Segmental Retaining Wall Blocks: Design Guide

Segmental retaining walls (SRW) use dry-stacked interlocking concrete units — no mortar, no footing pour. They're the dominant residential retaining wall because they're DIY-friendly, flexible and well-documented in published design tables.

How SRW design works

Each block course steps back slightly (the batter), which shifts the wall's weight-line backward to help it resist overturning. Manufacturers publish design tables that show the maximum wall height for a given block and base-width configuration. The NCMA Design Manual (National Concrete Masonry Association) is the industry standard.

The geogrid threshold

For walls under about 3.5–4 ft with no surcharge, a gravity SRW (no geogrid) is usually adequate. Above that, or with a surcharge, you add geogrid: a polymer mesh laid between block courses and extending back into the compacted backfill. The geogrid ties the soil mass to the wall, turning the combined unit into what the NCMA calls a mechanical earth stabilization (MSE) system.

Our calculator determines whether gravity alone passes the stability checks or whether the solution requires geogrid, using the NCMA methodology.

Drainage is always required

Every SRW design assumes free-draining backfill. If water builds up behind the blocks, the actual lateral pressure can far exceed the design value. Install drain rock, a 4-inch perforated pipe to daylight, and filter fabric on every SRW wall — the calculator includes these in the materials takeoff.

Past 4 ft

Reinforced SRW walls above 4 ft need an engineer's stamp in most jurisdictions, even though the block system itself is well-documented. The engineer specifies the geogrid type, length and placement for your specific soil.