Retaining Wall Block Calculator
Count the wall blocks, courses, and cap blocks for a segmental retaining wall from the wall length, height, and block face size.
Reviewed by the ToolNestr Editorial Team — July 2026
How the retaining wall block calculator works
A segmental retaining wall is built from interlocking blocks stacked in level rows called courses. The calculator works out how many blocks fit in one course by dividing the wall length by the block's face length, rounding up so the ends are covered. It then divides the wall height by the block height to find how many courses tall the wall is.
Multiplying blocks per course by the number of courses gives the wall blocks. Most walls finish with a separate cap course — flat capstones bonded on top — so the calculator adds one more course of caps when you leave that option checked. The result is split into wall blocks and cap blocks because they are often different products ordered separately.
Remember to bury the bottom course. Good practice sets the base course below grade, roughly one-tenth of the wall height, on compacted gravel, so include that buried height in your input. This tool counts blocks only; base gravel, drainage stone, and geogrid reinforcement for taller walls are ordered separately.
Segmental retaining wall design and construction guidance referenced here follows NCMA (National Concrete Masonry Association) Segmental Retaining Wall standards.
Enter wall size
Wall length in feet and total height in inches, including the buried base course.
Enter block size
Use your block's face length and height from the box or spec sheet.
Read the counts
Get wall blocks and cap blocks, plus blocks per course and course count.
The formula explained
Blocks per course
= wall length ÷ block length. A 30 ft wall with 12″ blocks: 360⁄12 = 30 blocks.
Courses
= wall height ÷ block height. A 36″ wall with 6″ blocks: 36⁄6 = 6 courses.
Totals
Wall blocks = 30 × 6 = 180; caps = 30; grand total = 210 blocks.
Worked example
A 45 ft garden wall, 24″ tall, using 18″ long × 8″ high blocks, with caps.
Building tips
Start with a solid base
The whole wall rides on the base course. Set it dead level on a compacted gravel pad; a base that is out of level multiplies its error all the way up the wall.
Drain behind the wall
Water pressure topples retaining walls. Backfill with drainage stone and run a perforated pipe at the base so water escapes instead of building up behind the blocks.
Set back and reinforce
Most SRW systems step each course back slightly to lean into the slope. Walls over about 4 feet usually need geogrid layers tied into the backfill.
Check permits
Taller walls and walls holding a slope commonly need a permit and an engineered design. Confirm the threshold locally before you start stacking.
Real-world context
A backyard terrace wall is a typical mid-size job for this calculator. A 60 ft wall at 18 inches tall, built from standard 12″ long × 4″ high blocks, needs 60 × 12 ⁄ 12 = 60 blocks per course and 18 ⁄ 4 = 5 courses (rounded up), for 300 wall blocks. Adding one cap course of 60 more brings the total to 360 blocks — a big enough order that most homeowners call ahead to confirm the supplier has that quantity of a single color and style on hand.
A small garden bed edging shows how quickly the numbers shrink. A 15 ft run at 8 inches tall, using the same 12″ × 4″ block, needs 15 blocks per course and 2 courses, for 30 wall blocks plus 15 caps — 45 blocks total, small enough to fit in the back of a pickup and often cheaper to buy loose than palletized.
At the tall end, an 80 ft engineered wall 48 inches high, using larger 18″ × 6″ blocks, needs 80 × 12 ⁄ 18 = 54 blocks per course (rounded up) and 48 ⁄ 6 = 8 courses, for 432 wall blocks plus 54 caps, or 486 total. A wall this tall sits well above the roughly 4 ft threshold where most jurisdictions require an engineered design, a permit, and geogrid soil reinforcement behind the blocks — the block count here is only part of the material list.
Common misconceptions
"The wall height I build above ground is all I need to plan for." Leave out the buried course and you'll come up short. Standard SRW practice buries the bottom course below grade — roughly one-tenth of the total wall height — on a compacted gravel base for stability. That buried course needs blocks too, so it belongs in the height you enter, not just the visible portion.
"Stacking blocks straight up is fine as long as they're level." Most SRW systems are designed to lean back into the slope. Each course typically sets back a fraction of an inch from the one below it, and this batter, combined with geogrid reinforcement on taller walls, is what actually resists the soil pressure pushing outward — a perfectly vertical stack of blocks is more prone to tipping over time.
Related calculators
Frequently asked questions
How many blocks do I need for a retaining wall?
Divide the wall length by the block face length to get the blocks per course, then divide the wall height by the block height to get the number of courses. Multiply the two together for the wall blocks, and add one course of cap blocks on top.
What size are retaining wall blocks?
Segmental retaining wall (SRW) blocks vary by brand, but common face sizes are about 12 or 18 inches long and 4, 6, or 8 inches high. Check the box or spec sheet for your exact block and enter those dimensions.
Do I need a buried base course?
Yes. Standard practice buries the bottom course about one-tenth of the wall height, or at least a few inches, on a compacted gravel base. Include that buried course in your height so it is counted.
When does a retaining wall need engineering?
Many codes require an engineered design and a permit for walls over about 4 feet, or for any wall holding back a slope or surcharge. Taller walls also need geogrid reinforcement. Always confirm with your local building department.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.