Retaining Wall Calculator
Estimate segmental wall blocks by course, including buried courses, optional cap length, base aggregate, and drainage-zone aggregate.
Total run of this wall section. Calculate unlike wall sections separately and add the material lists.
Enter the exposed wall height above the front grade. Buried courses are added separately below.
Choose the face length and height of the wall unit. Whole blocks are counted by course, not by an averaged blocks-per-square-foot rate.
How to Use
- Enter visible height - use the wall height above the front grade. The calculator adds the selected buried courses instead of pretending they are part of the visible face.
- Choose the actual block face - whole blocks are counted across each course and whole courses are counted vertically, so 12×6, 18×8, and 18×12 units no longer share one generic course assumption.
- Set buried courses - use the embedment required by your wall system or project design.
- Enter cap length if known - caps are calculated from their real coverage length rather than assuming every cap is 12 inches.
- Check base and drainage dimensions - open Advanced and replace the planning defaults with the dimensions specified for the actual wall.
A 20 foot wall with 3 feet of visible height, 12×6 inch wall blocks, one buried course, and a 10 percent order allowance needs 154 wall blocks in this course-based model. That is 6 visible courses plus 1 buried course, with 20 blocks in each course before allowance. Cap quantity depends on the cap length you enter. With the default 18×6 inch base and 12 inch drainage zone, the aggregate estimate is about 0.56 yd³ for the base and 2.59 yd³ behind the full 3.5 foot block stack.
Typical Segmental Wall Material Zones
Why Course-Based Counting Matters
Retaining-wall blocks are installed in whole courses. An area-only blocks-per-square-foot shortcut can miss the extra material created by course rounding and can ignore the buried course entirely. This calculator rounds the visible height up to whole courses, adds the selected buried courses, then rounds the wall length to whole blocks in every course before applying the order allowance.
That matters when block sizes change. A 12×6 inch face uses a 6 inch course height, an 18×8 uses an 8 inch course height, and an 18×12 uses a 12 inch course height. The old idea that every wall had two courses per vertical foot is only true for a 6 inch high unit.
12×6 Block Example With One Buried Course
| Visible height | 10 ft wall | 20 ft wall | 50 ft wall | 100 ft wall |
|---|---|---|---|---|
| 1 ft: 2 visible + 1 buried | 33 | 66 | 165 | 330 |
| 2 ft: 4 visible + 1 buried | 55 | 110 | 275 | 550 |
| 3 ft: 6 visible + 1 buried | 77 | 154 | 385 | 770 |
| 4 ft: 8 visible + 1 buried | 99 | 198 | 495 | 990 |
| 5 ft: 10 visible + 1 buried | 121 | 242 | 605 | 1,210 |
Includes a 10% order allowance and assumes 12 in block face length, 6 in block face height, and one full buried course. Change block size, embedment, or allowance in the calculator for the actual wall.
Base and Drainage Aggregate
The base result uses the entered base width and compacted depth. The drainage-zone result uses the entered aggregate width multiplied by the full calculated block-stack height, including buried courses. These are material volumes only; they do not design the wall foundation, drainage system, or reinforced soil zone.
CMHA guidance describes free-draining gravel fill behind segmental retaining wall units and notes that drainage features depend on site-specific groundwater and wall conditions. A drain pipe is therefore not treated here as an automatic requirement for every garden wall. The wall designer or system detail should determine where drainage components belong.
Retaining Wall Formulas
The calculator uses whole-course geometry:
Total courses = visible courses + selected buried courses
Blocks per course = ceil(wall length × 12 ÷ block face length)
Wall blocks = ceil(blocks per course × total courses × (1 + allowance %))
Cap blocks = ceil(wall length × 12 ÷ entered cap length)
Base aggregate = length × base width × base depth, converted to cubic yards
Drainage-zone aggregate = length × drainage width × total block-stack height, converted to cubic yards
Course rounding makes the block count intentionally different from a simple wall-face-area division. Final wall design still depends on the actual block system, site geometry, soils, drainage, surcharge, and reinforcement requirements.
Design and Installation References
- CMHA guidance says segmental retaining wall design depends on site geometry, soils, surcharges, drainage, and wall-system properties. It recommends qualified engineering for walls or conditions that exceed the system and project design limits, and notes that drainage features are site-specific.CMHA - Segmental Retaining Wall Design
- Allan Block residential installation guidance uses a compacted aggregate base, buried block, and wall rock behind the units, while its drain-pipe requirements vary with wall height, reinforcement, soil, and drainage conditions. Use the actual product instructions rather than treating one generic cross-section as universal.Allan Block - Basic Retaining Wall Installation
Next Steps
Once the block takeoff is set, quantify the other site materials separately:
Related Calculators
Retaining-wall takeoff connects blocks, aggregate, and grading. Browse the full construction calculator collection.
Frequently Asked Questions
Why does this calculator count a buried course?
A buried base course still has to be purchased. The calculator therefore keeps visible wall height separate from embedment and adds the buried courses you select. Use the actual wall-system or design requirement for embedment rather than assuming one course always applies.
Why does block size change the course count?
The face height controls how many whole courses are needed. A 6 inch high block uses two visible courses per vertical foot, while 8 inch and 12 inch units use different course counts. The calculator rounds up to whole courses before counting blocks.
How are cap blocks calculated?
Enter the actual cap coverage length in inches. The calculator divides wall length by that cap length and rounds up. If the cap product is not selected yet, leave the field blank instead of assuming one cap per foot.
How much drainage gravel do I need?
The calculator multiplies wall length by the drainage-zone width you enter and by the full block-stack height, including buried courses. CMHA guidance commonly describes at least 12 inches of gravel fill behind segmental units, but the site-specific wall design and product system control the final detail.
Do I always need a perforated drain pipe?
No universal yes/no answer belongs in a material calculator. Drainage requirements depend on wall height, groundwater, soil, reinforcement, grading, outlets, and the wall-system design. Follow the approved wall detail or manufacturer/engineer requirements for the actual site.
When does a retaining wall need engineering?
Height is only one trigger. CMHA recommends engineering when total design height or project conditions exceed prescriptive/system limits, and special conditions such as surcharge loads, poor foundations, live loads, slopes, utilities, or complex drainage can require engineering at lower heights. Check local requirements and the wall-system design limits.
Does this calculator work for timber, poured concrete, or natural-stone walls?
No. The calculation is specifically a segmental block material takeoff based on whole block courses. Timber, poured concrete, gabion, and natural-stone retaining systems need different quantity and structural workflows.