Voids in Mineral Aggregate (VMA): Formula and Minimums

Voids in mineral aggregate, almost always shortened to VMA, is the share of a compacted asphalt sample that is not stone. Picture the aggregate skeleton with the binder and air stripped out: the gaps left between the particles are the VMA. That space is what holds the asphalt binder and the small pockets of air a mix needs, so a mix design lives or dies by it. This guide explains what VMA includes, how to calculate it from lab numbers, the Superpave minimums, how it relates to air voids and VFA, and what pushes it up or down.

The short answer

Definition: VMA is the volume between aggregate particles in compacted mix, counting both the air voids and the binder that has not soaked into the stone, written as a percent of the sample’s total volume.
Formula: VMA = 100 − (Gmb × Ps ÷ Gsb), where Gmb is the mix bulk specific gravity, Ps the percent aggregate by weight of mix and Gsb the aggregate bulk specific gravity.
Minimums: Superpave sets a floor by nominal maximum aggregate size, from 16.0 percent for 4.75 mm mixes down to 11.0 percent for 37.5 mm mixes. Your state may add its own tweaks.
Why it matters: with air voids fixed near 4 percent, VMA decides how much room is left for binder. Too little VMA means thin binder films and a less durable mix.

What VMA counts and what it leaves out

A compacted sample has three parts by volume: aggregate, binder and air. Some binder also seeps into the pores of the stone. That absorbed binder behaves like part of the rock, so VMA does not count it. What VMA does count is everything else between the particles:

Air voids (Va): the tiny pockets of air between coated particles. See our guide to air voids in asphalt for targets and field checks.
Effective binder: the binder that coats the stone and glues it together, as opposed to the binder lost into the aggregate’s pores.

That is the same definition written into AASHTO R 35, the Superpave volumetric design practice: the room between the particles of compacted paving mix, made up of air voids plus effective binder and stated as a share of the whole specimen’s volume. Because VMA uses the aggregate’s bulk specific gravity (Gsb), the calculation treats any pore the binder could fill as part of the stone, which is how the absorbed binder drops out.

The VMA formula, step by step

You need three numbers, all of which usually show up on a mix design or a quality control report:

Gmb, the bulk specific gravity of the compacted mix, from AASHTO T 166 or a similar method. Our guide to bulk specific gravity of asphalt covers the test.
Ps, the percent of aggregate by total mix weight. It is simply 100 minus the binder content (Pb). A mix with 5.5 percent binder has a Ps of 94.5.
Gsb, the combined bulk specific gravity of the aggregate blend, measured on each stockpile and averaged by its share of the blend.

Then plug them in: VMA = 100 − (Gmb × Ps ÷ Gsb). The part in parentheses is the percent of the sample’s volume taken up by stone. Whatever is left over is VMA.

A worked example (made-up numbers)

Say a lab puck has a Gmb of 2.448, the mix carries 5.5 percent binder (so Ps is 94.5) and the blended aggregate has a Gsb of 2.680. The stone takes up 2.448 × 94.5 ÷ 2.680 = 86.3 percent of the volume, so the VMA is 100 − 86.3 = 13.7 percent. These values are invented for illustration and match the example in our Gmb guide.

If the same mix has a Rice value (Gmm) of 2.540, its air voids are 100 × (2.540 − 2.448) ÷ 2.540 = 3.6 percent. Subtract that from the VMA and about 10.1 percent of the volume is effective binder. As a share of the VMA, that binder fills 10.1 ÷ 13.7, or roughly 74 percent of the space. That last figure is the VFA, covered below. All of this is our arithmetic, not data from a real project.

Superpave minimum VMA by aggregate size

Superpave ties the minimum VMA to the nominal maximum aggregate size (NMAS), which is one sieve size larger than the first sieve that holds back more than 10 percent of the aggregate. Finer mixes get a higher floor because small particles have more surface area to coat. The table below lists the AASHTO M 323 values as printed in Kansas State University’s 2025 Superpave volumetric handbook and on Pavement Interactive’s Superpave mix design page:

Nominal maximum aggregate size	Minimum VMA
4.75 mm	16.0%
9.5 mm (3/8 in)	15.0%
12.5 mm (1/2 in)	14.0%
19.0 mm (3/4 in)	13.0%
25.0 mm (1 in)	12.0%
37.5 mm (1 1/2 in)	11.0%

These minimums apply at the design air void level, normally 4.0 percent at the design number of gyrations. Many state DOTs adjust them, so check the spec book for the job. Our example mix, at 13.7 percent, would pass as a 19.0 mm mix but fall short as a 12.5 mm mix. The broader design system is explained in our Superpave guide.

VMA, air voids and VFA work as a set

The three volumetric numbers are tied together. Air voids are a slice of the VMA, and the rest of the VMA is binder. Voids filled with asphalt (VFA) expresses that binder slice as a percent of the VMA:

VFA = 100 × (VMA − Va) ÷ VMA

Agencies set a VFA range that depends on design traffic, generally from the mid-60s up to about 80 percent. Pavement Interactive notes that VFA is somewhat redundant because it follows from the other two. The FHWA makes the practical point in its tech brief on Superpave gyratory levels: the VMA requirement is what really controls binder content. With a 14 percent VMA minimum and 4 percent air voids, about 10 percent of the mix volume is left for binder. Raise the VMA floor and the mix must carry more binder. Lower it and the mix can get by with less.

What raises or lowers VMA

Gradation. A gradation that sits right on the maximum density line packs the stone as tightly as possible. The Kansas State handbook calls that a gradation to avoid, since it leaves very little space for binder films thick enough to last. Moving the blend above or below that line opens up more VMA.
Particle shape and texture. Angular, rough crushed stone tends to resist packing more than smooth rounded gravel, which affects how much space is left between particles.
Dust content. Fines passing the No. 200 sieve can fill the gaps between larger particles. That is one reason Superpave also checks a dust-to-binder ratio.
Compaction effort. More gyrations squeeze the skeleton tighter. The FHWA brief warns that cutting gyration levels without changing the volumetric criteria lets weaker aggregate structures pass, and it suggests agencies that want more binder raise the VMA floor or lower the design air voids instead.

Small input errors move VMA a lot

Because VMA comes from a subtraction, a small error in either specific gravity shows up quickly. In our example, a Gsb that reads 0.010 too high (2.690 instead of 2.680) bumps the calculated VMA from 13.7 to 14.0 percent. A Gmb that reads 0.010 too high (2.458) drops it to 13.3 percent. That swing of about 0.3 points either way is enough to turn a pass into a fail, which is why labs re-check aggregate specific gravities when a source or stockpile changes, and why Gmb weighings are done carefully. The numbers here are our own calculation from the made-up example.

Does VMA change how many tons you order?

Not directly. Tonnage depends on how dense the finished mat is, which comes from Gmb and the compaction you achieve in the field. VMA is a design check that tells you whether the mix has room for enough binder. If you are estimating material for a driveway or lot, start with our asphalt tonnage calculator and the density figures in asphalt density in lbs per cubic foot.

Questions people ask

What are voids in mineral aggregate?

VMA is the space between the aggregate particles in a compacted asphalt sample. It includes the air voids and the binder that coats the stone, but not binder absorbed into the stone, and it is written as a percent of the sample's volume.

How do you calculate VMA?

Use VMA = 100 - (Gmb x Ps / Gsb). Gmb is the bulk specific gravity of the compacted mix, Ps is the percent aggregate by weight of mix (100 minus binder content) and Gsb is the bulk specific gravity of the aggregate blend.

What is the minimum VMA for a 12.5 mm mix?

Under the AASHTO M 323 Superpave criteria the minimum is 14.0 percent for a 12.5 mm nominal maximum aggregate size. A 9.5 mm mix needs 15.0 percent and a 19.0 mm mix needs 13.0 percent. State specs can differ.

What is the difference between VMA and air voids?

Air voids are only the pockets of air in the compacted mix. VMA is the air voids plus the effective binder. At 4 percent air voids and 14 percent VMA, about 10 percent of the volume is binder.

What is VFA in asphalt?

Voids filled with asphalt is the share of the VMA taken up by effective binder: VFA = 100 x (VMA - Va) / VMA. Agencies set an allowed VFA range that depends on design traffic.

How can you increase VMA?

The usual lever is gradation: moving the blend away from the maximum density line leaves more room between particles. Aggregate shape, dust content and compaction level also affect it. Check specific gravities first, since errors there shift the calculated VMA.

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