The short answer
Bulk density is the weight of pellets that fills a given volume, air gaps included. Most unfilled pellets run about 30–45 lb/ft³ (480–720 kg/m³): polyolefins near 30–37, styrenics and PC near 36–45, and dense or glass-filled resins about 45–55. Regrind and flake run lower. It is measured by ASTM D1895 and used to size hoppers, dryers, silos and gaylord loads. Pellet shape changes it, so measure your own lot when it matters.
Bulk density vs. solid density
Solid density (specific gravity) is the mass of the plastic itself. It sets part weight and is what the data sheet lists. Bulk density is the mass of a heap of pellets divided by the volume the heap occupies. Air fills the gaps, so for round or cylindrical pellets the bulk density is usually about 50–65% of the solid density. Two resins with the same solid density can differ in bulk density because their pellets are cut differently.
Units and conversions
- 1 lb/ft³ = 16.02 kg/m³
- 1 g/cm³ = 1 kg/L = 1,000 kg/m³ = 62.43 lb/ft³
- Example: 35 lb/ft³ = 561 kg/m³ = 0.56 g/cm³
Plants in the U.S. mostly use lb/ft³. Equipment from Europe is often sized in kg/m³ or kg/L.
Typical bulk density by resin
| Resin | Bulk density, lb/ft³ | Bulk density, kg/m³ | Solid density, g/cm³ |
|---|---|---|---|
| PP | 30–36 | 480–580 | 0.90–0.91 |
| HDPE | 33–37 | 530–590 | 0.94–0.97 |
| LDPE / LLDPE | 31–36 | 500–580 | 0.91–0.94 |
| PS / HIPS | 36–41 | 580–660 | 1.04–1.05 |
| ABS | 37–42 | 590–670 | 1.03–1.07 |
| PMMA | 35–42 | 560–670 | 1.18–1.19 |
| PC | 40–45 | 640–720 | 1.19–1.21 |
| PC/ABS | 38–43 | 610–690 | 1.12–1.17 |
| PA6 / PA66 | 38–45 | 610–720 | 1.13–1.15 |
| PA6 / PA66, 30% glass | 44–52 | 700–830 | 1.35–1.38 |
| POM (acetal) | 48–55 | 770–880 | 1.41–1.42 |
| PBT | 45–50 | 720–800 | 1.30–1.32 |
| PET (crystallized pellets) | 50–55 | 800–880 | 1.33–1.40 |
| PPS, 40% glass | 50–60 | 800–960 | 1.65–1.68 |
| PEEK | 42–50 | 670–800 | 1.30–1.32 |
| TPU | 35–40 | 560–640 | 1.10–1.25 |
| PVC compound pellets | 45–52 | 720–830 | 1.30–1.45 |
| Regrind (runners, parts) | 25–40 | 400–640 | as base resin |
| Flake (bottle, sheet) | 15–30 | 240–480 | as base resin |
Typical poured bulk density of standard pellets. Values vary by grade and especially by pellet size and shape (cylinder, lens, sphere), and tapped or vibrated material packs denser. Use these numbers for estimates; measure your lot for equipment sizing.
How it is measured
ASTM D1895 covers apparent density, bulk factor and pourability of plastics. In short, the material is poured through a standard funnel into a container of known volume, the top is struck off level, and the contents are weighed. Method A covers fine granules and powders and corresponds to ISO 60. Method B covers pellets and other coarse granules that do not pour well through the Method A funnel. Method C covers bulky flake and fibers, measured under a light load, and is identical to ISO 61.
On the plant floor you can get a working number with a bucket: measure its inside volume, weigh it empty, fill it with pellets without shaking, strike it level and weigh it again. Pellet weight divided by volume is the bulk density. A 5-gallon pail holds about 0.67 ft³, so 24 lb of pellets in a full pail is about 36 lb/ft³.
What changes it
- Pellet shape and size. Small, uniform, round pellets pack tighter. Long strand-cut cylinders and irregular pellets leave more air.
- Fillers. Glass fiber and minerals raise solid density, so bulk density rises with them. A 30% glass nylon runs roughly 15–20% heavier per cubic foot than unfilled nylon.
- Regrind and flake. Irregular granules and thin flake trap far more air. Regrind from runners and parts is typically 25–40 lb/ft³; bottle flake runs about 15–30 lb/ft³, and loose film fluff can be well under 10 lb/ft³.
- Fines and blends. Dust fills gaps and can raise the figure slightly, but it also makes the material flow and convey worse. Regrind and pellets of different bulk density separate in hoppers and loaders.
- Settling. Vibration in transit packs pellets down a few percent, which is why a full gaylord appears to settle.
Sizing a dryer hopper
A dryer hopper must hold enough resin to give every pellet its full drying time at the rate the press uses it. By weight that is throughput × drying time. Divide by bulk density to get the volume.
Example: a press uses 100 lb/hr of PC that needs 4 hours at 120°C / 248°F. The hopper must hold 400 lb, or 480 lb with a 20% margin. At 42 lb/ft³ that is about 11.4 ft³. Switch to a blend with a lot of regrind at 30 lb/ft³ and the same 480 lb needs 16 ft³, so an undersized hopper shortens residence time and the resin goes in wet. The dryer and hopper calculator does this math (it defaults to 35 lb/ft³; enter your measured value). Drying temperatures and times are in the resin drying guide.
Gaylord fill weight
A standard gaylord is about 40 × 48 in at the base. Filled about 36 in deep, it holds roughly 40 ft³ of pellets: about 1,200 lb of polypropylene at 30 lb/ft³, or about 1,600 lb of polycarbonate at 40 lb/ft³. Resin gaylords are commonly packed to a net weight in the range of roughly 1,000–1,600 lb, so a dense resin such as acetal or glass-filled nylon simply fills the box less deep. Always check the net weight on the quote and the packing list. More on package sizes in gaylord vs. LTL vs. truckload.
Silos, bins and conveying
- Silos and bins are volume-limited. A silo sized for 40,000 lb of PBT will not hold 40,000 lb of polypropylene.
- Vacuum loaders and conveying lines are rated in lb/hr for a given bulk density. Light flake and regrind move fewer pounds per hour and can bridge in hoppers.
- Volumetric feeders and blenders meter by volume. If you change resin or regrind share, recalibrate by weight, or use a gravimetric blender.
What to send us
- The grade you run, or its data sheet
- Package you can receive (bags, gaylord, bulk bag, bulk truck)
- Quantity and ship-to city
We can give the packed net weight per gaylord for the grade you order, and every lot ships with a COA. We aim to send your quote within one to three business days.
Common questions
What is the bulk density of plastic pellets?
Most unfilled pellets fall between about 30 and 45 lb/ft³ (480–720 kg/m³). Polyolefins sit at the low end, around 30–37 lb/ft³; denser resins such as PBT, PET and acetal, and glass-filled grades, run about 45–55 lb/ft³. Regrind and flake run lower.
What is the difference between bulk density and density?
Density (solid or specific gravity) is the mass of the plastic itself, with no air. Bulk density is the mass of a pile of pellets divided by the volume it fills, including the air between pellets, so it is usually only about 50–65% of the solid density.
How is bulk density of plastics measured?
ASTM D1895 pours the material through a standard funnel into a container of known volume, strikes it level and weighs it. Pellets are usually measured by Method B; Method A corresponds to ISO 60. In the plant, a bucket of known volume and a scale give a usable number.
Why does bulk density matter for a dryer?
A dryer hopper must hold throughput times drying time, by weight. The hopper is a fixed volume, so a lighter material such as regrind needs a bigger hopper to get the same residence time.
How do I convert lb/ft3 to kg/m3?
Multiply lb/ft³ by 16.02 to get kg/m³. For g/cm³ (the same as kg/L), divide lb/ft³ by 62.43.
What is the bulk density of regrind?
Regrind from runners and parts typically runs about 25–40 lb/ft³, bottle and sheet flake about 15–30 lb/ft³, and loose film fluff can be well under 10 lb/ft³. It depends on the grinder screen, particle shape and how much dust is present.
Why do gaylords of different resins weigh different amounts?
The box volume is roughly fixed, so a denser resin fits more pounds in the same box. Resin gaylords are commonly packed to a net weight of roughly 1,000–1,600 lb, and the heavier resins simply fill the box less deep.
Tell us the grade and package you need
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province