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.

What is Class 5 gravel, and is it the same as 21AA?

Class 5 is the Minnesota/Upper-Midwest DOT name for a dense-grade aggregate base — crushed stone or gravel blended with fines that compacts hard. It's functionally the same material as Michigan's 21AA, Illinois' CA6, and the generic ABC/crusher run: a compactable base, not a clean top-course stone. A cubic yard weighs about 2,650 lb (1.33 tons), same as 21AA.

What do gravel 'classes' mean (Class 1 to Class 5)?

"Class" is a DOT spec for a graded aggregate, and the numbering varies by state — it is not a national size scale like the AASHTO #-numbers. In many states higher classes are finer, denser base blends: Class 5 and Class A are common dense-grade base materials with fines that compact; lower classes or clean grades are coarser. Always confirm the local spec with your supplier, because 'Class 2' in California is not 'Class 2' in Minnesota.

What's the difference between 21AA, 22A, and 23A?

All three are Michigan MDOT dense-grade base aggregates topping out near 1 inch, but they differ in how much is crushed rock. 21AA is 100% crushed, so it locks up hardest — the pick for driveways and paver bases. 22A blends crushed stone with natural gravel; it costs a little less and works for general base. 23A is mostly natural (uncrushed) gravel with fewer fractured faces, so it's cheaper but won't compact as tight — it's a shoulder and sub-base gravel. For a driveway or patio you want 21AA or 22A, not 23A.

Is 21AA gravel crushed or natural?

21AA is 100% crushed stone. Every piece has fractured, angular faces, which is what lets it interlock and pack to near-concrete hardness under a compactor. That's the main thing separating it from 23A, which is largely natural rounded gravel. The crushed faces are why 21AA is specified for load-bearing bases rather than the rounded gravels that shift under traffic.

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.

21AA vs 22A vs 23A (Michigan grades)

Michigan drivers see three MDOT numbers at the yard. They're not interchangeable — the difference is how much of the mix is crushed rock versus natural gravel:

Grade	Make-up	Top size	Weight/yd³
21AA	100% crushed, dense-graded	1"	2,650 lb
22A	Crushed + natural gravel blend	1"	2,650 lb
23A	Mostly natural gravel, fewer crushed faces	1"	2,600 lb

21AA is the most crushed and locks up hardest, so it's the pick for driveways and paver bases. 22A costs a bit less and works for general base. 23A is a cheaper shoulder/sub-base gravel — fine under a driveway but it won't lock as tight. All three follow gradation limits set by sieve analysis under ASTM C136/C136M.

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