Retaining Wall Calculator
Build your retaining wall with confidence. This calculator estimates the number of blocks, cap stones, and the volume of gravel backfill needed, plus an optional cost breakdown for materials.
Total horizontal length of the retaining wall
Height of wall from base to top
Price of each retaining wall block
Depth of gravel drainage behind the wall
How many blocks do I need for a retaining wall?
Calculate total wall face area (length x height). Divide by the face area of one block. For example, a 20ft x 4ft wall with 8x16 inch blocks: area = 80 sq ft, block face = 0.89 sq ft, blocks needed = 80 / 0.89 = 90 blocks. Add 5-10% for cuts and breakage. Cap blocks are calculated separately along the wall length.
What depth of gravel is needed behind a retaining wall?
Standard gravel backfill depth is 12 inches behind the wall blocks. This drainage layer prevents hydrostatic pressure buildup. Use 3/4-inch clean crushed stone or drainage aggregate. For walls over 4ft tall, a perforated drain pipe at the base is recommended. The gravel should extend from the base up to within 6 inches of the top.
How do I calculate gravel volume for retaining wall drainage?
Gravel volume = wall length x gravel depth x wall height (all in feet). For a 20ft wall with 12in (1ft) gravel depth and 4ft height: volume = 20 x 1 x 4 = 80 cubic feet = 2.96 cubic yards. Weight is roughly 1.4 tons per cubic yard of drainage gravel.
Should I always include a cap row on a retaining wall?
Cap rows are strongly recommended for both appearance and wall integrity. Caps provide a finished look, protect the hollow cores of the top blocks from debris and water, and help lock the wall together. Without caps, water enters the block cores and can cause freeze-thaw damage. Caps also prevent weeds and dirt from accumulating in the cores.