How deep should the first course be buried?

The first course (base course) should be fully buried below grade for stability. This is typically one full block height — 4 to 6 inches depending on block size. The base should sit on a compacted gravel trench at least 6 inches deep.

Do I need a permit for a retaining wall?

Most jurisdictions require a building permit for retaining walls over 4 feet tall (measured from the bottom of the footing to the top of the wall). Walls over 4 feet typically also need engineering by a licensed professional. Always check local codes.

How Many Retaining Wall Blocks Do I Need?

Q: How many retaining wall blocks do I need?

Divide your wall length (in inches) by the block width to get blocks per course. Multiply by the number of courses (wall height in inches divided by block height, plus one buried base course). A 20-foot wall that's 3 feet high with standard 16×6 blocks needs about 120 blocks.

February 19, 2026 · By DIY Calc

Quick answer: Divide wall length by block width for blocks per course, then multiply by total courses (including one buried base course). A 20-foot long, 3-foot high wall with 16" blocks needs about 120 blocks plus 15 cap blocks.

Skip the math — use our free Retaining Wall Calculator →

The Formula

Blocks per course: Wall length (inches) ÷ block width (inches), round up
Number of courses: Wall height (inches) ÷ block height (inches), round up
Add buried base course: Add 1 more course for stability
Total blocks: Blocks per course × total courses
Cap blocks: Same as blocks per course (if using caps)

Example: 20ft Long × 3ft High Wall, 16×6×12 Blocks

Blocks per course: (20 × 12) ÷ 16 = 15 blocks
Visible courses: (3 × 12) ÷ 6 = 6 courses
Plus buried base: 7 courses total
Total blocks: 15 × 7 = 105 blocks
Cap blocks: 15 caps

Common Block Sizes

Standard (12×4×8"): Smaller, lighter blocks. Good for short walls. About 30 lbs each.
Large (16×6×12"): Most popular for DIY retaining walls. About 50 lbs each.
XL (18×6×12"): Fewer blocks needed, premium look. About 65 lbs each.
Small (8×4×12"): For garden borders and low walls. About 25 lbs each.

Don't Forget the Base and Drainage

The blocks are the most visible part, but what happens behind and below the wall determines whether it lasts 5 years or 50.

Gravel Base Trench

Dig a trench 6 inches deep and twice the block depth wide. Fill with compacted crushed gravel (not rounded stone). This provides drainage and a level surface for the first course.

Gravel Backfill

Behind the wall, fill with 12 inches of crushed gravel from the base to within 6 inches of the top. This prevents water pressure (hydrostatic pressure) from building up behind the wall — the #1 cause of retaining wall failure.

Drainage Pipe

Lay 4-inch perforated drain pipe at the base of the wall, behind the first course, surrounded by drainage aggregate. The pipe should daylight (exit) at the end of the wall or into a drainage area.

Setback and Batter

Retaining wall blocks are designed with a built-in setback — each course steps back about 3/4" to 1" from the course below. This lean (called "batter") uses gravity to resist the force of the soil behind the wall. Don't try to stack blocks perfectly vertical.

Adhesive

Apply landscape block adhesive between the top two courses and between the cap blocks and the top course. One tube covers about 10 linear feet. This prevents the top blocks from being knocked off by weather, animals, or people sitting on the wall.

When You Need an Engineer

DIY retaining walls should generally stay under 4 feet tall. Taller walls need:

A building permit
Engineering by a licensed professional
Possible geogrid reinforcement
Proper surcharge calculations (especially if there's a driveway, structure, or slope above)

Cost Estimates

Blocks: $3–$6 each depending on size and style
Cap blocks: $4–$8 each
Gravel (per cubic yard): $35–$60 delivered
Drainage pipe (per 10ft): $8–$15
Adhesive: $5–$8 per tube

A typical 20-foot × 3-foot wall costs $500–$1,000 in materials for a DIY build.

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