Nominal Maximum Aggregate Size (NMAS): Definition and Lifts

Nominal maximum aggregate size, almost always shortened to NMAS, is the number that names an asphalt mix. When a plant sells a "12.5 mm" or "19 mm" mix, it is talking about NMAS. The figure tells you roughly how coarse the stone in the mix is, and that controls how thin you can lay it, how it looks and how it compacts. This guide covers the exact Superpave definition, how to read it off a gradation, how it differs from maximum size and why it matters for lift thickness.

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Quick facts

Superpave definition: the sieve size just above the first sieve holding back over 10 percent.
Maximum aggregate size: one sieve size larger than the NMAS.
Standard mix sizes: 4.75, 9.5, 12.5, 19.0, 25.0 and 37.5 mm.
Lift rule of thumb: compacted thickness of 3 to 4 times the NMAS.
Scope: AASHTO M 323 notes these definitions apply to Superpave mixes only.

How to find NMAS from a gradation

Start with a sieve analysis and work from the largest sieve down, looking at the percent retained on each sieve (not cumulative). The first sieve that holds more than 10 percent is your trigger. The NMAS is the next sieve size up. Here is a made-up blend:

Sieve	% passing	% retained on this sieve
25.0 mm (1 in)	100	0
19.0 mm (3/4 in)	100	0
12.5 mm (1/2 in)	96	4
9.5 mm (3/8 in)	82	14
4.75 mm (No. 4)	58	24

The 12.5 mm sieve holds only 4 percent. The 9.5 mm sieve holds 14 percent, making it the first to hold back over 10. One size larger than 9.5 mm is 12.5 mm, so this is a 12.5 mm NMAS mix, and its maximum aggregate size is 19.0 mm.

NMAS vs maximum aggregate size

The two are easy to mix up. NMAS describes the largest stone present in a meaningful amount. Maximum size is one sieve larger and is the size that everything passes. Concrete specifications use ASTM-style definitions built around the sieves that a grading is permitted or required to pass, so a concrete "nominal maximum size" may not match the Superpave label for the same stone. Always check which system a spec is using.

Why NMAS matters

Lift thickness

If a lift is too thin for its stone, the largest particles bridge between the roller and the layer below. The mat will not reach density, and the rollers may crack the stone. Asphalt magazine summarizes the Asphalt Institute guidance as a minimum compacted lift of four times NMAS for dense-graded mixes, with fine-graded mixes able to go down to about three times. In practical terms:

NMAS	About 3 × NMAS	About 4 × NMAS
9.5 mm (3/8 in)	1 1/8 in	1 1/2 in
12.5 mm (1/2 in)	1 1/2 in	2 in
19.0 mm (3/4 in)	2 1/4 in	3 in
25.0 mm (1 in)	3 in	4 in

Thicker sections are often placed in more than one lift; see why two lifts beat one thick lift.

Volumetric requirements

AASHTO M 323 ties the minimum voids in mineral aggregate to NMAS. The minimums are 11.0 percent for 37.5 mm, 12.0 for 25.0 mm, 13.0 for 19.0 mm, 14.0 for 12.5 mm, 15.0 for 9.5 mm and 16.0 for 4.75 mm. Finer mixes need more room for binder because their aggregate has more surface area.

Texture and use

Smaller-NMAS mixes give a tighter, smoother surface and suit thin overlays and driveways. Larger-NMAS mixes are stiffer, carry heavy loads well and are used in base and binder courses, but they segregate more easily and are harder to hand-work. Our 19 mm mix guide and the asphalt mix types chart compare common choices.

From NMAS to tons

Once you know the mix and lift, the order is area × thickness × density. A made-up example: a 12 × 50 foot driveway with a 2-inch lift of 12.5 mm surface mix is 600 square feet × (2 ÷ 12) = 100 cubic feet. At an assumed 145 pounds per cubic foot compacted, that is 14,500 pounds, or about 7.3 tons. The asphalt tonnage calculator and driveway asphalt calculator do this for you, and the base calculator handles the stone underneath.

Questions people ask

What is nominal maximum aggregate size?

In Superpave asphalt mix design, you find the nominal maximum aggregate size by working down the sieve stack to the first one catching over a tenth of the blend, then stepping up one size. It names the mix, as in a 12.5 mm or 19 mm mix.

What is the difference between NMAS and maximum aggregate size?

In Superpave terms, the maximum aggregate size is simply one sieve size larger than the NMAS. A 12.5 mm NMAS mix has a 19 mm maximum size, and all of the aggregate passes that larger sieve.

Is NMAS defined the same way for concrete?

No. Superpave's definitions apply only to Superpave mixes, as AASHTO M 323 notes. Concrete aggregate specs use ASTM-style definitions tied to the sieves a grading is permitted or required to pass, so the same stone can carry a different label.

How thick should an asphalt lift be compared with NMAS?

A common rule from the Asphalt Institute is a compacted lift at least four times the NMAS for dense, coarse-graded mixes, and at least three times for finer-graded mixes. For a 12.5 mm (1/2 inch) mix, that works out to about 1 1/2 to 2 inches.

What NMAS is used for driveways?

Surface mixes of 9.5 mm or 12.5 mm are common for driveways because they give a tight, smooth finish in thin lifts. Larger 19 mm or 25 mm mixes are used in lower base or binder layers. Local mix names vary, so ask the plant.

How does NMAS affect the minimum VMA?

Smaller NMAS mixes must have more voids in the mineral aggregate. AASHTO M 323 sets minimums ranging from 11.0 percent for a 37.5 mm mix to 16.0 percent for a 4.75 mm mix.

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