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

Walls over 4 ft usually need an engineer

Cap blocks give the wall a finished top

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Read the full Retaining Wall Cost Guide 2026

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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

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.

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