Retaining Wall Calculator
Calculate blocks, courses, gravel backfill, and cost for your retaining wall project.
Quick Answer
For standard 16"×6" blocks, a 20 ft long × 3 ft tall wall needs about 90 blocks (99 with 10% waste), 15 cap blocks, and ~1.5 yd³ of gravel backfill — block units alone commonly run about $4–$20 per sq ft of wall face (approximately $240–$1,200 for a 20 × 3 ft wall); use the calculator's itemized estimate for your dimensions and block price.
Wall Dimensions
Block Size
Enter wall dimensions and block size to see results.
💡 Permit and engineering rules are local, and height is only one design factor — check local codes.
How to Calculate Retaining Wall Blocks
Formula
(Wall Length ÷ Block Width) × (Wall Height ÷ Block Height) = Blocks
Convert wall length and height to inches. Divide length by block face width for blocks per row. Divide height by block height for number of courses. Multiply to get total blocks. Add 10% for cuts and waste.
Example
For a 20 ft long × 3 ft tall wall using 16" × 6" blocks: 240" ÷ 16" = 15 blocks per row. 36" ÷ 6" = 6 courses. Total: 15 × 6 = 90 blocks (99 with 10% waste).
Tips & Best Practices
- •Dig a trench 6 inches deep and twice the block depth wide. Fill with compacted gravel.
- •Bury the first course at least halfway — this is critical for stability.
- •Backfill behind segmental block walls with free-draining gravel (not soil), at least 12 inches behind the units, and follow your wall system or engineered plans for the details. Add landscape fabric to prevent soil migration.
- •Install a drain pipe at the base behind the wall if your wall system or design calls for one, and grade the site so surface water is directed away from the wall.
- •Step back each course slightly (about ½ inch) for a natural batter angle.
Frequently Asked Questions
How much does a retaining wall cost?+
Block units alone commonly run about $4–$20 per sq ft of wall face (approximately $240–$1,200 for a 20 ft × 3 ft, 60 sq ft wall). A complete DIY wall also needs base and drainage aggregate, caps, fabric, delivery and potentially reinforcement, so total material cost depends heavily on the wall system and site. A straightforward installed modular/interlocking wall is approximately $15–$35 per sq ft (approximately $900–$2,100 for the same wall); site preparation, drainage, reinforcement, engineering, delivery and difficult access can push a project above this range. Use the calculator's itemized estimate for your dimensions and selected block price.
Do I need a permit for a retaining wall?+
Permit rules are local. As a reference point, the 2024 International Residential Code exempts retaining walls not over 4 feet from a building permit when measured from the bottom of the footing to the top of the wall, unless the wall supports a surcharge. Your jurisdiction may use a different code edition or more restrictive rules, so check locally before building.
How much gravel do I need behind a retaining wall?+
For segmental retaining wall systems, CMHA guidance calls for at least 12 inches of free-draining gravel fill behind the facing units; follow your wall-system manufacturer or engineered plans for the exact detail. The calculator also estimates a 6-inch deep gravel base extending 6 inches beyond the block depth as a planning assumption.
What type of block is best for retaining walls?+
Interlocking concrete blocks (like Allan Block or Versa-Lok) are the most popular for DIY. They stack without mortar and use a lip or pin system for setback. Natural stone is more attractive but harder to work with.
Common Retaining Wall Projects
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Get Local Quotes →How Many Retaining Wall Blocks Do I Need?
Divide the wall length by the block length to get blocks per row, then multiply by the number of courses (rows of height). A 20 ft wall that is 3 ft high with 16"×6" or 12"×8" blocks (about 0.67 sq ft of face each) needs approximately 90 blocks, or 99 including a 10% waste allowance.
Use the table below to quickly estimate block count for common wall sizes using standard 12"×8" retaining wall blocks.
| Wall Size (L × H) | Blocks (incl. 10% waste) | Gravel Backfill | Block Units Alone |
|---|---|---|---|
| 10 ft × 2 ft | ~33 | ~0.5 yd³ | $80–$400 |
| 20 ft × 3 ft | ~99 | ~1.5 yd³ | $240–$1,200 |
| 30 ft × 3 ft | ~149 | ~2.2 yd³ | $360–$1,800 |
| 40 ft × 4 ft | ~264 | ~4 yd³ | $640–$3,200 |
Block counts are estimated from wall-face area and include 10% waste; the calculator rounds to whole courses, so its result for your dimensions may be slightly higher. The block-unit column is a broad comparison range of $4–$20 per square foot of wall face (material only — no base, drainage gravel, caps, fabric, geogrid, delivery, excavation, labor, engineering or permits). Use the calculator's itemized estimate for your dimensions and selected block price.
How Much Does a Retaining Wall Cost?
Block units alone commonly run about $4–$20 per square foot of wall face. A straightforward installed modular/interlocking concrete wall is approximately $15–$35 per square foot, and labor alone is approximately $10–$25 per square foot.
For a 20 ft × 3 ft wall (60 sq ft of face), that is approximately $240–$1,200 for block units alone, or approximately $900–$2,100 installed. A complete DIY wall also needs base and drainage aggregate, caps or finishing units, fabric, delivery and potentially reinforcement, so total cost depends heavily on the wall system and site. Excavation, difficult access, weak soils, drainage work, reinforcement, engineering and taller walls can raise the total substantially.
Concrete Block Walls
Most popular DIY option. Interlocking blocks (no mortar): block units alone commonly run about $4–$20 per sq ft of wall face, with premium systems toward the upper end. Often a practical DIY choice for shorter, straight walls. Check your wall system's height limits.
Natural Stone Walls
Premium look with fieldstone or flagstone. $8–$20 per sq ft of face. More labor-intensive but no two walls look alike.
Timber / Railroad Tie Walls
Budget-friendly at $5–$10 per sq ft. Easier to install on curves. Service life depends on treatment, moisture, drainage and soil contact — roughly 20–40 years under favorable conditions.
Do I Need Gravel Behind a Retaining Wall?
Yes. For segmental retaining wall systems, CMHA guidance calls for at least 12 inches of free-draining gravel fill behind the facing units. Follow the wall-system manufacturer or engineered plans for the exact material and drainage detail.
Drainage is one of the most important factors in retaining wall performance. These steps describe a typical segmental block wall; details vary by wall system and site:
The gravel zone and drain pipe are intended to manage incidental water behind the wall. They are not a substitute for controlling surface runoff or significant groundwater. Grade the site so water is directed away from the wall and use the drainage design required for the site, as CMHA design guidance describes. Drainage details depend on wall height, soil, groundwater, slopes and the wall system; reinforced walls and sites with poor drainage typically require more deliberate drainage design.
Permits, Wall Height and Engineering
Permit rules are local. As a reference point, the 2024 International Residential Code exempts retaining walls not over 4 feet from a building permit when measured from the bottom of the footing to the top of the wall, unless the wall supports a surcharge. Your jurisdiction may use a different code edition or more restrictive rules, so check locally before building.
Height is only one design factor. Soil conditions, slopes, groundwater, wall geometry and surcharge loads such as driveways, structures or stored materials can increase wall forces even on a shorter wall. A surcharge is an added load above or behind the wall; driveways, parked vehicles, buildings, pools and slopes can materially change the design. Where local requirements do not specify otherwise, CMHA recommends an engineered design for segmental retaining walls with a total design height over 4 feet. Retaining-wall design height may include more than the visible wall face, depending on embedment and site geometry. For walls with significant surcharge, poor soils, drainage problems, steep slopes or unusual geometry, professional design may be appropriate at a lower exposed wall height.
Geogrid requirements depend on the retaining-wall system, wall height, soil, slopes and surcharge loads. Follow the block manufacturer's reinforcement chart or an engineered wall design rather than using one universal height threshold. A taller wall does not automatically use the same geogrid layout as another wall of the same height; soil and loading conditions matter.
Before You Order
- ☐Permit and engineering rules are local — check with your building department before buying anything
- ☐Plan your drainage layer (gravel + fabric behind the wall) before ordering block
- ☐Measure total face area (length × height), not just the footprint
- ☐Taller walls, poor soils or surcharge loads may require geogrid per your block manufacturer's chart or an engineered design — this affects your block count
- ☐Check block availability and lead time locally before committing to a color or style
Ordering in bulk?
Crushed stone base and crusher run are available for bulk delivery — typically more cost-effective than bags for a full wall project. Compare local pricing at AggregateMarkets.
Prices and availability vary by location. Use code MEADOWLARK for 5% off your first bulk order.