2A Modified Stone: Size, Weight and How Many Tons

2A modified stone is the everyday name for PennDOT No. 2A aggregate, the dense-graded crushed stone that Pennsylvania road crews, municipalities and driveway contractors spread under pavement. It runs from about 2 inches down to dust. All those sizes together let it compact into a tight, firm layer, which is why it sits under so many asphalt driveways, concrete slabs and parking pads in Pennsylvania and nearby states. Plan on roughly 1.8 tons of stone for every cubic yard of finished, compacted base. Below is what the name means, how 2A compares with the other stone a Pennsylvania yard sells, where it works and where it does not, and how to figure the tons for your job.

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What “2A” and “modified” mean

The 2A label comes from PennDOT Publication 408, the state’s construction specification book. Section 703 lists the sizes and gradations for coarse aggregate, and No. 2A is the one written for subbase, the stone layer that carries the load under a road. The word “modified” is not part of the spec table. Quarries and yards add it to make clear you are getting the blend with fines, not a washed stone. So “2A,” “2A modified,” “2A mod” and “2A subbase” usually mean the same product, though it never hurts to ask for the gradation sheet.

The Section 703 limits for 2A work out to roughly the following. Check the current edition of Publication 408 or your quarry’s test report if a plan calls for a certified material.

Sieve opening	Share passing (by weight)
2 inch	100%
3/4 inch	52 to 100%
3/8 inch	36 to 70%
No. 4 (about 3/16 inch)	24 to 50%
No. 16	10 to 30%
No. 200 (dust)	0 to 10%

Those ranges are wide. One load of 2A can look coarse and open while another looks almost sandy, depending on the quarry and how much fine material made it into the pile. The fines are the part that lets the stone pack, so a 2A on the low end of the dust range drains better but will not lock up as hard, and one on the high end packs tight but holds more water.

2A vs other Pennsylvania stone

Product	What it is	Fines?	Typical use
2A modified	About 2 inches down to dust	Yes	Base under asphalt, concrete and pavers; gravel driveway base
2B	A name many PA yards still use for clean stone close to #57	No	Drainage, pipe bedding, clean top layer
2RC	A 2A-sized blend with looser limits that can include reclaimed material such as crushed concrete	Yes, varies widely	Fill, trench backfill, low-cost base where allowed
DSA	Driving Surface Aggregate, a dense blend designed as a top layer for dirt and gravel roads	Yes, controlled	Wearing surface on unpaved roads and lanes

Outside Pennsylvania, the same job is done by other state names: 304 limestone in Ohio, 21A in Virginia, CA6 in Illinois, Item 4 in New York and crusher run in much of the South. They differ in the details, but all are dense-graded stone with fines meant to compact. Our dense grade aggregate driveway guide covers how this whole family behaves.

What 2A modified is good for

Base under an asphalt driveway. This is its main job. Six to eight inches of compacted 2A over firm, well-drained soil is a common residential choice, with more on soft clay. See paving over a gravel driveway if you are reusing an old stone surface.
Base under concrete and pavers. Slabs, sidewalks, patios and shed pads all benefit from a compacted layer that will not settle unevenly. The gravel base under a concrete slab guide covers depth.
Gravel driveways and parking pads. 2A packs into a drivable surface on its own, and many rural lanes are nothing but 2A that gets touched up every few years.
Trench backfill and general fill. Many municipalities allow 2A around and over utility trenches because it compacts without long settling. Follow the local spec when a permit is involved.

Where 2A falls short

As a top surface, 2A has a few habits. Rain and traffic slowly wash the fines out of the top inch, which leaves loose stones that roll under tires and feet. On slopes it can gully, and in dry weather it throws dust. On a road or long lane that only gets stone, DSA usually holds up better because its blend was designed to stay put. On a driveway, many owners lay 2A as the base and finish with a thin layer of a smaller, cleaner stone, or they pave it. Because 2A holds water in its fines, it is also a poor pick for a French drain or anywhere you need water to pass through quickly. Use a clean stone there.

How much 2A modified weighs

Loose 2A, the way it sits in the truck or the pile, commonly weighs about 1.4 to 1.5 tons per cubic yard. It shrinks when you compact it, so a cubic yard of finished, rolled base takes more stone than that, often around 1.8 tons, depending on the rock and moisture. Limestone, sandstone, slag and gravel versions all get sold as 2A, and each weighs a little differently, so use your supplier’s figure when they give one. For other materials, see our gravel weight per yard chart.

How many tons of 2A you need

Measure the area in square feet and decide on the compacted depth in inches. Then:

Tons ≈ area (sq ft) × compacted depth (in) × 0.0055
Area one ton covers ≈ 180 ÷ compacted depth (in), in square feet

That factor already builds in the extra stone needed for compaction, based on about 1.8 tons per compacted cubic yard. Add another 5 to 10 percent if your edges are irregular or the subgrade is soft.

Compacted depth	Approx. area one ton covers	Tons per 100 sq ft
4 inches	about 45 sq ft	about 2.2
6 inches	about 30 sq ft	about 3.3
8 inches	about 23 sq ft	about 4.4
10 inches	about 18 sq ft	about 5.5

Example: a 12 by 60 foot driveway base, 6 inches compacted. 12 × 60 = 720 square feet. 720 × 6 × 0.0055 = about 23.8 tons. A tri-axle dump truck often carries somewhere around 20 to 25 tons, so this is roughly one full load. Our guide to tons of gravel in a dump truck explains how loads vary.

Example: a 10 by 16 foot shed pad, 4 inches compacted. 10 × 16 = 160 square feet. 160 × 4 × 0.0055 = about 3.5 tons. Order 4 tons to cover edges and leveling.

How to install a 2A base

Strip sod, topsoil and soft spots down to firm ground, and shape the subgrade so water drains away.
On clay or wet soil, roll out a woven geotextile first so the stone does not push down into the mud. A geogrid can help on very soft ground.
Spread the 2A in lifts of about 3 to 4 inches. Thicker lifts are hard to compact all the way through with a plate compactor.
Keep the stone damp, not soaked, and compact each lift until a footprint barely shows. Compacting a gravel base in lifts walks through the passes.
Finish with a crown or a steady cross slope so rain runs off rather than soaking in.

Buying tips

Say the full name. Ask for “2A modified” or “2A subbase.” If you just ask for “2A gravel,” a yard may assume you know which one you want.
Ask about the rock. Limestone 2A is common in central Pennsylvania, while other regions may sell sandstone, gravel or slag versions. They all compact, but color and dust differ.
Check whether it is certified. For a permit or a road job, you may need PennDOT-approved material from a listed source. For a home driveway, a quarry’s standard 2A is usually fine.
Compare delivered prices. Stone is cheap at the scale and expensive to haul, so the distance from the quarry often moves the price more than the stone itself.

Questions people ask

What is 2A modified stone?

It is PennDOT No. 2A aggregate, a dense-graded crushed stone from about 2 inches down to dust. Yards add the word modified to show it is the blend with fines. It compacts into a firm base for driveways, slabs and roads.

Is 2A modified the same as crusher run?

They do the same job. Crusher run, 304, 21A, CA6 and Item 4 are other regional names for dense-graded base stone with fines. Gradations differ a little by state spec and quarry, so compare the product sheets if a plan names one.

How much does a ton of 2A modified cover?

At about 1.8 tons per compacted cubic yard, one ton covers roughly 45 square feet at 4 inches compacted, 30 square feet at 6 inches, or 23 square feet at 8 inches.

How much does a yard of 2A modified weigh?

Loose 2A commonly runs about 1.4 to 1.5 tons per cubic yard. Compacted in place it takes more stone per yard, often around 1.8 tons. Moisture and rock type change the number, so use your supplier's figure when you have it.

Can I use 2A modified for a driveway?

Yes. It is one of the most common driveway bases in Pennsylvania and packs into a drivable surface on its own. Left as the top layer it loses fines over time and can get loose or dusty, so many owners top it with smaller stone or pave it.

What is the difference between 2A and 2B stone?

2A has fines and compacts hard. 2B is a name many Pennsylvania yards use for a clean stone close to #57, with the fines removed so water drains through it.

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