How many concrete blocks do I need for a wall?

Divide the total square footage of your wall by the face area of one block. A standard 8×8×16 block has a face area of 0.89 sq ft (accounting for mortar joints), so you need 1.13 blocks per square foot. Multiply by 1.10 for 10% waste on corners and cuts. A 20-foot long by 4-foot tall wall = 80 sq ft × 1.13 × 1.10 = 99 blocks.

How many cinder blocks do I need for a 100 sq ft wall?

For a 100 sq ft wall using standard 8×8×16 blocks, you need approximately 113 blocks before waste. Add 10% waste (11 blocks) for a total of 124 blocks. Also order about 3 bags of mortar mix (one bag covers 40-50 blocks).

What is the difference between a cinder block and a concrete block?

Cinder blocks were originally made with cinder aggregate (coal ash) and are no longer manufactured. What most people call a cinder block today is actually a concrete masonry unit (CMU) made from Portland cement, aggregate, and water. The terms are used interchangeably in modern construction, but technically all new blocks are concrete blocks.

How many courses of block equal one foot in height?

Standard 8x8x16 blocks with 3/8-inch mortar joints stack at exactly 8 inches per course. That means 1.5 courses per foot, or 3 courses per 2 feet. A 4-foot wall requires 6 courses. A 6-foot wall requires 9 courses. This makes planning very straightforward.

How much does a concrete block wall cost?

Material cost for standard 8×8×16 CMU blocks runs $2.50–$4.00 per block. A 20×4 ft wall (99 blocks) costs $250–$400 in materials plus about $15 in mortar. Installed cost by a mason runs $10–$15 per sq ft, so that same wall costs $800–$1,200 professionally installed.

How much mortar do I need for concrete blocks?

One 70-pound bag of mortar mix covers approximately 40-50 standard 8×8×16 blocks using 3/8-inch joints. Add 10-15% for waste. For 100 blocks, order 3 bags. For 200 blocks, order 5-6 bags. Pre-mixed mortar bags are more convenient for small projects; bulk masonry mortar is more economical for large walls.

Retaining Wall Calculator: Blocks & Concrete

🧱 BLOCK CALCULATOR

WALL LENGTH (ft)

WALL HEIGHT (ft)

BLOCK SIZE

WASTE FACTOR

Slab Thickness by Use Case

How thick should your concrete slab be? Code-recommended dimensions for any project

Application	Min Thickness	Recommended	Reinforcement
Sidewalk / Walkway	3.5"	4"	Optional fiber
Patio	4"	4"	Mesh recommended
Residential Driveway	4"	5"	Mesh or rebar grid
Garage Floor	4"	5–6"	Rebar grid required
RV/Truck Pad	6"	6–8"	Heavy rebar grid
Shed Foundation (small)	4"	4"	Mesh
Pool Deck	4"	4–5"	Mesh + control joints
Hot Tub Pad	4"	6"	Rebar grid

For freeze-thaw climates (Northeast, Midwest, Mountain West), add 1 inch to the recommended thickness. Always check your local building code — some jurisdictions require 6" minimum for driveways regardless of use.

Slab Pour Checklist

A repeatable order-of-operations that prevents the most common DIY slab mistakes

Excavate to depth. Slab thickness + base depth + 2" for forms = total dig depth. For a 4" slab on 4" base, dig 10" below final grade.
Compact subgrade. Native soil should be compacted to 95% before adding base. Don't skip this on clay or recently disturbed soil.
Lay 4" of crushed stone or roadbase. Compact in 2" lifts. 95% compaction is the target.
Set forms. 2x4 lumber for 3.5" slabs, 2x6 for 5" slabs. Stake every 4 feet. Check for level and proper slope (¼" per foot away from buildings).
Place vapor barrier and reinforcement. 6 mil poly under interior slabs. Wire mesh or rebar grid raised 1.5–2" off the base on chairs.
Pour and screed. Pour from far end working back. Screed level with the forms using a 2x4. Don't over-work the surface.
Float, edge, and joint. Bull float once water sheen disappears. Edge the perimeter with an edging tool. Cut control joints every 8–10 feet.
Final finish and cure. Broom finish for traction or trowel for smooth. Cover with plastic for 7 days minimum, lightly mist daily.
Wait before loading. Foot traffic 24–48 hours. Vehicles 7 days. Heavy equipment 28 days.

Concrete Block Types and Coverage

How many blocks per square foot for each standard CMU size

Block Type	Nominal Size	Face Area	Blocks per Sq Ft	Common Use	Est. Price Each
Standard CMU (8×8×16)	8" × 8" × 16"	0.89 sq ft	1.13	Walls, foundations, retaining	$2.50–$4.00
Half Block (8×8×8)	8" × 8" × 8"	0.44 sq ft	2.25	Corners, ends, openings	$2.00–$3.00
Jumbo (12×8×16)	12" × 8" × 16"	0.89 sq ft	1.13	Load-bearing walls, thicker walls	$4.00–$6.50
Partition (4×8×16)	4" × 8" × 16"	0.89 sq ft	1.13	Interior non-load-bearing	$1.50–$2.50
Split Face (8×8×16)	8" × 8" × 16"	0.89 sq ft	1.13	Decorative retaining walls, landscape	$5.00–$8.00
Retaining Wall Block	6"–8" × varies	varies	~1.0	Landscape retaining walls, edging	$3.00–$7.00

Mortar Requirements for Concrete Block

How much mortar mix you need — one bag goes further than most people think

One 70-pound bag of mortar mix covers approximately 40-50 standard 8×8×16 blocks. Standard joint thickness is ⅜ inch. Thicker joints (½ inch) use more mortar and are common for rough or irregular block work. Always estimate 10-15% extra for dropped mortar and waste.

Blocks per bag (standard ⅜" joint)

Blocks per bag (thick ½" joint)

+10%

Waste factor to always add

Retaining Wall Design Rules

Critical structural guidelines — get these wrong and the wall will fail

The 1:8 Rule

For every 1 foot of wall height, the base width should be at least 1/8 of the height. A 4-foot wall needs a base at least 6 inches wide. Taller walls need proportionally wider bases to resist soil pressure.

Batter (Lean Back)

Most segmental retaining walls should lean back into the slope at 1 inch per foot of height. A 3-foot wall leans back 3 inches. This "batter" counteracts the soil pressure pushing against the wall face.

Drainage is Critical

Most retaining wall failures are caused by water pressure, not soil pressure. Install a 6-inch layer of clean gravel behind the wall and use perforated pipe at the base to drain water away. Without drainage, hydrostatic pressure will push the wall over.

Height Limits

Most municipalities require a permit for retaining walls over 3 or 4 feet tall. Walls over 4 feet should be engineered by a professional. DIY segmental block walls are generally safe up to 3 feet without engineering — check your local code.

Retaining Wall Material Comparison

Concrete block, timber, poured concrete, and natural stone — costs and pros/cons

Material	Cost per Sq Ft	Max DIY Height	Lifespan	Best For
Segmental block (Allan, Versa-Lok)	$15–$35	3–4 ft	50+ yrs	Landscape, yard terracing
Concrete CMU block	$20–$40	3 ft (DIY)	50+ yrs	Structural, heavy load
Poured concrete	$30–$60	Hire out	50+ yrs	Commercial, engineered
Pressure-treated timber	$10–$20	4 ft	20–30 yrs	Budget, garden beds
Natural stone	$25–$80	3 ft	100+ yrs	Decorative, premium look

Concrete Slab FAQ

Common questions about calculating and pouring concrete slabs