosting fire resistance without adding excessive dead load to floor slabs.

Unbraced Height Limits & Ceiling Deflection Connections (IBC 2109)

Maximum Allowable Partition Heights

Under the International Building Code (IBC Section 2109), interior non-bearing masonry partitions are governed by a maximum height-to-thickness ratio ($h/t \le 30$):

Standard 4-Inch CMU (3-5/8" actual): Maximum allowable unbraced height between floor slab and structural ceiling is 9 feet 1 inch (108 inches).
With Horizontal Pilasters / Bond Beams: Unbraced height can be extended to 12 feet when reinforced horizontally with ladder mesh every 8 inches vertically.
Non-Bearing Rule: Never apply vertical roof or floor framing loads directly to 4-inch CMU partition walls.

Slotted Deflection Track Connection

In commercial concrete and steel frame buildings, overhead floor slabs deflect under live occupant loads. The top of a 4-inch CMU partition must never be mortared tightly to the underside of the upper floor deck:

Deflection Gap: Leave a 1/2" to 3/4" gap between the top block course and the overhead deck.
Slotted Angle Clips: Fasten slotted steel angle clips to the deck, allowing the building structure to deflect vertically while providing lateral out-of-plane restraint to the wall top.
Firestopping Seal: Pack the joint with mineral wool batting and seal with elastomeric fire-rated silicone sealant (UL 2079).

Exterior Cavity Wall Design: 4-Inch CMU Backup Systems

In commercial architecture, 4-inch CMUs frequently serve as the interior structural backup wall behind exterior clay brick or architectural stone veneer assemblies:

Continuous Insulation Plane

A 2-inch continuous layer of rigid polyiso (R-13) or XPS (R-10) is fastened directly to the exterior face of the 4" CMU backup, satisfying ASHRAE 90.1 energy envelope codes.

1-Inch Drainage Air Cavity

A minimum 1-inch clear air space between rigid insulation and exterior brick veneer allows penetrating wind-driven rainwater to drain freely down to base weep screeds.

Adjustable Wire Ties

Hot-dipped galvanized 2-piece wire eye-and-pintle anchors are embedded in the 4" CMU mortar joints every 16" vertical by 24" horizontal to transfer lateral wind loads.

Floor Slab Dead Load Math & Acoustic Sound Transmission (STC)

Dead-Weight Advantage Over 8-Inch Block

A standard hollow 4-inch CMU wall weighs only 22 to 26 lbs per square foot of wall face (compared to 38 to 43 lbs/sq ft for an 8-inch wall). On multi-story commercial buildings with thousands of linear feet of corridor demising walls, selecting 4-inch CMU saves hundreds of tons of structural dead load, preventing excessive floor slab beam sagging and foundation footings over-sizing.

Acoustic Sound Transmission (STC 44–48)

Unlike drywall assemblies that transmit low-frequency sound and vibrate under door slams, a 4-inch hollow concrete block wall achieves an STC rating of 44 to 46 unpainted, and STC 47 to 49 when painted on both sides with high-build latex block filler to seal surface pores.

2026 Real Cost Breakdown for 4-Inch CMU Partition Walls

US National Contractor Averages

Cost Item	Unit Price Range	Cost per Sq Ft Wall Face	Contractor Specifications
4" CMU Partition Blocks	$1.65 – $2.15 / block	$2.05 – $2.65	1.125 blocks/sq ft + 10% waste allowance (1.24 units)
Type N or S Masonry Mortar	$9.00 – $12.50 / 80-lb bag	$0.35 – $0.48	Narrow 3-5/8" bed joints use ~30% less mortar than 8" CMU
Horizontal Joint Wire (9-ga)	$18 – $26 / 50-ft roll	$0.30 – $0.45	Installed in bed joints every 16 inches vertically (every 2nd course)
Top Slotted Deflection Clips	$3.50 – $5.00 / clip	$0.45 – $0.70	Anchored to deck every 32" to 48" along top course
Masonry Labor & Tender	$55 – $80 / crew-hour	$7.50 – $11.00	High productivity: masons lay 130–160 four-inch units per day
Turnkey Installed Partition	All Materials & Labor	$10.65 – $15.28	Complete non-bearing partition including joint tooling

Pallet Packaging, Unit Counts & Flatbed Logistics

High Pallet Density

Standard delivery pallets carry 135 to 150 units of 4-inch CMUs arranged in 5 banded courses. One pallet provides approximately 120 to 133 square feet of wall area.

Pallet Weight (~3,240 lbs)

A pallet of medium-weight 4-inch CMUs weighs approximately 3,100 to 3,450 lbs, enabling commercial flatbeds to transport up to 14 full pallets per delivery run.

Elevated Slab Staging

When distributing 4-inch block pallets on upper concrete floor slabs, always stage pallets directly above primary structural beams to prevent temporary slab point-load deflection.

Explore Other CMU Block Dimensions & Tools

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6-Inch CMU Dimensions

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12-Inch CMU Dimensions

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Breeze Block Sizes

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Frequently Asked Questions About 4-Inch Concrete Blocks

What is the actual size of a 4-inch concrete block?

The actual manufactured dimensions of a nominal 4x8x16-inch CMU are exactly 3-5/8 inches wide, 7-5/8 inches high, and 15-5/8 inches long (92mm × 194mm × 397mm). The 3/8-inch difference allows for standard horizontal bed and vertical head mortar joints.

Can 4-inch CMU blocks be used for load-bearing walls?

Under the International Building Code (IBC Section 2109), 4-inch CMUs are generally classified as non-load-bearing due to their narrow web thickness. They are primarily engineered for interior corridor fire partitions, curtain walls, acoustic separations, and exterior brick veneer backup systems.

How much does a 4-inch concrete block weigh?

Under ASTM C90, a hollow 4-inch CMU weighs approximately 18 to 20 lbs in lightweight expanded aggregate, 21 to 23 lbs in medium weight, and 24 to 26 lbs in normal weight gravel concrete. A 100% solid 4-inch block weighs roughly 31 to 34 lbs.

What is the maximum unbraced height of a 4-inch block partition wall?

Building codes establish a maximum height-to-thickness (h/t) ratio of 30 for interior non-bearing masonry partitions. Based on actual 3-5/8 inch thickness, the maximum unsupported height between structural ceiling decks or horizontal stiffeners is approximately 9 feet 1 inch (108 inches).

What is the fire resistance rating of a 4-inch CMU wall?

Under IBC Chapter 7 and NCMA TEK 07-01D, an un-grouted hollow 4-inch CMU wall provides a 1-hour fire resistance rating with siliceous or limestone gravel aggregates, and up to a 1.5 to 2-hour rating when manufactured with lightweight expanded shale or pumice aggregate.