How Many Blocks for a Retaining Wall?

There's a course you can't see

A 20 ft wall, 24 inches above ground, built from 12 × 6 inch blocks:

100 blocks — not 80.

The difference is the buried first course. Standard practice is to set the bottom row fully below grade on a compacted base, which anchors the wall against soil pushing it forward. That's five courses bought, not four — 25% more, and it's very easy to leave out of an estimate and impossible to leave out of the build.

The arithmetic

Blocks per course — wall length in inches ÷ block length, rounded up:

20 ft × 12 = 240 in ÷ 12 = 20 blocks

Courses — exposed height ÷ block height, rounded up, plus the buried one:

24 ÷ 6 = 4, + 1 = 5 courses

Total: 20 × 5 = 100 blocks, plus 20 caps (one more course's worth, usually a different product).

Rounding is charged per course

Make that wall 20 ft 6 in and you need 21 blocks per row rather than 20.5.

Across five courses that's five extra blocks, not one — because the rounding applies to every row independently.

Which means it's often worth nudging the wall length to land on a whole number of blocks, if the design allows. Offcuts from split blocks are less useful than they look.

The part a calculator can't answer

Wall height is a safety question, not an arithmetic one.

Most jurisdictions require an engineered design and a permit above roughly 3 or 4 feet — and the threshold drops if there's a slope above the wall, a driveway, or anything else surcharging it.

A retaining wall holds back a large mass of soil that gets substantially heavier when saturated, and the failure mode is sudden rather than gradual. This page counts blocks. It says nothing about whether your wall will stand, and no calculator can.

Ask your building department before you build, not after.

Drainage is what actually fails walls

Not the block count. Almost never the block count.

Walls fail because water collected behind them with nowhere to go — and saturated soil pushes far harder than dry soil does. The aggregate and the perforated pipe behind the wall aren't optional extras a tight budget can drop. They're the part doing the work the blocks get credit for.

And it's worth deciding where the water goes at the end of that pipe before you start, rather than discovering it's your neighbour's lawn.

What else isn't in the count

• Compacted gravel base — depth depends on your soil
• Drainage aggregate and pipe
• Geogrid reinforcement, where the design calls for it
• Adhesive for the cap row

Setback

Most segmental blocks are designed so each course steps back slightly from the one below, leaning the wall into the slope.

That doesn't change the block count — but it does mean the wall isn't vertical. Over a tall wall the top sits back from the bottom by a noticeable amount, which is worth checking against where you actually want the finished face before you set the base course.

Curves

Not accurately handled here. Blocks fan on a curve, so the effective count changes with the radius and with which way the curve turns — an outside curve needs more blocks per foot, an inside curve fewer. Many block systems have a taper designed for exactly this.

Use this for the straight runs and check your manufacturer's guidance for the rest.

Before any of it

Locate the utilities. Then confirm the property line, especially if the wall is near it — a retaining wall is not a thing you want to move afterwards.

Print the build sheet. The block count is one column; the checklist beside it is the half that decides whether the wall is still there in ten years.