Retaining Wall Block Calculator
Enter your wall height, soil type, and block size to get the minimum base width, footing depth, block count, and a stability safety factor. Nothing uploaded.
Stability verdict
StableLearn more: Retaining wall design and soil mechanics
Why retaining walls fall over
Soil exerts a horizontal force on any wall that contains it. This force increases with height, with soil density, and dramatically with water content. A wall that passes the dry-soil stability calculation can fail catastrophically when saturated by rain - which is why drainage behind the wall is not optional.
Wall design in three steps
Enter wall height and soil type (loose, sandy, clay, or compacted gravel), each with different unit weight and friction properties. Specify block dimensions and any surcharge (foot traffic or vehicle load). The calculator returns stability verdict, minimum base width, footing depth, total block count, and safety factor.
FAQ
How wide should the base of a retaining wall be?
A gravity retaining wall (concrete blocks stacked without reinforcement) needs a base width of approximately 50-70% of the wall height to resist overturning. For a 1m wall, the base should be at least 500-700mm wide. Clay soil requires a wider base than granular soil. The calculator computes the minimum base width for your specific soil type and surcharge conditions.
Why do retaining walls fail?
The most common cause of retaining wall failure is water pressure buildup behind the wall. Saturated soil exerts 2-3x more lateral force than dry soil. A perforated drainage pipe and gravel backfill behind the wall is essential - the calculator includes a prominent reminder about this. The second most common cause is insufficient base width or footing depth.
Do I need planning permission for a garden retaining wall?
In most UK and US jurisdictions, garden retaining walls under 1m (0.5m adjacent to a highway) don't require planning permission. Walls over 1m should ideally be designed by a structural engineer. The calculator shows a warning when the wall exceeds 1.2m and recommends professional consultation.