How many blocks do I need for a retaining wall?

Multiply your wall length by its height to get the face area in square feet, then divide by the face area of one block. A standard 12 x 4 inch block covers 0.33 sq ft, so a 20 ft long, 3 ft tall wall needs roughly 180 blocks. Add 5% extra for cuts and breakage.

Do retaining walls need a permit?

Often, yes. Many municipalities require a permit for retaining walls above 3 to 4 feet tall, and some require engineered drawings. Always check your local building code before starting, especially if the wall supports a slope, driveway, or structure.

What goes behind a retaining wall?

Drainage gravel and a perforated drain pipe. Backfill at least 12 inches behind the wall with clean crushed gravel, and lay a perforated pipe at the base sloped to daylight. Proper drainage prevents hydrostatic pressure, the most common cause of wall failure.

How deep should the base be?

Excavate and compact at least 6 inches of gravel base, extending roughly 6 inches in front of and behind the block. The first course of blocks should also be partially buried, about 10% of the wall height, for stability.

How tall can I build a DIY retaining wall?

Most segmental block manufacturers rate unreinforced gravity walls at 3 to 4 feet maximum. Anything taller typically requires geogrid reinforcement, engineering, and permits. When in doubt, hire a professional for walls over 4 feet.

Retaining Wall Calculator

Calculate retaining wall blocks, cap blocks, base gravel, and drainage backfill for your wall project. Works for standard segmental wall blocks.

Enter Your Wall Dimensions

Wall Length (ft)

Wall Height (ft)

Walls over 4 ft (1.2 m) usually need an engineer

Block Size (W×H face)

Cap Row

Cap blocks give the wall a finished top

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How to Estimate Retaining Wall Blocks

Our retaining wall calculator estimates the blocks, cap blocks, base gravel, and drainage backfill you need for a segmental block wall. Multiply the wall length by its height to get the face area, divide by the face area of one block, and add 5% for cuts and breakage. Don't forget that the first course should be partially buried, so measure your finished wall height from grade.

Base and Drainage Explained

Base gravel: At least 6 inches of compacted crushed gravel under the first course, extending in front of and behind the blocks
Drainage backfill: 12 inches of clean gravel directly behind the wall, the full height of the wall
Drain pipe: Perforated pipe at the base of the backfill, sloped so water exits at the ends

Drainage is the most important part of a retaining wall. Water trapped behind the wall creates hydrostatic pressure that pushes blocks out of alignment and is the leading cause of wall failure.

When to Call a Pro

DIY-friendly gravity walls top out around 3 to 4 feet. If your wall is taller, supports a slope or driveway, sits near a property line, or holds back poorly draining soil, hire an engineer or experienced contractor. Many municipalities require permits and engineered drawings for walls above 3 to 4 feet.

Frequently asked questions

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