Retaining Wall Block Calculator
Building a retaining wall? Enter the wall length and height to see how many blocks you need, plus cap blocks and base gravel for a complete install.
Retaining Wall Block Systems
Modular retaining wall blocks are engineered for DIY installation. Each block weighs 40-80 pounds and includes a lip or pin system that locks it to the course below. The built-in setback creates a batter (backward lean) that resists soil pressure without needing mortar or rebar for walls under 4 feet.
Common systems include Allan Block, Versa-Lok, and Keystone. They vary in face texture, size, and interlock design, but all follow the same installation principles: level base, compacted backfill, and proper drainage. Some blocks are hollow and can be filled with gravel or concrete for added weight and strength.
Cap blocks finish the top course with a flat, smooth surface. They're typically half the depth of standard blocks and sit horizontally to cover the cores and provide a clean edge. Use construction adhesive to secure caps so they don't shift when someone sits on the wall or bumps it with a lawnmower.
Base and Drainage Essentials
A retaining wall is only as strong as its foundation. Excavate below grade to undisturbed soil or frost depth, whichever is deeper. Install 6-12 inches of crushed gravel base, depending on wall height. Compact it in 2-inch lifts with a plate compactor. A soft base allows settling, which leads to cracks and failure.
Drainage is non-negotiable. Water buildup behind a retaining wall creates hydrostatic pressure that can push even a well-built wall forward. Install a perforated drain pipe (4-inch corrugated with a sock) at the base, behind the blocks, sloped 1/4 inch per foot to drain to daylight or a sump. Backfill the area directly behind the blocks with 12 inches of clean gravel to create a drainage zone.
Use geotextile fabric to separate backfill gravel from native soil. This prevents soil from migrating into the gravel and clogging drainage. Wrap the fabric around the gravel zone but keep it away from the drain pipe—fabric can clog the perforations over time if it contacts the pipe directly.
When to Use Geogrid Reinforcement
Walls over 3 feet high, walls on slopes, and walls supporting heavy surcharges (driveways, buildings) need geogrid reinforcement. Geogrid is a high-strength plastic mesh that extends back into the soil behind the wall, tying the wall to a large mass of retained earth. This massively increases the wall's resistance to overturning and sliding.
Install geogrid every 2-3 courses, depending on wall height and soil conditions. Roll it out over the course of blocks, extending at least 60% of the wall height back into the fill. For a 6-foot wall, geogrid should extend 3-4 feet into the backfill. Pin it to the blocks with stakes or pins provided by the manufacturer, then bury it with compacted fill.
Geogrid installation requires careful compaction. Each 6-inch lift of backfill must be compacted before adding the next layer or the next course of blocks. This takes time but ensures the geogrid develops the tension it needs to function. Walls above 4 feet should be designed by a geotechnical engineer who specifies geogrid type, spacing, and length based on soil testing and load calculations.
Frequently Asked Questions
How high can a retaining wall block be without engineering?
Most jurisdictions allow up to 4 feet without an engineer's stamp. Walls above 4 feet need geotechnical analysis, reinforcement design, and a building permit. Don't guess on tall walls—failures cause property damage and injury.
What is the base for a retaining wall?
Excavate below grade and install 6-12 inches of compacted gravel base. The base should extend at least 6 inches beyond the front and back of the wall. Proper base compaction prevents settling and ensures drainage.
Do I need to glue retaining wall blocks?
Most modular retaining wall systems rely on weight, interlock, and setback for stability—no adhesive required. Use masonry adhesive on cap blocks to prevent shifting. For walls over 3 feet, add geogrid reinforcement between courses.
How much setback do I need per course?
Most block systems have a built-in setback of 3/4 to 1 inch per course. This backward lean (batter) helps the wall resist soil pressure. Walls without setback are weaker and more likely to fail.
What drainage goes behind a retaining wall?
Install a perforated drain pipe at the base, behind the wall, sloped to daylight. Backfill with 12 inches of gravel behind the blocks to allow water to drain to the pipe. Without drainage, hydrostatic pressure will push the wall over.