How many blocks do I need for a retaining wall?

Divide wall area (length × height) by the face area of one block (typically 0.67–1.0 sq ft). Add 10% for waste, plus blocks for the buried base course and cap blocks on top.

Do I need gravel behind a retaining wall?

Yes — always. Drainage gravel behind the wall prevents hydrostatic pressure buildup that can topple even a well-built wall. Use crushed stone or gravel, not native soil, as backfill.

How tall can a retaining wall be without a permit?

Most jurisdictions allow walls up to 3 or 4 feet without a permit. Walls over 4 feet typically require engineered drawings and a building permit. Check your local building department.

How deep should the base course of a retaining wall be buried?

Bury at least one full block course (6–8 inches) below grade. For walls over 3 feet, bury 10–12 inches. The buried course anchors the wall and prevents sliding.

Retaining Wall Block Calculator: How Many Blocks Do I Need?

Wall area divided by block face area gives you your count. But the base course, cap blocks, and gravel backfill are where most estimates go wrong.

Measure the Wall Area

Calculate retaining wall area by multiplying wall length by wall height. For a wall 40 feet long and 3 feet tall: 40 × 3 = 120 square feet of wall face.

Standard retaining wall blocks (such as Allan Block or Versa-Lok) have a face area of about 0.67 to 1.0 square feet. Divide wall area by block face area to get block count.

For 120 square feet with blocks that cover 0.75 sq ft each: 120 / 0.75 = 160 blocks. Add 5 to 10% for waste, cuts, and damaged blocks: order 170 to 180 blocks.

Base Course and Cap Blocks

The base course is buried below grade to anchor the wall. You typically bury at least one course (6 to 8 inches) for walls under 4 feet. This adds material that won't be visible.

For a 40-foot wall with 12-inch wide blocks, the base course needs 40 linear feet of blocks. If your block is 12 inches long, that's 40 additional blocks — about 25% more than your visible count.

Cap blocks finish the top of the wall. They're flat-topped pieces designed to match the wall block. One cap block per 12 inches of wall length: a 40-foot wall needs 40 cap blocks.

Free tool: Retaining Wall Calculator →

Gravel Base and Backfill

Every retaining wall needs a compacted gravel base (4 to 6 inches of crushed stone) and gravel backfill behind the wall for drainage. Skipping this step is the number one reason retaining walls fail.

Base gravel: wall length × base width (typically 2 feet) × 0.5 feet deep = cubic feet, divided by 27 for cubic yards. A 40-foot wall needs about 1.5 cubic yards of base gravel.

Backfill gravel: wall length × 1 foot wide × wall height = cubic feet behind the wall. A 40-foot × 3-foot wall needs about 4.5 cubic yards of drainage gravel behind it.

Wall Height and Permit Requirements

Walls under 3 feet typically don't require permits in most jurisdictions. Walls 3 to 4 feet are a gray area — check local codes. Walls over 4 feet almost always require an engineered design and permit.

Taller walls need batter (backward lean) — typically 1 inch of setback per foot of height. Most interlocking block systems have a built-in batter angle when you set the blocks correctly.

For walls over 4 feet or retaining significant soil loads, use geogrid reinforcement — layers of mesh embedded in the backfill every 2 feet of wall height. Your block manufacturer has specific grid specs for their system.

Cost to Build a Retaining Wall

Concrete block retaining walls cost $20 to $50 per square face foot installed. A 40-foot × 3-foot wall (120 sq ft) runs $2,400 to $6,000 professionally installed.

Material-only cost for standard retaining wall block runs $8 to $15 per block. With base gravel, cap blocks, and drainage gravel, material cost runs $12 to $20 per square face foot.

DIY retaining walls are common for walls under 3 feet. The work is labor-intensive (excavation, compaction, leveling each course) but manageable over a weekend for a 20 to 30-foot wall.

Related free calculators

Stop estimating by hand

ProBuildCalc scans a room with your iPhone's LiDAR and builds the square footage, material takeoff, and a blueprint automatically.

Get started free Free calculators

FAQ

How many blocks do I need for a retaining wall?: Divide wall area (length × height) by the face area of one block (typically 0.67–1.0 sq ft). Add 10% for waste, plus blocks for the buried base course and cap blocks on top.
Do I need gravel behind a retaining wall?: Yes — always. Drainage gravel behind the wall prevents hydrostatic pressure buildup that can topple even a well-built wall. Use crushed stone or gravel, not native soil, as backfill.
How tall can a retaining wall be without a permit?: Most jurisdictions allow walls up to 3 or 4 feet without a permit. Walls over 4 feet typically require engineered drawings and a building permit. Check your local building department.
How deep should the base course of a retaining wall be buried?: Bury at least one full block course (6–8 inches) below grade. For walls over 3 feet, bury 10–12 inches. The buried course anchors the wall and prevents sliding.