How many blocks do I need for a retaining wall?

Courses times blocks per course. A 20 ft wall 24" high in 12" × 4" block is 6 courses above ground plus one buried, 7 in all, at 20 blocks a course: 140 blocks and 20 caps, before waste.

How tall can a retaining wall be without an engineer?

The 2021 IRC (R404.4) requires a wall that holds back more than 48" of soil, and is not braced at the top, to be designed by accepted engineering practice. A slope or a driveway above the wall can bring that need in lower. That is the model code; ring the building department for the rule where you are.

Why does a retaining wall need gravel behind it?

Water. Saturated soil pushes on a wall far harder than dry soil, and water trapped behind a wall is what tips most of them over. A zone of clean stone behind the block with a perforated pipe at the bottom lets it drain away instead of building up.

Retaining wall block calculator

Enter the wall's length and height and the size of the block. This counts the courses, blocks and caps, and the gravel base and drainage stone the wall stands on and drains through.

Wall length

Height above ground

The soil it holds back

Block length

Block height

Block depth

Advanced settings1 buried · 12" caps · 6" base · 12" drainage stone · 5% waste

Courses buried

Cap length

Base depth

Compacted gravel under the first course

Base wider than block by

Drainage stone behind

Waste allowance

Blocks to buy

147blocks

Courses

Per course

Caps

Under and behind it

Base gravel: 0.59 cu yd
Drainage stone: 1.73 cu yd
Perforated drain pipe: 20 ft
Wall, buried courses included: 28"

Checks

All 2 checks passShow the numbers

Holds back no more than 48" of soil — passes24" is within the 48" the 2021 IRC (R404.4) allows before a wall has to be engineered. A slope or a driveway above the wall can change that.
At least one course below grade — passes1 course buried, so the wall is keyed into the ground.

The 48" limit is the 2021 IRC's, and it is a default rather than the law where you live — ring the building department before you dig. The block maker's own guide sets the base, the setback and the drainage for its block.

Where to buy this

147 wall blocksHome Depot Lowe's
20 capsHome Depot Lowe's
0.59 cu yd baseHome Depot Lowe's
20 ft drain pipeHome Depot Lowe's

This searches the materials, not the block you sized — buy caps from the same maker and line as the blocks.

Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.

Worked example: a 20 ft wall, 24" high

24" of exposed wall in 4" block is 6 courses, plus 1 buried, 7 in all; 20 ft is 20 12" blocks a course, so 147 blocks with 5% waste and 20 caps.

Under it, 0.59 cu yd of base gravel; behind it, 1.73 cu yd of drainage stone and 20 ft of drain pipe.

It passes all 2 of the building code and layout checks the calculator applies.

Blocks: 147
Caps: 20
Base gravel: 0.59 cu yd
Drainage stone: 1.73 cu yd

These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.

About retaining walls

How many blocks do I need for a retaining wall?: Courses times blocks per course. A 20 ft wall 24" high in 12" × 4" block is 6 courses above ground plus one buried, 7 in all, at 20 blocks a course: 140 blocks and 20 caps, before waste.
How tall can a retaining wall be without an engineer?: The 2021 IRC (R404.4) requires a wall that holds back more than 48" of soil, and is not braced at the top, to be designed by accepted engineering practice. A slope or a driveway above the wall can bring that need in lower. That is the model code; ring the building department for the rule where you are.
Why does a retaining wall need gravel behind it?: Water. Saturated soil pushes on a wall far harder than dry soil, and water trapped behind a wall is what tips most of them over. A zone of clean stone behind the block with a perforated pipe at the bottom lets it drain away instead of building up.