How much gravel for a retaining wall?
A 20 ft by 3 ft wall needs about 2.96 cu yd — roughly 4.1 tons.
0.74 cu yd of compacted base under the first course, and 2.22 cu yd of clean stone in a chimney behind the wall. They are two different materials doing two different jobs.
Gravel by wall size
| Wall | Base (cu yd) | Drainage (cu yd) | Total | Tons |
|---|---|---|---|---|
| 10 x 2 ft | 0.37 | 0.74 | 1.11 | 1.6 |
| 20 x 2 ft | 0.74 | 1.48 | 2.22 | 3.1 |
| 20 x 3 ft | 0.74 | 2.22 | 2.96 | 4.1 |
| 30 x 3 ft | 1.11 | 3.33 | 4.44 | 6.2 |
| 40 x 3 ft | 1.48 | 4.44 | 5.93 | 8.3 |
| 20 x 4 ft | 0.74 | 2.96 | 3.7 | 5.2 |
| 40 x 4 ft | 1.48 | 5.93 | 7.41 | 10.4 |
Assumes a 24 in wide trench with 6 in of compacted base, and a 12 in chimney of drainage stone for the full height of the wall. Tons at about 1.4 tons per cubic yard, which is where crushed stone usually lands — yards sell by the ton, so that is the number you will be quoted on.
Two gravels, two jobs
The base is a compactable material — crushed stone with fines still in it, sold as 3/4 in minus, Class 5, or road base depending where you are. The fines are the point: they let it lock together under a plate compactor into something that behaves like soft rock. Clean stone will not do this. It stays loose and the wall settles into it unevenly.
The drainage chimney behind the wall is the opposite — washed, angular, no fines, typically 3/4 in clean or #57 stone. Its job is to let water fall straight through to a drain pipe instead of saturating the soil behind the wall. Saturated soil is the thing that pushes walls over: water roughly doubles the pressure the wall has to hold.
Ask for them by the right name and you will get the right material. Ask for "gravel" and you will get whichever one the yard sells most of.
Common questions
- What size gravel goes under a retaining wall?
- 3/4 in minus, or the local equivalent — crushed, angular, WITH fines so it compacts. Not pea gravel and not clean stone; rounded or fine-free material never locks up and the wall will settle.
- What size gravel goes behind it?
- 3/4 in clean or #57 washed stone, no fines. It must drain freely. Wrapping the chimney and the drain pipe in filter fabric keeps the soil behind from washing into it and clogging it over a few seasons.
- How much crushed stone under a retaining wall?
- 6 in compacted, in a trench about 24 in wide — twice the depth of a standard block, so the base extends in front of and behind the wall rather than just under it. For a 20 ft wall that is 0.74 cu yd regardless of the wall's height.
- Do I need a drain pipe as well?
- On anything holding real soil, yes. A perforated pipe at the base of the chimney, sloped to daylight, gives the water somewhere to go. The stone collects it; the pipe removes it. Stone alone just holds it lower down.
- Is the base the same for a taller wall?
- The base gravel is, because it depends on the trench, not the height. The drainage stone is not — it runs the full height, so it grows with the wall. That is why a 40 x 4 ft wall needs 5.93 cu yd of drainage against 1.48 cu yd of base.
- How many tons should I order?
- Multiply cubic yards by about 1.4. Order the base and the drainage stone separately and have them dropped in separate piles — mixing them ruins both.
Related answers
Or run any size through the retaining wall calculator.