How many cinder blocks for a retaining wall?

Multiply wall length × total height (exposed + buried), divide by 0.89 sq ft per standard block, and add 10% waste. A 20 ft × 4 ft retaining wall needs approximately 99 blocks.

Do retaining walls need drainage?

Yes. All retaining walls must have drainage to prevent hydrostatic pressure. Install a perforated drain pipe at the base and 12 inches of gravel backfill behind the wall.

How tall can a cinder block retaining wall be without engineering?

Most building codes allow retaining walls up to 3–4 ft without engineering. Walls over 4 ft typically require a permit, professional engineering, and structural reinforcement.

Retaining Wall Specialist

Retaining Wall
Calculator

Estimate blocks, mortar, rebar, grout, drainage gravel, drain pipe, and footing concrete for any retaining wall — from garden terraces to engineered earth retention.

Retaining Wall Type

Wall Length (ft)

Exposed Height (ft)

Buried Courses

Block Width

Waste (%)

Retaining Wall Estimate

Enter wall dimensions to see estimates

Retaining Wall Tip

Buried courses provide a stable base below grade. Drain gravel and pipe estimates assume a 1 ft wide drainage column behind the wall.

Retaining Wall Quick Reference

Garden Terrace

Height:1–2 ftBlock:8"Rebar:NoneDrainage:Gravel only

Gravity Wall

Height:2–3 ftBlock:12"Rebar:NoneDrainage:Pipe + gravel

Reinforced CMU

Height:3–6 ftBlock:8" or 10"Rebar:Every 32"Drainage:Pipe + gravel

Cantilevered

Height:6–12 ftBlock:12"Rebar:EngineeredDrainage:Full system

Wall Type	Height Range	Block Width	Rebar	Drainage
Garden Terrace	1–2 ft	8"	None	Gravel only
Gravity Wall	2–3 ft	12"	None	Pipe + gravel
Reinforced CMU	3–6 ft	8" or 10"	Every 32"	Pipe + gravel
Cantilevered	6–12 ft	12"	Engineered	Full system

How to Plan a Retaining Wall

Retaining walls resist lateral soil pressure, making proper planning critical for safety and longevity. Unlike freestanding walls, retaining walls must account for drainage, buried courses, and soil loading.

Cinder Block Retaining Wall Calculator (2026)

Determine the soil type, slope grade, and total height to retain. Mark the wall line and check for underground utilities.

Cinder Block Retaining Wall Calculator (2026)

Excavate for the footing and 1–2 buried block courses. The trench should be level and extend below the frost line.

Cinder Block Retaining Wall Calculator (2026)

Cast a concrete footing at least twice the block width. Allow 24–48 hours to cure before laying blocks.

Cinder Block Retaining Wall Calculator (2026)

Stack courses with Type S mortar. Insert vertical rebar every 32" and add horizontal bond beams every 2–3 courses.

Cinder Block Retaining Wall Calculator (2026)

Place perforated drain pipe at the footing base. Backfill with 12" of clean gravel and add filter fabric.

Cinder Block Retaining Wall Calculator (2026)

Backfill in 12" lifts, compacting each layer. Add a capstone or stucco finish to the top course.

Critical: Drainage Requirements

A retaining wall without proper drainage will fail. Hydrostatic pressure from trapped water is the #1 cause of retaining wall collapse.

Perforated drain pipe — 4" diameter at the footing base, sloped 1% to daylight or a catch basin.
Gravel backfill — 12" of clean ¾" washed stone behind the wall, full height.
Filter fabric — Wrap gravel in geotextile to prevent soil migration into the drainage layer.
Weep holes — For CMU walls, leave unmortared joints every 4–6 ft in the first course above grade.

Types of Retaining Walls

🪨

Cinder Block Retaining Wall Calculator (2026)

Relies on sheer mass to resist soil pressure. Built with wide 12" blocks and no rebar. Limited to 3 ft height. Best for garden terraces.

🧱

Cinder Block Retaining Wall Calculator (2026)

Standard approach for 3–6 ft walls. Uses vertical rebar every 32", grouted cores, and horizontal bond beams. Most common residential choice.

🏗️

Cinder Block Retaining Wall Calculator (2026)

Engineered design with an L-shaped or T-shaped footing. The footing uses the weight of backfill soil to resist overturning. For walls 6–12 ft+.

⛰️

Cinder Block Retaining Wall Calculator (2026)

Multiple shorter walls (2–3 ft each) separated by flat terraces. Reduces pressure on each wall section. Great for slopes and landscaping.

Permits required for walls over 4 feet

Most jurisdictions require a building permit and engineered plans for retaining walls over 4 ft tall, or any retaining wall that supports a surcharge (driveway, building, etc.). Fines for unpermitted retaining walls can be significant.

Cinder Block FAQs

Everything you need to know about calculating masonry materials.

For a standard 8x8x16 block, there are approximately 1.125 blocks per square foot. This includes the standard 3/8" mortar joint. If you are using 4-inch high blocks, you will need 2.25 blocks per square foot.

A standard pallet typically holds 75 to 90 blocks, depending on the manufacturer and the block size. An 8x8x16 pallet usually has 75 blocks, weighing approximately 2,800 lbs.

As a general rule, you will need about 8 to 10 cubic feet of mortar for every 100 standard 8x8x16 blocks. This translates to roughly 3 to 4 bags of pre-mixed mortar (80lb bags).

While the terms are used interchangeably, concrete blocks (CMUs) are made of crushed stone and sand, making them very heavy and strong. True cinder blocks use coal cinders as an aggregate, making them lighter and less structural. Most modern 'cinder blocks' are actually concrete blocks.

Yes, always add 5% to 10% for waste. This accounts for blocks that arrive broken, mistakes during cutting for corners or ends, and any architectural adjustments needed on-site.