How Much Sub-Base (Crushed Stone) Do I Need?

The short answer

For a residential driveway, plan on 4–6 inches of compacted crushed stone; for a patio, 4 inches; for a shed base, 3–4 inches. The formula is the same as every bulk-material job: length × width × depth in feet, divided by 27 gives cubic yards, then multiply by your stone's weight — 1.4 tons per cubic yard for typical crushed stone — to get tons. A 20 × 40 ft driveway at 4 inches needs 9.88 cubic yards ≈ 13.8 tons. One catch: those are compacted depths, and you order loose material — with crusher run, add 15–20% to reach your finished depth.

What a sub-base actually does

Every paved or gravel surface that lasts is a stack of layers, and the sub-base is the load-bearing one — a bed of crushed stone laid directly on graded soil, under the surface you actually see and walk on. Its job is to spread point loads across a wider area so the soil underneath doesn't compress, rut, or pump up through the surface. Car tires, paver slabs, and shed floors all concentrate weight into small footprints; the sub-base turns those footprints into a distributed load the ground can actually carry.

Names vary and that causes real confusion. Paver installers tend to call the crushed-stone layer the “base” and the inch of sand above it the “bedding layer.” Driveway builders often say “sub-base” for the bottom stone and “base” for the middle layer in a three-layer build. A supplier may call the same material crusher run, road base, dense grade, 21A, or ABC stone. Don't get hung up on the label — ask whether the material has fines (the rock dust that lets it compact into a rigid bed). If it does, it works as a sub-base; if it's clean washed stone with no dust, it's a drainage layer, not a structural one. (In the UK this layer is called hardcore or MOT Type 1, but the math — depth × area, then convert to tons — is identical.)

The formula, then the depths

Sub-base quantity is volume math with a unit conversion at the end:

1. Measure length and width in feet. 2. Pick your compacted depth from the table below and convert inches to feet (divide by 12). 3. Multiply: length × width × depth in feet, then divide by 27 — that's cubic yards of compacted material. 4. Multiply by your stone's tons per cubic yard for the tonnage, then add the compaction adjustment (15–20% for crusher run) to get the loose quantity you actually order.

“Order the loose equivalent” assumes crusher run or road base compacted to about 17% — the midpoint of the 15–20% compaction loss these materials need (see our settlement and ordering guide for the full tables). Clean #57 stone barely compacts at all, so its loose and compacted volumes are nearly the same — but clean stone also makes a weaker sub-base, which is why crusher run is the default choice below.

Cubic yards to tons, by stone type

Suppliers usually quote sub-base by the ton and deliver by the yard, so you need both numbers. Tons per cubic yard depend on the stone — these are this site's published bulk densities, and the calculator uses the same values, so the page, the tables, and the tool can never disagree:

In practice: if your supplier sells “crusher run,” “dense grade,” or “road base,” use 1.4 tons per yard. Across every common crushed stone the real range is only 1.325–1.45 tons/yd³, so an estimate built on 1.4 lands within a few percent either way. Run the volume through our gravel calculator or gravel-to-tons converter and pick your stone type from the density dropdown — the weights follow automatically.

Worked examples

Driveway, 20 × 40 ft, 4 in compacted crusher run. Area: 800 sq ft. Depth: 4 ÷ 12 = ⅓ ft. Volume: 800 × ⅓ = 266.7 cu ft = 9.88 cubic yards compacted. At 1.4 tons/yd³: 9.88 × 1.4 = 13.8 tons. Add the ~17% compaction adjustment: 9.88 × 1.17 = 11.56 loose yards → order 12 yards (≈ 16.8 tons at 1.4). That's the full order — compacted depth, loose delivery, and tons in one pass.

Patio, 12 × 16 ft, 4 in compacted sub-base. 192 sq ft × ⅓ ft = 64 cu ft = 2.37 cubic yards compacted (3.3 tons at 1.4). Loose with the 15% adjustment: 2.37 × 1.15 = 2.73 → order 3 yards. Our paver base guide adds the bedding-sand inch on top of this.

Shed base, 10 × 12 ft, 4 in. 120 sq ft × ⅓ ft = 40 cu ft = 1.48 cubic yards (≈ 2.1 tons). Loose: 1.48 × 1.15 = 1.70 → order 2 yards. One delivery, one job — see our shed base guide for the full build.

Quick per-foot rule. One square foot at 4 inches deep is 0.0124 cubic yards — about 35 lb (0.017 tons) of crushed stone at 1.4 t/yd³. For a rough price check, multiply your square footage by 35 lb and by 1.4 at 6 inches instead.

Which material to actually order

The name on the ticket matters less than the particle mix. What you want is angular crushed stone with fines — stone broken by a crusher, with rock dust and chips mixed in. The dust fills the voids between the larger pieces under the plate compactor, and that's what turns loose stone into a rigid, almost pavement-like bed. Sold regionally as crusher run, ¾-inch minus, dense grade, road base, 21A, or ABC — they're the same idea in different quarries. For a side-by-side on the decorative side of the family, see our crushed stone vs pea gravel guide.

Recycled concrete aggregate (RCA) is worth asking about: it compacts well, drains acceptably, and often costs 20–40% less than virgin stone. The one caution is consistency — ask whether it's screened to a uniform size and free of rebar and trash, especially for the top inch under pavers. Decomposed granite makes a beautiful path surface but a mediocre structural sub-base; it needs about 20% extra loose to compact and still stays softer than crusher run. And skip pea gravel and river rock for structure entirely — round stones roll instead of interlocking, which is exactly what you don't want under load.

The compaction adjustment, explained

This is the step that strands DIYers a yard short. A 4-inchcompacted layer of crusher run needs roughly 4.6–4.8 inches of loose material to get there — the 15–20% the fines give up as they pack into the voids. Clean #57 stone is the opposite: it barely compacts, so loose and finished volumes are nearly equal, but you've traded away the interlock that makes a sub-base work. The rule: order loose volume, design to compacted depth, and spread the loose stone in 2 to 3-inch lifts, compacting each lift with a plate compactor (mist lightly with water between passes — damp stone compacts tighter than dry).

For driveways, see our full driveway gravel guide for the three-layer build (large base stone, #57 middle, crusher run top) with tons tables by driveway size. For ordering strategy — why the buffer beats a second delivery — our settlement guide covers the economics.

Common mistakes

Ordering compacted yards instead of loose yards. The 15–20% shortfall shows up exactly when the truck is leaving. Dumping all the stone at once and compacting the top. The bottom stays soft and settles for years — work in lifts. Using clean decorative stone as structure. Pea gravel and river rock belong on top of a path, not under a driveway. Skipping fabric on soft soil. Without a separator, the sub-base sinks into clay and you effectively lose an inch of paid-for stone. Forgetting the crown. A dead-flat driveway holds water; crown it about ¼ inch per foot from center to edge so it sheds.