Unit Weight of Aggregate: Test, Formula and Tons per Yard
The unit weight of aggregate is how much a cubic foot of dry stone, gravel or sand weighs when it is packed into a container, voids and all. Labs call it bulk density, and the standard test is ASTM C29 (AASHTO T 19). It matters well outside the lab, because it is the number that links a volume, such as a cubic yard in a bin or a box on a plan, to the tons that show up on a scale ticket. This guide explains what the number means, how the test is run, how to do the math, how to get voids from it and how to turn it into tons per cubic yard.
The short answer
- Definition: unit weight is the mass of aggregate that fills a container of known volume, divided by that volume. The volume counts the particles plus the air spaces between them.
- Formula: unit weight = (mass of measure full of aggregate − mass of empty measure) ÷ volume of the measure. Results are reported in lb/ft³ or kg/m³.
- Three ways to fill the measure: rodding for aggregate up to 1-1/2 inch nominal size, jigging for larger stone, and loose shoveling only when a spec asks for loose unit weight.
- Typical values: the Portland Cement Association gives roughly 75 to 110 lb/ft³ (1,200 to 1,750 kg/m³) for the aggregates commonly used in normal-weight concrete.
- To get tons per cubic yard: multiply lb/ft³ by 27 and divide by 2,000. A result of 100 lb/ft³ works out to 1.35 tons per cubic yard.
What unit weight actually measures
Every pile of rock is part rock and part air. Specific gravity describes the rock alone: how heavy the particles are compared with the same volume of water. Unit weight describes the pile. Because the gaps between particles are part of the volume, two materials made of the same stone can have quite different unit weights. A single-size stone leaves big gaps and weighs less per cubic foot. A well-graded mix, where smaller pieces fill the spaces between larger ones, packs tighter and weighs more.
ASTM still uses the old term “unit weight” in its title, in quotation marks, and notes that the property is really mass per unit volume, which is density. Pores inside individual particles are not counted as voids by the test. Only the open space between particles is.
Equipment for the test
The test needs very little: a rigid cylindrical metal measure, a scale, a straight steel tamping rod, a shovel or scoop, and a piece of plate glass for calibration. The California DOT version of the method (California Test 212) describes the rod as 5/8 inch in diameter and about 24 inches long with a rounded tip.
The measure has to be big enough for the largest stone in the sample. C29 sizes it by nominal maximum aggregate size:
| Nominal maximum size | Measure capacity |
|---|---|
| 1/2 inch (12.5 mm) | 1/10 ft³ (2.8 L) |
| 1 inch (25.0 mm) | 1/3 ft³ (9.3 L) |
| 1-1/2 inch (37.5 mm) | 1/2 ft³ (14 L) |
| 3 inch (75 mm) | 1 ft³ (28 L) |
| 4 inch (100 mm) | 2-1/2 ft³ (70 L) |
| 5 inch (125 mm) | 3-1/2 ft³ (100 L) |
A smaller stone can always go in a larger measure. The reverse gives a misleading number, because big particles cannot nest properly against the walls of a small container.
Calibrating the measure
Nobody trusts the stamped volume. To calibrate, fill the measure with room-temperature water, slide the glass plate across the top to push out bubbles and excess water, and weigh it. The net weight of water divided by the density of water at that temperature gives the true volume. At 70°F water weighs about 62.30 lb/ft³. Many technicians keep a factor, F, which is one over the volume, so that every later result is simply net aggregate weight times F. C29 calls for recalibrating at least once a year.
How the test is done
Start with a representative sample dried to constant mass, usually in an oven at 230°F, plus or minus 9°F. Then fill the measure using the method that fits the material.
Rodding (most aggregate up to 1-1/2 inch)
- Fill the measure about one-third full and level the surface with your fingers.
- Rod the layer 25 times, spreading the strokes evenly. On this first layer, do not let the rod hit the bottom hard.
- Fill to two-thirds, level and rod 25 more times, pushing just hard enough for the rod to reach the layer below.
- Overfill, rod a final 25 times, then level the top so that pieces sticking up roughly make up for the hollows below the rim.
- Weigh the full measure and subtract the empty weight.
Jigging (larger stone)
For aggregate over 1-1/2 inch, each third of the measure is settled by lifting alternate sides about 2 inches and letting the measure drop onto a firm floor, 50 drops per layer (25 on each side). The impact packs the large pieces in a way a rod cannot.
Shoveling (loose unit weight)
For loose unit weight, the aggregate is poured from a shovel held no more than 2 inches above the rim until the measure overflows, then leveled. Loose values come out lower than rodded values for the same material, so always note which method a number came from.
Worked example
Here is the arithmetic with made-up but realistic lab readings for a 3/4 inch crushed stone tested in a 1/2 ft³ measure:
| Step | Reading or result |
|---|---|
| Water to fill measure at 70°F | 31.20 lb |
| Volume = 31.20 ÷ 62.301 | 0.5008 ft³ (F = 1.997) |
| Measure + rodded stone | 63.40 lb |
| Empty measure | 14.10 lb |
| Net stone | 49.30 lb |
| Unit weight = 49.30 × 1.997 | 98.4 lb/ft³ |
That falls inside the PCA’s usual range. In practice the lab runs more than one fill and reports the average.
Voids in aggregate from unit weight
Once you know the unit weight and the bulk specific gravity of the particles (from ASTM C127 for coarse aggregate or C128 for fine aggregate), C29 lets you calculate how much of the measure is empty space:
% voids = 100 × [(S × W) − M] ÷ (S × W)
Here S is the dry bulk specific gravity, W is the density of water (62.3 lb/ft³ or 998 kg/m³) and M is the unit weight. If the stone above has a specific gravity of 2.65, then S × W is 165.1 lb/ft³, the weight a solid block of that rock would have. The voids come out to 100 × (165.1 − 98.4) ÷ 165.1, or about 40 percent.
For context, the PCA puts void content at about 30 to 45 percent for coarse aggregate and 40 to 50 percent for fine aggregate. Angular, crushed particles leave more voids. Larger, well-graded material leaves fewer. Concrete designers care because paste has to fill those gaps.
The test reports dry unit weight. If you need it on a saturated surface-dry basis, C29 multiplies by (1 + absorption ÷ 100). With 0.8 percent absorption, the 98.4 lb/ft³ example becomes about 99.2 lb/ft³.
Converting unit weight to tons per cubic yard
A cubic yard is 27 cubic feet and a short ton is 2,000 pounds, so:
tons per cubic yard = unit weight (lb/ft³) × 27 ÷ 2,000
| Unit weight (lb/ft³) | Tons per cubic yard |
|---|---|
| 75 | 1.01 |
| 85 | 1.15 |
| 90 | 1.22 |
| 95 | 1.28 |
| 100 | 1.35 |
| 105 | 1.42 |
| 110 | 1.49 |
The worked example, 98.4 lb/ft³, is about 1.33 tons per cubic yard, so a 20-ton load of that stone would fill roughly 15 cubic yards at the same packing. To go the other way, from tons to yards, divide instead of multiply; our tons to cubic yards for gravel guide walks through it.
Why the lab number will not match your truck
C29 itself warns against treating a lab result as a purchase conversion without thought. It says the compaction in a truck bed or stockpile is unknown compared with the test, and that hauled material usually carries absorbed and surface moisture while the test is run dry. Damp sand in particular bulks up and takes more room than the same sand dry. A few practical consequences:
- Loose and rodded values bracket reality. Freshly dumped stone sits closer to loose unit weight. Rolled base sits above the rodded value.
- Moisture adds weight. A wet load weighs more per yard than the dry test value, and you pay for water on a ton basis.
- Compacted base is a different test. In-place density for a compacted base or subgrade comes from a Proctor curve and field density tests, not C29. See our standard Proctor test guide.
- Ask the supplier. Quarries know the weight per yard of their own products. Use their number when you have it, and plan a margin for waste and compaction.
Where unit weight gets used
- Concrete mix design. The common ACI absolute-volume method sets the amount of coarse aggregate using its dry-rodded unit weight. This is the main reason ready-mix labs run the test.
- Buying and selling by volume. C29 notes that the result can be used to set mass and volume relationships in purchase agreements, with the moisture and compaction caveats above.
- Checking a source. A shift in unit weight from the same pit can flag a change in grading or particle shape before it causes trouble.
- Estimating material. Contractors use a weight per yard to turn plan volumes into tons. Our crushed stone calculator and gravel calculator do that conversion for common materials.
Common mistakes
- Using a measure that is too small for the stone size, which lowers the result.
- Testing a damp sample and reporting it as dry unit weight.
- Skipping calibration and trusting the nominal volume of a dented or worn measure.
- Mixing up methods, such as comparing one supplier’s loose value with another’s rodded value.
- Confusing unit weight with specific gravity. Specific gravity times 62.4 gives the weight of solid rock, not of a pile. A pile always weighs less because of the voids.
Questions people ask
What is the unit weight of aggregate?
It is the weight of dry aggregate needed to fill a container of known volume, divided by that volume. The volume includes the air spaces between particles, so it describes a pile of stone or sand rather than solid rock.
Is unit weight the same as bulk density?
Yes. ASTM C29 uses both names. Unit weight is the older term, and the property is really mass per unit volume, reported in lb/ft3 or kg/m3.
What is dry rodded unit weight?
It is the unit weight measured after the aggregate is placed in three layers and each layer is rodded 25 times. It is the value used for coarse aggregate in the common ACI concrete mix design method.
What is a typical unit weight for aggregate?
The Portland Cement Association lists about 75 to 110 lb/ft3 (1,200 to 1,750 kg/m3) for aggregates commonly used in normal-weight concrete. Grading, particle shape and rock type move it within that range.
How do I convert unit weight to tons per cubic yard?
Multiply lb/ft3 by 27 and divide by 2,000. For example, 100 lb/ft3 equals 1.35 tons per cubic yard.
How do you calculate voids from unit weight?
Use percent voids = 100 x (S x W - M) / (S x W), where S is the dry bulk specific gravity, W is the density of water (62.3 lb/ft3) and M is the unit weight.
Related guides
- How much does a yard of crushed stone weigh?
- How much does a yard of sand weigh?
- How much does a yard of pea gravel weigh?
- Sieve analysis of aggregate
- Crushed stone vs gravel
- Concrete mix ratio
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