Gravel Calculators
Retaining Wall Gravel Calculator
The gravel behind a retaining wall is structural drainage — a narrow column of washed stone that keeps water pressure off the wall. Model it as a vertical trench: wall length × backfill width × wall height, pre-set to washed #57.
A note on this estimate
Drainage pipe, fabric and weep holes are not computed here. Walls over about 4 ft, or walls holding back slopes or driveways, need engineered design — this tool sizes the gravel, not the wall.
Last updated: 7 November 2026 · Report an error
Why the backfill zone matters
Most retaining walls don't fail from soil weight — they fail from water weight. Saturated soil behind a wall pushes far harder than dry soil, and frozen saturated soil pushes harder still. A 12"+ column of washed stone with a perforated pipe at the footing turns that water into flow it can handle.
Worked example: a 40-foot wall, 4 feet tall, with a 12-inch drainage zone: 40 × 1 × 4 = 160 ft³ = 5.93 yd³ of washed #57 ≈ 8.0 tons — modest money next to rebuilding a bulged wall.
Backfill width by wall height
The gravel behind a retaining wall is not fill — it is the drainage zone that keeps water pressure off the wall's back face, and water pressure is what pushes most failed garden walls over. Twelve inches of clean drainage stone directly behind the wall is the minimum worth building; taller walls hold back more soil and more water, and earn a wider zone.
| Wall height | Gravel zone width | Notes |
|---|---|---|
| Up to 2 ft | 12 in | Garden and edging walls; drain pipe optional on free-draining soil |
| 2–4 ft | 12–18 in | Perforated drain pipe at the base becomes worth its cost |
| 4–6 ft | 18–24 in | Pipe at the base, fabric separation, and an engineered wall design |
| Over 6 ft | Engineered | Outside this calculator — the wall structure itself needs design |
Worked example: a 20 ft wall, 3 ft of retained height, with the standard 12-inch drainage zone: 20 × 3 × 1 = 60 ft³ = 2.22 yd³ of washed #57, which is 3.00 tons. Double the wall to 30 ft at 4 ft high and the same zone takes 120 ft³ = 4.44 yd³ ≈ 5.99 tons — the zone scales with the wall face, which is why backfill is so often under-ordered when a wall grows during the build.
Material choice is the whole game here: the zone must be clean, washed, open-graded stone (#57 or similar) so water falls through it to the drain pipe. Crusher run and native excavated soil — the two things builders most often substitute — both hold water against the wall instead of relieving it. Keep a layer of filter fabric between the stone and the soil behind it, compact the backfill in lifts beside (never directly against) the wall face, and give the collected water somewhere to go: a pipe that daylights, or weep holes through the face.
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Retaining wall FAQ
Why does a retaining wall need gravel behind it?
Water is what fails most retaining walls. Hydrostatic pressure pushing on the back of the wall can exceed the soil pressure the wall was designed to hold. A column of free-draining gravel plus a perforated drain pipe relieves that pressure by letting water escape instead of building up.
How wide should the gravel backfill zone be?
At least 12 inches of washed stone directly behind the wall, with a perforated drain pipe at the base. Some designs call for more. Treat the drainage zone as a narrow trench running the wall's length — the calculator models it exactly that way.
What gravel do you put behind a retaining wall?
Washed angular stone — #57 is typical — never crusher run or soil. Fines retain water and clog drainage paths. Wrap the zone in filter fabric to keep soil particles from migrating into the stone over time.