Retaining Wall Drainage Calculator
Trapped water is the number one cause of retaining wall failure — it adds thousands of pounds of hydrostatic pressure behind the wall. Use this free retaining wall drainage calculator to estimate the length of perforated drain pipe and the volume of gravel backfill your wall needs, plus weep hole spacing for taller walls.
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Planning estimates only. Actual drainage needs vary with soil type, slope, rainfall, and local code — consult a soils report or engineer before building.
How to use this calculator
Measure your wall's length and height in feet, and enter them above. The gravel backfill width and drain rock depth default to 1 ft each, the standard drainage zone behind most segmental retaining walls — adjust them if your wall design calls for a deeper or wider zone.
Results update instantly as you type. The drain pipe figure tells you how much perforated pipe to buy (it runs the full wall length), the gravel figures tell you how much clean backfill stone to order, and the weep hole count applies to walls over 4 ft tall.
How it works
Every retaining wall needs a continuous perforated drain pipe at its base, so the pipe length equals the wall length. The gravel zone is a rectangular trench behind the wall, so its volume is a simple box calculation:
Pipe length (ft) = wall length (ft)
Gravel (cu ft) = length × backfill width × drain rock depth
Gravel (cu yd) = gravel (cu ft) ÷ 27
For weep holes, the rule is one row spaced every 8 ft on walls taller than 4 ft, with a second row added when the wall exceeds 6 ft:
Weep holes = roundup(length ÷ 8) × rows, where rows = 2 if height > 6 ft, 1 if height > 4 ft, and 0 otherwise.
These are planning estimates. Real drainage needs depend on soil type (clay drains far worse than sand), the slope above the wall, rainfall, and local building code. A soils report or structural engineer has the final word.
Frequently asked questions
How much drain pipe do I need behind a retaining wall?
One continuous run of perforated drain pipe (usually 4-inch) along the base of the wall for the full wall length. For example, a 30 ft wall needs 30 ft of pipe. Buy one extra coupling per 10 ft section, since pipe is commonly sold in 10 ft sticks.
How much gravel do I need for retaining wall backfill?
Multiply wall length by backfill width by drain rock depth. With the common 1 ft wide by 1 ft deep gravel zone, gravel in cubic feet equals the wall length — so a 30 ft wall needs 30 cu ft, which is 30 divided by 27, or 1.11 cubic yards.
Do I need weep holes in my retaining wall?
Weep holes are recommended for walls over 4 ft tall. Place one row every 8 ft along the wall; walls over 6 ft tall should get a second row. Walls 4 ft and under generally drain well enough through the gravel and base pipe alone.
What kind of pipe and gravel should I use for wall drainage?
Use 4-inch perforated drain pipe (with a filter sock in silty soil) laid at the base of the wall, just behind the first course, and backfill with 3/4-inch clean or washed gravel — never crusher run with fines, which clogs the pipe. Daylight the pipe to a downhill outlet.