Class 5. CR6. Flex base. 21A. To a homeowner these look like model numbers; to a yard they are recipes — precise statements about what sizes of stone are in the pile, how much dust it carries, and how hard it will compact. This guide translates the spec names into buyer language, so the name on your quote stops being a mystery and starts being a checklist.

What a base material actually is

Every spec gravel in this article is a dense-graded aggregate base, and three properties define it. Gradation: the pile contains every size from a stated maximum down to dust, in controlled proportions — no gaps. Spread and compacted, the small sizes fill the voids between the large ones. Fines: the dust share (the fraction passing the finest sieve, No. 200) is limited but deliberately present — a few percent up to about a tenth — because that dust is the mortar that lets the layer pack into a single interlocked mass instead of a loose pile of separate stones. Crushing: a minimum share of the stone must be mechanically crushed, because crushed faces are angular and angular faces grip; round pit-run gravel in the same gradation compacts noticeably worse. Add water and a compactor and those three properties produce the layer roads and driveways actually stand on. It is also why the fashionable substitutes fail: clean washed stone has no fines, so it shifts; pea gravel has no angular faces and no gradation, so it rolls. Drainage stone and base stone are opposite recipes wearing similar names.

Quantities follow the same math as everything else on the hub — area × depth ÷ 27 for yards, × density for tons — with base ordered against its compacted density, about 1.60 tons per yd³, and sold by the ton. The Gravel Calculator‘s Custom density setting (about 118.5 lb per ft³) reproduces that figure; the spec decoders below tell you which pile to point it at.

Class 5, decoded

Class 5 is the aggregate-base grade of the Minnesota Department of Transportation, Standard Specification 3138, and the name travels across the Upper Midwest. In buyer terms it promises: a nominal top size of about ¾ inch with everything passing a 1-inch screen; at least 10% crushed material, so the pile has angular faces to lock; and fines capped at about 10% passing the No. 200 sieve — present enough to bind, limited enough to drain and resist frost heave. Class 5 is the default base under Minnesota driveways, farmyards and township roads, and the paving-grade sibling (Class 6, with a 15% crushing minimum) sits directly under pavement. When a class 5 gravel calculator asks only for length, width and depth, this recipe is the reason it does not have to ask what the stone is: the spec already answered.

CR6, decoded

CR6 — written CR-6 — is Crusher Run Aggregate under the Maryland State Highway Administration’s standard specifications (§901), the mid-Atlantic’s standard base stone. The name tells the process: stone as it comes off the crusher, blended from roughly 1½ inches down to dust, unscreened into separate sizes, so the full gradation is in the pile by construction. Maryland uses it under roads, shoulders and driveways; Virginia’s graded base 21A is the close sibling one state south, and suppliers near the border often stock to both. If class 5 is the Upper Midwest’s recipe card, CR6 is the Chesapeake’s — same dense-graded idea, locally audited proportions.

The same material, other name tags

The western entry explains another common search: “3/4 road base” — the name a 3/4 road base calculator uses — simply states the top size in the title: a base graded to a ¾-inch maximum, the Class 2 style common west of the Rockies. Different top sizes (¾ in, 1 in, 1½ in) change the pile’s look more than its behaviour; what matters for a driveway is that the gradation is dense and the fines are in the recipe at all.

Reading a gradation in one minute

A gradation table looks forbidding and is actually one idea repeated: down the left, sieves from coarse to fine; across, the percentage of the pile (by weight) that passes each sieve. “100 at 1 inch” means every stone fits through a 1-inch screen. “35–70 at ⅜ inch” means between a third and two-thirds of the pile is smaller than ⅜ inch. Read top to bottom, a dense-graded base shows a smooth staircase — a bit of everything at every size — while a clean drainage stone shows a cliff: nearly everything passes one sieve and nearly nothing passes the next. The last row, the No. 200 (the dust sieve, 0.075 mm), is the one to find first: a base spec keeps it in single figures or just above — enough dust to bind, never enough to turn to mud — and a spec with 0–2% there is a drainage product wearing a base product’s name. The other phrase worth decoding is “minus”: “¾-inch minus” means top size ¾ inch and everything below it, dust included — the minus is the fines, and the fines are the point.

Compaction, moisture and lifts: why the same pile performs differently

Two driveways can order identical class 5 and age completely differently, because spec is only half the story — placement is the other half. Base compacts properly when it is damp (dry dust will not bind; soup will not bear weight — the spec’s fines want the moisture in between) and when it is placed in lifts of about 3 inches, each compacted before the next lands, because a plate compactor or roller only densifies so much depth at a pass. The weight story follows from the same physics: a loose cubic yard of crusher run in the truck runs roughly 1.3–1.45 tons, while the same yard rolled into place is the 1.50–1.70-ton compacted figure this article orders against. That gap is not fudge — it is the air you paid to squeeze out — and it is exactly why base is specified, sold and ordered by compacted tons for a finished depth, never by the loose yards that happened to fit in the truck.

Flex base: the Texas entry

Texas calls the same layer flexible base, specified under TxDOT Item 247 in grades by intended duty — crushed stone, crushed concrete, gravel or slag blends, graded and compacted to a density the spec polices. A flex base calculator (the trade also squashes it to flexbase calculator) is therefore not a different tool; it is the base-material workflow with Texas vocabulary, and the quantities come out of the same area × depth × 1.60-tons-per-yard chain as class 5 in Minnesota or CR6 in Maryland.

Base under slabs, pavers and asphalt

The spec family earns its keep out of sight. Under a concrete slab — a garage floor, a shed base — a compacted 4-inch layer of graded base gives the pour a level, free-draining platform that will not pump mud up through the joints: a 20 × 20 ft slab’s base is 400 sq ft × 0.333 ft = 133 cu ft = 4.94 yd³ ≈ 7.90 tons at 1.60, after which the slab itself is the Concrete Calculator‘s department. Under pavers, the same base at the same 4 inches carries the patio, with an inch of bedding sand doing the fine levelling. Under asphalt, the base is usually the thicker half of the build and the hot-mix layer is a different material with different density maths entirely — that side of the family has its own tool, the Asphalt Calculator. The pattern to notice: the base is the constant. Surfaces change with fashion and budget; the dense-graded, compacted layer underneath is what every one of them stands on.

How much base do I need?

The measuring half of the job — tape, rectangles, split shapes for L-drives and aprons — is what a driveway measurement calculator does, and the hub’s multi-area form is built for exactly that split. After measuring, the workflow is one line: yards = length × width × (depth ÷ 12) ÷ 27; tons = yards × 1.60. Whichever name your yard uses — class 5 calculator, cr6 calculator, aggregate base calculator or base material calculator searches all land on this same workflow, and a base rock calculator yards figure is just the yd³ line of it. Worked on the standard 60 × 12 ft driveway: a 6-inch base is 13.33 yd³ ≈ 21.33 tons; a 4-inch base is 8.89 yd³ ≈ 14.22 tons. Two ordering rules follow from the physics. Order against the compacted figure and add roughly a tenth — the pile in the truck is looser than the layer it becomes. And place base in lifts of about 3 inches, compacting each: the tonnage is the same, the driveway is not.

Which spec should you actually ask for? Almost always: your own state’s. Specs are regional because aggregates are — a yard produces to the recipe its state audits and its quarries can make from local rock, so a Minnesota yard has class 5, a Maryland yard has CR6, and neither can sell you the other’s pile no matter what a drawing from the internet says. If plans drawn elsewhere name a foreign spec, do not order the name; describe the recipe — graded base, such-and-such top size, fines in — and let the yard match it to their graded base, which is what their ticket will call it. The name is a dialect. The gradation is the language.

Frequently asked questions

What is class 5 gravel made of?

Crushed gravel or stone graded to the Minnesota DOT’s Spec 3138: everything passes a 1-inch screen, at least 10% of the stone is crushed so the faces are angular, and no more than about a tenth is fine dust. It is a recipe — gradation, crushing and a fines limit — not a particular rock.

What is the difference between class 5 and CR6?

The idea is identical — a dense-graded, compactable base — but the specs are regional. Class 5 is Minnesota’s (Spec 3138, top size around ¾–1 inch); CR6 is Maryland’s crusher-run grade (§901), blended from roughly 1½ inches down. A yard stocks its own state’s spec, and for a driveway base they are interchangeable in behaviour.

How much road base do I need for a driveway?

Measure length × width, choose 4–6 inches of base, and convert at the compacted density of about 1.60 tons per yd³. A 60 × 12 ft driveway at 6 inches is 13.33 yd³, roughly 21.33 tons; at 4 inches, 8.89 yd³, about 14.22 tons.

What is flex base?

The Texas name for graded flexible base — crushed stone, gravel or slag blends specified under TxDOT Item 247. It is the same dense-graded base material other states sell as class 5, CR6 or crusher run, graded and compacted the same way.

Can I use pea gravel or clean stone as a base?

No — a base layer has to lock, and locking needs the full gradation: big stone, small stone and fines together. Clean single-size stone has voids that shift under load, and pea gravel’s rounded stones roll. Both drain well, which makes them good drainage stone — and poor foundations.