Estimating Concrete Blocks for a Wall: CMU, Cinder Block & Mortar Quantities

Estimating Concrete Blocks for a Wall: CMU, Cinder Block & Mortar

Concrete masonry units (CMU) — sold under trade names including cinder blocks, concrete blocks, and breeze blocks — are the most cost-effective structural masonry material for foundations, retaining walls, fences, and load-bearing construction.

Getting the right block count matters: under-ordering means a second delivery (often with a minimum fee); over-ordering leaves you with 40-lb blocks to return or dispose of. Mortar and grout quantities are equally important — running out of mortar mid-course forces you to cold-joint the work.

Use the Concrete Block Calculator to estimate block count and mortar for any wall. This guide explains the math behind it.

CMU Nominal Dimensions vs. Actual Dimensions

The nominal block size includes the mortar joint thickness (typically 3/8 inch). The actual (physical) block size is smaller.

Nominal Size (H×W×L)	Actual Size (H×W×L)	Joint Thickness
8×8×16	7⅝" × 7⅝" × 15⅝"	⅜" mortar
8×6×16	7⅝" × 5⅝" × 15⅝"	⅜" mortar
8×4×16	7⅝" × 3⅝" × 15⅝"	⅜" mortar
8×12×16	7⅝" × 11⅝" × 15⅝"	⅜" mortar
4×8×16 (half height)	3⅝" × 7⅝" × 15⅝"	⅜" mortar

Practical note: When calculating how many courses high a wall is, always use the nominal dimension (8 inches per course for standard 8×8×16). For a 4-foot wall: 4 × 12 / 8 = 6 courses.

The 8×8×16 is the most common block in U.S. residential and commercial construction. In some regions, 6×8×16 blocks are used for partition walls; 12×8×16 for heavy retaining walls and foundations.

Block Count Formula

The standard rule for 8×8×16 CMU with 3/8-inch mortar joints is:

1.125 blocks per square foot of wall face

This comes from: each block covers a face area of (8/12) × (16/12) = 0.667 ft × 1.333 ft = 0.889 ft²/block Therefore: 1 / 0.889 = 1.125 blocks/ft²

Block count = Wall face area (ft²) × 1.125

Add 5–10% for cuts, breakage, and waste.

Worked Example 1: Foundation Wall

Dimensions: 40 ft long × 8 ft high, one side

Wall face area: 40 × 8 = 320 ft²

Block count (8×8×16): 320 × 1.125 = 360 blocks

With 7% waste: 360 × 1.07 = 385 blocks → order 390 (next pallet quantity)

Courses: 8 ft / (8/12 ft) = 12 courses

Blocks per course: 40 ft / (16/12 ft) = 30 blocks per course 12 × 30 = 360 blocks ✓ (matches formula)

Worked Example 2: Garden Retaining Wall

Dimensions: 60 ft long × 4 ft high (6 courses of 8×8×16 block)

Wall face area: 60 × 4 = 240 ft²

Block count: 240 × 1.125 = 270 blocks

With 5% waste: 270 × 1.05 = 284 blocks → order 285

Blocks per course: 60 / (16/12) = 45 blocks 6 courses: 45 × 6 = 270 blocks ✓

Block Count Reference Table (8×8×16 CMU)

Wall Area (ft²)	Blocks (no waste)	+5% Waste	+10% Waste
50	56	59	62
100	113	119	124
200	225	236	248
500	563	591	619
1,000	1,125	1,181	1,238
2,000	2,250	2,363	2,475

Mortar Quantity for CMU Walls

CMU mortar occupies the bed joints (horizontal) and head joints (vertical) between blocks. It does not fill the block cores — that's grout (discussed below).

Standard mortar consumption for 8×8×16 CMU:

Approximately 0.05 ft³ of mortar per block placed (face shell bedding, standard bed and head joints)
Or approximately 7 bags of 60-lb mortar mix per 100 blocks

For the 360-block foundation wall example:

Mortar volume: 360 × 0.05 = 18 ft³
60-lb mortar mix bags: 360 × 7/100 = 25.2 bags → order 26 bags
Or specify by cubic feet: 18 ft³ = 0.67 yd³ of pre-mixed mortar

Standard mortar mix (Type S, per ASTM C270):

1 part Portland cement : 0.5 part lime : 4.5 parts masonry sand (by volume)
Or use pre-bagged Type S mortar mix — just add water

Type S mortar (1,800 psi) is the standard for below-grade, retaining wall, and exterior CMU. Type N mortar (750 psi) is used for interior, above-grade, and non-structural applications.

Grout for Core Fill

Grouted cores provide additional compressive and tensile capacity to a CMU wall. Structural requirements specify which cores must be grouted (typically those with vertical rebar, corners, jambs, and bond beams).

For fully grouted walls (every core filled), estimate grout volume per block:

Block Size	Core Volume per Block
8×8×16 (2-core)	~0.060 ft³ = 0.002 yd³
8×8×16 (3-core)	~0.045 ft³ per core
12×8×16 (2-core)	~0.130 ft³ = 0.005 yd³

For a fully grouted 360-block wall: Grout volume = 360 × 0.060 = 21.6 ft³ = 0.80 yd³

At $150/yd³ for pea gravel grout: **$ 120 material cost** for full core fill.

For partially grouted walls (specific cores only), count only the grouted cores using the rebar layout — typically every 24–48 inches and at all corners.

Cinder Block vs. Concrete Block vs. CMU — What's the Difference?

The terms are used interchangeably in common speech, but there is a technical distinction:

Term	Technical Meaning
CMU (Concrete Masonry Unit)	The industry-standard term. Includes all hollow or solid masonry units made from Portland cement, aggregate, and water.
Concrete block	A CMU made with normal-weight aggregate (limestone, granite, river gravel). Density: ~35–50 lb per 8×8×16 block.
Cinder block	Historically, a CMU made with coal cinder aggregate (a byproduct of coal combustion). True cinder blocks are largely obsolete since the 1980s; the term now refers loosely to any hollow CMU.
Lightweight CMU	Made with expanded shale, clay, or slate aggregate. Typically 28–38 lb per block vs. 38–52 lb for normal weight.

For the Cinder Block Calculator and CMU Block Calculator, the same 1.125 blocks/ft² factor applies — the nominal dimensions are identical across the terminology variations.

How Mortar Joint Thickness Affects Block Count

Standard joint thickness is 3/8 inch. Thick joints (½ inch) or thin joints (¼ inch) affect block count:

Joint Thickness	Face area per block	Blocks per ft²
¼"	0.888 ft²	1.127
⅜" (standard)	0.889 ft²	1.125
½"	0.891 ft²	1.122

The difference is negligible — joint thickness matters much more for mortar volume than for block count. Thicker joints use more mortar and place the wall face slightly out of square over height.

Typical CMU Wall Cost

Item	Unit Cost	200 ft² Wall (225 blocks)
8×8×16 CMU blocks	$2.50–$ 4.00 each	$563–$ 900
Type S mortar (16 bags)	$12–$ 18/bag	$192–$ 288
Labor (masonry contractor)	$18–$ 30/ft²	$3,600–$ 6,000
Total installed	$22–$ 36/ft²	$4,400–$ 7,200

CMU wall construction is labor-intensive. Splitting jobs across courses with proper lead time reduces overtime costs.

Frequently Asked Questions

How many concrete blocks do I need for a 10×10 shed foundation?

A 10×10 shed perimeter wall, one block wide and two courses high (16 inches total height): Perimeter = (10 + 10 + 10 + 10) = 40 LF. Wall area = 40 × (16/12) = 53.3 ft². Blocks = 53.3 × 1.125 = 60 blocks (+5% waste = 63).

What is the weight of a standard 8×8×16 concrete block?

A standard normal-weight hollow 8×8×16 CMU weighs approximately 38–44 lb. Lightweight versions weigh 28–35 lb. Solid-core (not hollow) 8×8×16 blocks weigh 50–60 lb.

Do I need rebar in a concrete block retaining wall?

For retaining walls over 3 feet high (measured from bottom of footing to top of wall), most codes require vertical rebar in grouted cores plus horizontal rebar in bond beams at regular intervals. For residential retaining walls under 3 feet, check your local code — some jurisdictions require a structural engineer review regardless of height.

How many blocks come on a pallet?

Standard pallets hold 80–90 blocks of 8×8×16 CMU. Confirm with your supplier before ordering — some pallets hold 70 or 100. Pallet pricing typically provides a 5–10% discount over per-unit pricing.

Use the Block Calculators

Standard concrete block: Concrete Block Calculator
Cinder block (hollow CMU): Cinder Block Calculator
CMU grout and mortar: CMU Block Calculator

Visit Concrete Calculator Max for the full suite of masonry and concrete estimation tools.