Landscaping · updated 2026
Retaining Wall Calculator
Retaining wall material counts depend on the block size, cap size, and base depth you actually plan to use. Enter those dimensions to get courses, blocks to buy with waste, cap blocks, base gravel, and optional drain pipe. Walls over 4 ft tall often need engineering and permits; this calculator is for planning typical landscape walls.
TBD — enter your local block price
TBD — enter your local cap price
TBD — enter your local gravel price
TBD — enter your local pipe price
Show the math
Courses = ⌈3 × 12 ÷ 8⌉ = 5
Blocks per course = ⌈20 × 12 ÷ 16⌉ = 15
Blocks to buy = 5 × 15 × (1 + 10% waste) = 83
Caps = ⌈20 × 12 ÷ 12⌉ = 20
Base gravel = 20 × 1 × (6 ÷ 12) ÷ 27 × 1.4 tons/yd³ = 0.52 tons
Drain pipe = 20 ft when drainage is included. Cost inputs are local/TBD.
How to use the retaining wall calculator
Enter the finished wall length and height, then use the block dimensions printed on the product packaging. Courses are the wall height divided by block height, rounded up. Blocks per course are the wall length divided by block length, also rounded up. The default 10% waste allowance covers cuts, breakage, and pieces that do not fit cleanly at corners.
Turn on cap blocks when the wall has a finished top. Enter the cap length so the calculator can count the top course, and enter local block, cap, gravel, and pipe prices if you want a cost total.
Block types and what this assumes
This calculator assumes modular interlocking segmental wall blocks with a regular face and a separate cap course. Natural stone walls use irregular pieces and timber walls use posts, deadmen, and different drainage details, so neither should be substituted into these dimensions without a separate takeoff.
Interlocking systems also vary in setback, actual face dimensions, and manufacturer requirements. Use the product’s actual block height and length rather than relying on a nominal label.
Why cap blocks matter
Caps protect the top course, shed water, and make the wall look finished. Cap count is the wall length in inches divided by cap length, rounded up. A 20-ft wall with 12-in caps needs 20 caps before cuts; the separate cap cost line helps keep the finish from disappearing inside a single material total.
Corners, curves, and cuts can require extra caps. The basic count is a straight-run planning quantity, so add site-specific pieces after laying out the top course.
Base gravel and drainage
The base is not decorative fill. It provides a level, compacted seat for the first course and helps keep the wall from settling. The calculator uses wall length × base width × base depth, converts the volume to cubic yards, and multiplies by the 1.4-ton-per-yard planning assumption.
Drainage relieves hydrostatic pressure behind the wall. A perforated pipe at the base, clean gravel, and a suitable outlet are site details; use the drain toggle for a pipe-length planning quantity and confirm the design for your soil and wall height.
Height, cost, and site risk
Taller walls use more courses and put more pressure on the base and backfill. Local rules often treat walls over about 4 ft differently, and slopes, surcharge loads, poor soil, and property-line conditions can require engineering. Price per linear foot is therefore only a comparison measure; actual block, excavation, base, drainage, access, and labor costs remain local/TBD.
Get a supplier or installer quote that states the block system, cap profile, base depth, drainage, excavation, and disposal.
Common mistakes and assumptions
The frequent misses are no waste for cuts, a shallow or unlevel base, and no drainage outlet. The calculator counts material quantities for a regular straight wall; it does not design reinforcement, batter, geogrid, or a stepped hillside wall. Verify the manufacturer’s installation guide and local code before building.
What the numbers mean
The calculator converts your measurements into a practical purchase quantity using a documented coverage or spacing assumption.
Any whole-unit quantity is rounded up so the result does not leave you short during the project.
Use the cost range as a planning baseline and confirm the final quantity, availability, and delivery charge with your local supplier.
2026 material prices
Block, cap, gravel, and pipe prices are TBD; enter local prices for a planning total. 2026 U.S. national averages. Prices vary by region and retailer — confirm locally before buying.
Pro tips
- Drainage is the wall — use clean gravel behind the blocks and a perforated pipe at the base.
- Bury the first course and keep it level; every course above follows the foundation you set below.
- Use the actual block dimensions from the product label; nominal sizes vary by retaining-wall system.
- Step the wall base on a slope instead of sloping the blocks; each course needs a level seat.
- Glue cap blocks with landscape adhesive after the wall has settled and the top course is clean.
- Walls over 4 ft tall can require engineering or permits; verify local code before excavation.
Common questions
Frequently asked questions
How many blocks for a 20×3 retaining wall?
With 16×8-in blocks, 5 courses × 15 blocks per course = 75 raw blocks; with 10% waste, buy 83 blocks.
How many cap blocks do I need?
For 20 ft with 12-in caps, 20 caps cover the run. The calculator does not add cap waste unless you account for cuts.
How many blocks for a 24×4 wall?
With 16×8-in blocks, 6 courses × 18 blocks per course = 108 raw blocks; with 10% waste, buy 119 blocks.
How deep should the gravel base be?
The default is 6 in. Use the depth required by your block manufacturer and local soil conditions.
Do I need drainage behind a retaining wall?
Most landscape retaining walls need free-draining gravel and a perforated pipe to relieve water pressure.
What does a retaining wall cost?
Costs depend on local block, cap, gravel, and pipe prices. Enter your local prices above for a planning total.
Related calculators
Planning estimates only. Prices are 2026 U.S. national averages and vary by region — confirm with your local supplier before buying.