How many blocks do I need for a retaining wall?

Blocks per course is wall length in inches divided by block length; courses is wall height divided by block height. A 20 ft long, 3 ft high wall of 12 x 8 inch face block takes 20 blocks per course and 5 courses including one buried course — 100 blocks plus caps.

Should the first course of a retaining wall be buried?

Yes — bury one full course, or about 10 percent of the wall height, below finished grade on a compacted base. It resists the toe of the wall kicking outward.

How tall can a retaining wall be without an engineer?

Many jurisdictions allow up to 3 or 4 feet of exposed height without engineering, but the limit and permit threshold vary and any wall with a slope, driveway or structure above it is treated differently. Check with your building department before you build.

Do retaining walls need drainage stone behind them?

Always. A 12 inch wide column of clean crushed stone plus a perforated drain pipe at the base relieves hydrostatic pressure. Water pressure, not soil weight, is what pushes most failed walls over.

How many blocks in a course per foot?

A 12 inch long block gives 1 block per foot, a 16 inch block gives 0.75, and an 18 inch block gives 0.67. Multiply by the number of courses for the total.

Retaining Wall Block Calculator

Block count by course including a buried base course, cap blocks, base gravel, drainage stone and total material cost.

Quick answer

A 20 ft long, 3 ft high wall in SRW 12" x 8" x 8" takes 6 courses of 20 blocks — 126 blocks with 5% waste, plus 20 caps. Behind and below it you need 0.87 tons of base gravel and 3.15 tons of drainage stone. Materials come to about $850.

Disclosure: CementCalc is free to use and supported by ads and affiliate links. As an Amazon Associate we earn from qualifying purchases. Full disclosure.

Wall and block

Wall length(ft)

Exposed height(ft)

Block

47 lb each

Bury first course

Waste(%)

Base gravel depth(in)

6 in compacted, level

Drainage stone width(in)

12 in of clean stone behind the wall

Block price($ each)

Cap price($ each)

Stone price($/ton)

courses = ceil(height in / 8 in) | per course = ceil(length in / 12 in)

Material list

Blocks to buy

126

6 courses x 20 per course

Cap blocks

Top course

Base gravel

0.87tons

6 in compacted trench

Drainage stone

3.15tons

12 in column behind the wall

Wall face area

60sq ft

Block weight to move

2.82tons

120 blocks at 47 lb

Materials total

$850

Blocks per course by block length

Block	Face size	Blocks per ft of length	Courses per ft of height	Blocks per sq ft of face	Weight
SRW 12" x 8" x 8"	12 x 8 in	1	1.5	1.5	47 lb
SRW 12" x 4" x 8"	12 x 4 in	1	3	3	22 lb
SRW 16" x 6" x 12"	16 x 6 in	0.75	2	1.5	60 lb
CMU 16" x 8" x 8"	16 x 8 in	0.75	1.5	1.13	38 lb
CMU 16" x 8" x 12"	16 x 8 in	0.75	1.5	1.13	52 lb

Coverage figures for common segmental wall block and CMU sizes.

The base course decides the wall. Excavate a level trench, place and compact 6 inches of road base, and set the first course dead level in both directions — every course above inherits whatever error you leave there, and by the top it is obvious from the street.

Backfill with clean stone against the block, not with the soil you excavated, and run a perforated drain at the bottom of that stone daylighting to grade. Compact the retained soil in lifts behind the stone column.

Source & method: Block counts are geometry from nominal face dimensions; gravel and stone quantities use loose unit weights of 135 lb/ft³ for road base and 100 lb/ft³ for clean drainage stone. Block weights are typical values for concrete units of each size.

Wall building supplies

Links below are affiliate links. If you buy through them we may earn a commission at no extra cost to you.

Frequently asked questions

How many blocks do I need for a retaining wall?: Blocks per course is wall length in inches divided by block length; courses is wall height divided by block height. A 20 ft long, 3 ft high wall of 12 x 8 inch face block takes 20 blocks per course and 5 courses including one buried course — 100 blocks plus caps.
Should the first course of a retaining wall be buried?: Yes — bury one full course, or about 10 percent of the wall height, below finished grade on a compacted base. It resists the toe of the wall kicking outward.
How tall can a retaining wall be without an engineer?: Many jurisdictions allow up to 3 or 4 feet of exposed height without engineering, but the limit and permit threshold vary and any wall with a slope, driveway or structure above it is treated differently. Check with your building department before you build.
Do retaining walls need drainage stone behind them?: Always. A 12 inch wide column of clean crushed stone plus a perforated drain pipe at the base relieves hydrostatic pressure. Water pressure, not soil weight, is what pushes most failed walls over.
How many blocks in a course per foot?: A 12 inch long block gives 1 block per foot, a 16 inch block gives 0.75, and an 18 inch block gives 0.67. Multiply by the number of courses for the total.