Retaining Wall Block Calculator

How this is calculated

Blocks per course is the wall length in inches divided by the block length, rounded up. Courses is the exposed height divided by the block height, rounded up, plus one if you are burying the first course. Total blocks is those multiplied together, and the cap row is one more course's worth. The buried first course is why this is not just height divided by block height. Standard practice for a segmental wall is to set the bottom course fully below grade on a compacted base, which anchors the wall against the soil pushing it forward. On a 24 inch exposed wall of 6 inch blocks that is five courses bought, not four - a 25 percent difference that is very easy to leave out of an estimate and impossible to leave out of the build. Both counts round UP, and they round up per course, so an awkward wall length is paid for on every single row. A 20 foot 6 inch wall of 12 inch blocks needs 21 blocks per course rather than 20.5, and across five courses that is five extra blocks rather than one. ⚠️ WALL HEIGHT IS A SAFETY QUESTION, NOT AN ARITHMETIC ONE. Most jurisdictions require an engineered design and a permit above a certain height - commonly around 3 or 4 feet, sometimes lower with a slope or a surcharge above the wall. A retaining wall holds back a large mass of saturated soil, and the failure mode is sudden. This calculator counts blocks; it says nothing about whether your wall is safe, and no calculator can. Also not modelled, and all of it matters more than the block count: the compacted gravel base, drainage aggregate and pipe behind the wall, geogrid reinforcement where the design calls for it, the setback or batter each course steps back by, and adhesive for the cap row. Drainage is the one that fails walls - water behind a wall with nowhere to go is what pushes it over. The calculation also assumes a straight wall. Curves change the effective block count because the blocks fan, and corners need their own treatment.

Common questions

Why is there an extra course?

Because standard practice is to bury the bottom course fully below grade on a compacted base. It anchors the wall against soil pushing it forward, and without it the wall has nothing but its own weight resisting the load at the point where the load is greatest. On a 24 inch exposed wall of 6 inch blocks that is five courses rather than four - 25 percent more blocks, which is easy to leave out of an estimate and impossible to leave out of the build.

How tall can I build without an engineer?

That is a question for your local building department, not a calculator. Many jurisdictions require an engineered design and a permit above roughly 3 or 4 feet, and the threshold drops if there is a slope above the wall, a driveway, or anything else surcharging it. A retaining wall holds back a large mass of soil that gets much heavier when saturated, and the failure mode is sudden rather than gradual. Ask before you build, not after.

What is not included in the block count?

Everything that actually makes the wall work: the compacted gravel base, drainage aggregate and pipe behind it, geogrid reinforcement where the design calls for it, and adhesive for the caps. Drainage is the one that fails walls - water trapped behind a wall with nowhere to go generates far more pressure than the soil alone, and it is the most common cause of a wall leaning out.

Why does an awkward length cost so much?

Because the rounding applies per course, not once. A 20 foot 6 inch wall of 12 inch blocks needs 21 blocks per row rather than 20.5, and over five courses that is five extra blocks rather than one. It is usually worth adjusting the wall length slightly to land on a whole number of blocks if the design allows - the offcuts from splitting blocks are less useful than they look.

What about the setback?

Most segmental blocks are designed so each course steps back slightly from the one below, which leans the wall into the slope. That does not change the block COUNT, but it does mean the wall is not vertical, so the top sits back from the bottom by a noticeable amount over a tall wall - worth checking against where you actually want the finished face to be before you set the base course.

Does this work for a curved wall?

Not accurately. 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 than a straight run, an inside curve fewer. Many block systems are designed with a taper for exactly this. Use this for the straight sections and check your manufacturer's guidance for the curves.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Retaining Wall Block Calculator at https://www.bfcbrilliance.com/tools/retaining-wall-block-calculator.

What I entered:
- Wall length (ft): ___
- Height ABOVE ground (in): ___
- Block length (in): ___
- Block height (in): ___
- Bury the first course: ___

What it calculated:
- Blocks to buy: ___
- Blocks per course: ___
- Courses: ___
- Cap blocks: ___
- Total wall height built: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Where the job touches structure, gas, or electrics, tell me to check local building code and whether a permit or a licensed trade is required.

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