Retaining Wall Blocks: How Many and What Base?

Retaining walls fail at a genuinely high rate, and almost never because someone bought the wrong blocks. They fail because water built up behind them or the base moved. The blocks are the easy part.

Counting the blocks

Blocks = (wall height × wall length) ÷ block face area

A 3 ft × 20 ft wall is 60 sq ft. With standard 8 × 12 inch blocks (0.67 sq ft face), that's about 90 blocks. Add a cap row if you want a finished top.

Our retaining wall calculator takes height, length, block dimensions and whether you want caps, and handles the count including the buried course.

The base is the wall

Everything rests on this, literally. Get it wrong and nothing above it matters.

Excavate a trench wide enough for the block plus about 12 inches behind it for drainage stone.

6 inches of compacted crushed gravel, tamped in 2-inch lifts. Not sand — sand shifts. Not dirt. Crushed angular stone that locks.

Level it obsessively. This is where DIY walls are won or lost. Every error in the base course multiplies as you go up: an eighth of an inch out at the bottom is a visibly crooked wall six courses later, and there's no fixing it without starting over. Spend the time here — it's the least fun and most important hour of the job.

Our gravel calculator and crushed stone calculator size the base material.

Drainage is what actually kills walls

Here's the thing people underestimate. Soil is heavy. Saturated soil is dramatically heavier, and it exerts hydrostatic pressure that most DIY walls were never designed to resist.

A wall that holds dry soil comfortably can be pushed over by the same soil after a week of rain. Water is the enemy, and the entire job is giving it somewhere to go:

Drainage stone — 12 inches of clean angular gravel behind the full height of the wall. Not soil. Not fines. Clean stone that water falls straight through.

Perforated drain pipe at the base, holes down, sloped to daylight at one end. It has to actually exit somewhere — a pipe that ends in the middle of the backfill is decoration.

Geotextile between the drainage stone and the backfill soil, so fines don't migrate in and clog the stone over a few seasons.

Setback and backfill

Setback: each course should step back slightly, about an inch per course. This leans the wall into the hill so gravity works with you. Most segmental blocks have a built-in lip that creates the setback automatically — a genuinely good piece of design.

Backfill in lifts. Fill and compact 6–8 inches at a time as you go up, not all at once at the end. Dumping 3 feet of soil behind a finished wall shoves it forward.

Don't over-compact right behind the blocks. A plate compactor an inch from the wall can push it out of alignment. Compact the zone further back and hand-tamp near the face.

When you need an engineer

Rules of thumb, and check locally because they vary:

Over ~4 feet — most jurisdictions require an engineered design and a permit.

Any surcharge — a driveway, pool, structure, or parking above the wall changes the loads completely, at any height.

Slope above the wall — a hill behind it adds load that a flat-ground design doesn't account for.

Tiered walls — the upper wall loads the lower one. Two 3-foot walls close together are not two independent 3-foot walls; they behave more like one 6-foot wall.

A failed retaining wall doesn't just cost the rebuild — it can take the slope, the fence, and the neighbour's yard with it. That's when the engineering fee looks cheap.