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.

Step 1 · Backfill zone
Step 2 · Zone dimensions
Step 3 · Drainage stone

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.

This page estimates stone, not engineering. Walls over about 4 feet, walls holding back slopes or driveways, and anything with poor drainage behind it deserve a designed drainage system and possibly an engineer. The gravel is one part of it — pipe placement, weep holes, and fabric matter just as much. For the horizontal version of this idea, see the French drain calculator.

French Drain Gravel Calculator

The horizontal cousin: trench drainage runs.

Gravel Calculator

Any area, any material — the general tool.

Gravel Types and Sizes Guide

Which washed stone sizes drain best.

Gravel Calculator (Tons)

Plain tonnage with the density table.

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.