What's the difference between 21AA and 21A?

Mostly nomenclature. 21AA is the Michigan DOT name for the dense-grade base course. 21A is the Virginia / federal equivalent — same gradation envelope, same use cases.

Is 21AA the same as crusher run?

Functionally yes. Both are dense-grade mixes from 1" down to dust. "Crusher run" is the generic name; 21AA is a specific DOT spec ensuring a tighter gradation envelope.

How much does 21AA weigh per yard?

About 2,600–2,700 lbs per cubic yard loose; closer to 3,200 after compaction. The calculator uses the loose density to match what you'll have delivered.

Can I use 21AA as a driveway top course?

You can, but most builders don't. The fines wash out over time and the surface looks dusty. Use 21AA as the base + #57 (or a similar clean stone) as the top.

How deep does 21AA need to be for a driveway?

4 inches under a passenger-vehicle driveway, 6 inches for trucks or RVs. Always compacted in 2-inch lifts.

21AA is the Michigan DOT specification number for a dense-graded crushed-rock aggregate base. The number identifies a specific gradation (size mix) — 21AA runs from about 1 inch down to dust. The same product is called CA6 in Illinois, Class 5 in Minnesota, 304 in Ohio, and ABC or crusher run in the South. Functionally interchangeable.

Does 21AA need a top course?

Optional but recommended. Compacted 21AA forms a hard, drivable surface on its own — many rural driveways are 21AA only. A 2-inch top course of #57 stone or limestone makes the driveway look finished, drains better at the surface, and reduces dust kicked up by tires. For a one-pass build, just 21AA is fine; for a long-life surface, 21AA base + #57 top is the standard.

Can I use 21AA for a paver patio base?

Yes — 21AA, CA6, ABC, and crusher run are all interchangeable for paver bases. Compact 4 inches of 21AA in 2-inch lifts, then add 1 inch of bedding sand on top. The dense gradation gives pavers a firm, non-shifting foundation.

21AA Gravel Calculator — Yards & Tons

What 21AA actually is

21AA is a Michigan DOT specification for graded crushed aggregate used as road base. The same material under different names:

21AA — Michigan
CA6 — Illinois
Class 5 — Minnesota / Wisconsin
304 — Ohio
ABC — "Aggregate Base Course" in NC, SC, GA
Crusher Run — generic; widely used
MOT Type 1 — UK equivalent

All of these are dense-graded mixes containing fines that bind together when compacted. They're the right base material for driveways and paver patios because they pack to near-concrete hardness. They're the wrong material for French drains or anywhere drainage matters — the fines block water flow.

Compact 21AA in 2-inch lifts

21AA reaches 95% of its load-bearing capacity only after compaction. Lay 2-inch lifts and run a plate compactor between each. A single uncompacted 4-inch lift is a recipe for ruts.

Recommended depth & material

Use	Depth	Material	Notes
Light driveway base	4"	21AA / CA6 / Class 5	Under 2" of #57 top course.
Heavy / RV driveway base	6"	21AA / Crusher Run	Compacted in 2 lifts.
Paver patio base	4"	21AA / CA6	Topped with 1" bedding sand.
Walkway / path base	3"	21AA / Crusher Run	Compacted; finish with pea gravel or pavers.
Shed pad	4"	21AA	Alternative to #57 if you want a hard surface.

Ordering 21AA

Standard rectangular math, but with a higher buffer (15%) because 21AA compacts ~20% on installation. Order by the ton — that's how all major quarries quote dense-grade base.

Measure length × width. In feet. Use the metric toggle for UK MOT Type 1.
Set depth. 4 inches for most uses; 6 inches for heavy traffic.
Apply 15% buffer. 21AA loses ~20% of its loose volume to compaction. The 15% buffer covers compaction + spillage.
Order in tons. All major quarries quote 21AA per ton, not per yard.

Worked example — 1,200 sq ft paver patio base

A 30 × 40 ft paver patio needing 4 inches of 21AA: volume = 1,200 × (4/12) ÷ 27 = 14.8 yd³. At 2,650 lb/yd³, that's 19.6 tons. With a 15% buffer: 17.0 yd³ / 22.5 tons. At $35/ton delivered: $789. Plus 1 inch of bedding sand on top — about 4 yd³.

21AA Gravel Calculator