Drainage Gravel Sizes and Void Space: #57 Stone, Pea Gravel, Crusher Run and What Each Is For
Use clean, washed, angular stone about ¾ to 1 inch — #57 in most of the US — for French drains, dry wells and behind retaining walls. It stores water in roughly 40 percent of its volume and weighs about 1.4 tons a cubic yard. Pea gravel packs tighter and shifts; crusher run has fines that turn it into concrete-like fill. Anything with dust in it is not drainage stone.
Two things a drainage stone has to do
Gravel in a drain has exactly two jobs. It has to let water through fast — faster than the soil around it, or the drain is pointless — and it has to hold its shape under load without migrating into the pipe or settling into mud. Both jobs come down to the same property: clean, angular pieces of roughly one size, with the gaps between them left open. The gaps are the drain. Fill them with sand or rock dust and you have built a hard, wet lump.
That is why the stone the quarry calls washed or clean is the whole game, and why the cheapest bulk product on the price list — crusher run, ABC, "gravel with fines" — is the wrong one for every drainage use even though it is the right one for a driveway base.
The sizes, and what each is for
Stone is sold by a gradation number, not an inch size. The numbers below are the ASTM/AASHTO designations used across most of the US; local names vary, so the inch range is the thing to match.
| Name | Size | Angular / rounded | Best for | Avoid for |
|---|---|---|---|---|
| #57 stone | ¾ – 1 in | Angular, washed | French drains, dry wells, retaining wall zones, basin bedding | Nothing in drainage — the default |
| #67 stone | ¾ in, slightly finer | Angular, washed | Same as #57 where it's the local stock | — |
| #4 stone | 1 – 1½ in | Angular, washed | Large dry wells, deep trenches, high flows | Small trenches — hard to place around pipe |
| #8 / #78 stone | ⅜ in | Angular, washed | Bedding under pipe, pavers, small dry creek fill | Main drain fill — less void space, migrates more |
| Pea gravel | ⅜ in | Rounded, washed | Decorative surfaces, paths | French drains and dry wells — rolls, packs, less void |
| River rock | 1 – 3 in | Rounded | Dry creek beds, outlet aprons | Trench fill — big voids let soil in, rolls under load |
| Crusher run / ABC | Dust to ¾ in | Angular, with fines | Driveway and retaining wall base | Every drainage use — the fines lock it solid |
| Screenings / stone dust | Dust to ¼ in | Fines | Paver bedding | Anything that has to drain |
Angular matters as much as size. Crushed stone locks together and stays put; rounded gravel behaves like ball bearings, which is why a pea-gravel French drain settles into a dip in the lawn and why river rock walks out of a trench.
Void space — how much water stone actually holds
A hole full of stone is not a hole full of water. Only the gaps between the pieces hold water, and for clean uniform stone that is roughly 35 to 45 percent of the volume. The site's calculators use 40 percent for #57, which is why a dry well pit comes out about two and a half times the size of the water it stores.
| Material | Void space | 100 cu ft of it stores about |
|---|---|---|
| #4 clean stone | 40 – 45% | 320 gallons |
| #57 / #67 clean stone | 35 – 42% | 300 gallons |
| #8 clean stone | 33 – 40% | 275 gallons |
| Pea gravel, rounded | 28 – 35% | 235 gallons |
| River rock, mixed sizes | 25 – 35% | 220 gallons |
| Crusher run, compacted | 5 – 15% | 75 gallons — and it will not drain |
| Open plastic chamber | 90 – 95% | 700 gallons |
The last row is the case for a chamber-style dry well: the same hole stores more than twice the water, in exchange for buying the unit. The dry well calculator shows both options side by side.
Weight, volume and how it's sold
Bulk stone is priced by the ton and delivered by the cubic yard, so every order involves a conversion. Clean #57 runs about 1.35 to 1.45 tons per cubic yard loose — the site uses 1.4. Crusher run with its fines is heavier, about 1.5 to 1.6 loose and more once compacted. Bagged stone at a home centre is roughly 0.5 cubic feet per 50-pound bag, so a cubic yard is about 54 bags — the point at which bulk delivery is cheaper is around one cubic yard, which is a 20-foot trench.
| Per cubic yard | Per ton | |
|---|---|---|
| #57 / #67 clean stone | ~1.4 tons, ~2,800 lb | ~0.7 cu yd |
| Pea gravel | ~1.4 tons | ~0.7 cu yd |
| #4 stone | ~1.35 tons | ~0.75 cu yd |
| Crusher run | ~1.5 – 1.6 tons loose | ~0.65 cu yd |
The French drain gravel calculator and retaining wall calculator do the conversion and add a settling allowance, because a delivery that is a yard short is a second delivery fee.
Fabric: the other half of the system
Clean stone stays clean only if soil cannot get into it. Fine particles carried by water migrate into the voids over a few seasons, and a drain that took a weekend to build silts solid with no way to clean it. Non-woven geotextile between the stone and the native soil — around the trench, under and around a dry well, behind the stone zone of a retaining wall — is what keeps the voids open. It is a few dollars per running foot and the item most often skipped on a cheap job. What happens without it is on the clogged yard drain page.
FAQs
What size gravel is best for a French drain?
Clean, washed, angular stone from ¾ to 1 inch — #57 in most of the country, #67 where that is the local stock. It is large enough to leave open voids, small enough to place around a 4-inch pipe without leaving gaps, and angular enough to lock in place. Wrap the whole stone zone in non-woven filter fabric so the surrounding soil cannot silt it up.
Can I use pea gravel for drainage?
It drains, but it is a poor choice for a French drain or dry well. Rounded pieces pack more tightly than crushed stone, leaving less void space, and they roll under load, so the trench settles and the pipe shifts. It is fine as a decorative surface over a drain or in a dry creek bed. For the working part of any drain, use angular #57.
What is the void ratio of gravel?
For clean, single-size crushed stone, roughly 35 to 45 percent of the volume is air — the site's calculators use 40 percent for #57. Rounded pea gravel is closer to 30 percent, mixed sizes less, and crusher run with fines as little as 5 to 15 percent, which is why it does not drain. Engineers quote a void ratio (voids ÷ solids) rather than a percentage; 40 percent voids is a void ratio of about 0.67.
How much does a cubic yard of #57 stone weigh?
About 1.4 tons, or 2,800 pounds, loose in the truck. Suppliers quote anywhere from 1.35 to 1.45 depending on the rock. A 12-inch by 18-inch trench needs about 0.055 cubic yards per foot, so a 40-foot French drain takes roughly 2.2 cubic yards or 3 tons — a small dump-truck load, not a car-trunk job.
Do I need filter fabric with drainage gravel?
Yes, everywhere stone meets native soil. Water carries fine particles into the stone and they lodge in the voids; within a few seasons an unwrapped drain silts solid and cannot be flushed. Use non-woven geotextile, overlap it a foot at the top, and keep it between the stone and the soil — not between the stone and the pipe, where it would block the perforations.
Is crusher run good for drainage?
No. Crusher run, ABC and "gravel with fines" are graded to compact into a dense, near-solid base — exactly the property you want under a driveway or a retaining wall and exactly the opposite of what a drain needs. Compacted, it has 5 to 15 percent void space and drains about as well as the clay around it. Use it for the base course; use washed #57 for the drainage zone above it.
Sources consulted
- ASTM C33 / AASHTO M43 — standard size designations for coarse aggregate (#57, #67, #8, #4)
- ASCE Manual of Practice 77 — backfill void ratios for infiltration practices
- NC Department of Transportation — aggregate gradations and typical unit weights
- Site test data and supplier weight tickets for #57 washed stone in the NC Piedmont
By Under Your Home Team · Last reviewed 2026-09-07