Retaining Wall Block Calculator
Block and cap counts for a segmental retaining wall, base stone, and the height where you need an engineer.
6 courses × 40 per course, plus 5%
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How to work it out yourself
- 1
Bury the first course
The bottom course goes below grade — roughly one inch for every foot of wall, with 6 inches as a common minimum. It is what stops the base of the wall being pushed out. Include the buried course in your block count; it is real block you have to buy.
- 2
Courses, then blocks per course
Total height including the buried course, divided by block height, gives the number of courses. Wall length in inches divided by block length gives the blocks per course. Multiply the two and add about 5% for cuts at corners and ends.
- 3
Base stone twice the block width
The levelling pad is 6 inches of compacted crushed stone, and about twice as wide as the block is deep, so the wall is not standing on its own edge. That is where the wall is won or lost — an unlevel base shows in every course above it.
Worked example: A 40 ft wall, 36 in exposed, 12 × 8 in block
| Total height | 36 + 6 buried = 42 in |
| Courses | 42 ÷ 8 = 6 courses |
| Blocks per course | (40 × 12) ÷ 12 = 40 |
| Blocks + 5% | 6 × 40 × 1.05 = 252 |
| Cap blocks | (40 × 12) ÷ 12 = 40 |
| Base stone | 6 in deep × 20 in wide ≈ 3.1 tons |
Around 252 wall blocks, 40 caps and 3 tons of base stone, plus drainage stone and fabric behind the wall.
Four feet is where it stops being a landscaping job
Above roughly 4 ft of exposed height, most jurisdictions require an engineered design and a permit, and manufacturers require geogrid tied back into the soil every second or third course. Quoting a 5 ft wall as a straight block count means quoting a job you cannot legally build — and the engineering, the grid and the extra excavation are thousands, not hundreds. Check your local threshold before you price it, because it varies: some places set the line at 3 ft, some at 4, and surcharge from a driveway or slope above the wall lowers it further.
Turn this into a quote your client can accept
You have the quantities. What wins the job is a priced, branded estimate in your client’s hands before the other two bidders get back to them.
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- See the moment your client opens it
Common questions
How many blocks do I need for a retaining wall?
Divide the total wall height, including the buried course, by the block height for the number of courses, then divide the wall length by the block length for blocks per course, and multiply. A 40 ft wall 36 inches high in 12 × 8 in block takes about 252 blocks plus 40 caps.
How deep should the first course of a retaining wall be buried?
About one inch for every foot of wall height, with 6 inches as a practical minimum. On a slope, or where there is a surcharge above the wall, bury more.
How high can a retaining wall be without an engineer?
Commonly 4 ft of exposed height, but it varies by jurisdiction and some set the line at 3 ft. Any surcharge above the wall — a driveway, a slope, a structure — lowers the threshold. Check locally before quoting; it is the difference between a landscaping job and a permitted, engineered one.
What goes behind a retaining wall?
Free-draining crushed stone, at least 12 inches deep, separated from the soil with geotextile, and a perforated drain pipe at the base daylighting to somewhere sensible. Water pressure behind a wall is what pushes it over; the block is rarely the thing that failed.
Do retaining walls need geogrid?
Above about 3 to 4 ft, yes — usually every second or third course, extending back into the soil several feet. The exact spacing and length come from the block manufacturer's tables or the engineer, not from a rule of thumb.
Put this calculator on your own site
Free to use anywhere — a supplier’s resources page, a builder’s blog, a trade forum answer. It works on a phone, needs no account from your visitors, and we will not change it out from under you. All we ask is that you keep the credit line underneath, which is the second half of the snippet.
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