Table of Contents
Reviewed and updated by ICD - Week 43/2026
The density (specific gravity) of unreinforced PA plastic is about 1.13 to 1.14 g/cm³, slightly heavier than water so it sinks. BASF lists PA6 (Ultramid B3K) and PA66 (A3K) at the same 1130 kg/m³. Adding 30% glass fibre (PA66-GF30, A3WG6) raises it to 1360 kg/m³, about 20% heavier.
This article covers just one property, density, and why the figure differs by a few percent between articles. I take numbers from BASF datasheets and calculate the remaining points myself, marking which ones are my own calculations. General details on PA plastic are in the overview article.
Density of virgin PA plastic
Density is mass per unit volume, measured to ISO 1183. For unreinforced PA resin, the two common types give the same figure: BASF Ultramid B3K (PA6) and A3K (PA66) are both 1130 kg/m³, or 1.13 g/cm³. Many sources list PA66 at 1.14 to 1.15; that is from other makers or grades, about 1% apart, so for a specific calculation use the datasheet figure for the exact grade you are buying.
Two things are easy to misread. First, nylon absorbs moisture: at 23 °C and 50% RH, PA6 takes up 2.6 to 3.4% water and PA66 takes up 2.5 to 3.1% (BASF), so a part left in the open air weighs a few percent more than when freshly moulded. Second, datasheet figures are for uncoloured resin; resin with colour, lubricant or filler may differ slightly.
PA plastic sinks in water (water density is 1.0 g/cm³). This is a quick way to tell it from PE and PP: HDPE has a density of 958 kg/m³ (Borealis) so it floats, PP also floats, while PA and POM sink. This is only a rough sort, because resin with heavy filler also sinks.
Why glass-fibre PA is heavier
Load-bearing parts often use glass-fibre (GF) reinforced PA to raise stiffness and dimensional stability. Glass fibre is an inorganic material much heavier than plastic, so it pulls the density of the compound up. BASF lists PA66-GF30 (Ultramid A3WG6, 30% fibre by weight) at a density of 1360 kg/m³; the HRX version of the same range is 1370 kg/m³.
For other fibre contents I calculate with the rule of mixtures (1/ρ = fibre fraction/ρ fibre + resin fraction/ρ resin), taking base resin density as 1.13 and glass fibre as about 2.55 g/cm³. The result at GF30 matches the BASF figure, so the other points are reliable as a reference. The GF15 and GF50 figures are my own calculations, not manufacturer measurements.
| Grade | Density | Source |
|---|---|---|
| PA6, PA66 unreinforced | 1.13 g/cm³ | BASF B3K, A3K |
| PA66 + 15% glass fibre | about 1.23 g/cm³ | Rule of mixtures |
| PA66 + 30% glass fibre | 1.36 g/cm³ | BASF A3WG6 |
| PA66 + 50% glass fibre | about 1.57 g/cm³ | Rule of mixtures |
Consequence of changing material: with the same mould, switching from unreinforced PA (1.13) to GF30 (1.36) makes the part about 20% heavier. If you buy by the kg, the raw material cost per part rises almost in proportion, before counting the difference in resin price.
Comparing the density of PA with other materials
To see where PA sits, put it next to a few materials that are often compared. I use BASF datasheets for POM (the acetal plastic often confused with PA in gears), Borealis for HDPE, and the common physical figure of about 2.7 g/cm³ for aluminium. Mitsubishi Chemical also notes that PA weighs about one sixth of steel for the same volume.
| Material | Density | Note |
|---|---|---|
| HDPE (Borstar BB2588) | 0.958 g/cm³ | Floats in water; see also PE plastic |
| PA6, PA66 | 1.13 g/cm³ | Sinks in water |
| POM (Ultraform N2320) | 1.41 g/cm³ | About 25% heavier than PA |
| PA66-GF30 | 1.36 g/cm³ | 20% heavier than unreinforced PA |
| Aluminium | about 2.7 g/cm³ | About 2.4 times heavier than PA |
Two takeaways. PA is about 2.4 times lighter than aluminium (2.7 versus 1.13), so replacing an aluminium part with unreinforced PA can cut weight by more than half, but it also means losing stiffness; to keep stiffness you need a reinforced grade, which gives back some of the weight advantage. For gears, POM (1.41) is about 25% heavier than PA so it has more inertia, but POM absorbs very little moisture (0.2% at 50% RH per BASF) while PA absorbs 2.5 to 3.4%, so POM keeps its size more stably. For comparison with PP plastic, it also floats in water like HDPE.
How to calculate the weight of PA sheet and round bar
Engineering plastics are usually sold by the kg but ordered by size. The conversion formulas use metres for length and millimetres for thickness, with density ρ in g/cm³.
| Form | Formula | Example (ρ = 1.13) |
|---|---|---|
| Sheet | Weight (kg) = length (m) × width (m) × thickness (mm) × ρ | 1 m × 2 m sheet, 10 mm thick: 1 × 2 × 10 × 1.13 = 22.6 kg |
| Solid round bar | Weight (kg) = diameter (mm)² × 0.000785 × length (m) × ρ | 50 mm bar, 1 m long: 50² × 0.000785 × 1 × 1.13 = about 2.22 kg |
Both formulas are theoretical, based on nominal dimensions. Real extruded bar and sheet stock has dimensional tolerance and different moisture content, so the weighed mass usually differs by a few percent; if you need exact figures, ask the stock supplier batch by batch.
How density affects cost
Resin is sold by the kg, but a moulded part has a fixed volume. For the same 1 litre of mould volume: unreinforced PA uses about 1.13 kg of resin, POM about 1.41 kg, and PA66-GF30 about 1.36 kg. If two materials had the same price per kg, PA would be about 20% cheaper than POM per part of the same volume. In practice the two resins have different unit prices, so you must multiply by the price of each, and this article gives no prices.
Frequently asked questions
What is the density of virgin PA plastic?
About 1.13 g/cm³ for both unreinforced PA6 and PA66 per BASF (Ultramid B3K, A3K). Some makers list 1.14 to 1.15 for PA66.
Why is glass-fibre PA heavier?
Glass fibre is heavier than the base resin. BASF’s PA66-GF30 reaches 1.36 g/cm³, about 20% above unreinforced PA.
Does PA plastic float or sink in water?
It sinks, because a density of 1.13 is greater than water (1.0). PE and PP float.
How much lighter than aluminium is PA?
About 2.4 times by density (2.7 versus 1.13 g/cm³), for unreinforced PA.
How do you calculate the weight of PA plastic sheet?
Length (m) × width (m) × thickness (mm) × density (g/cm³). For example a 1 m × 2 m sheet 10 mm thick: about 22.6 kg at ρ = 1.13.
Does moisture absorption in PA change the weight?
Yes. At 23 °C and 50% RH PA6 takes up 2.6 to 3.4% water, so the part weighs a few percent more than when freshly moulded.
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References
- BASF, Ultramid A3K (PA66) Product Information, 09/2026: melting point 260 °C, saturated absorption 8 to 9%, equilibrium 2.5 to 3.1%, HDT A 75 °C, shrinkage 1.5/1.8%
- BASF, Ultramid A3WG6 PA66-GF30 (density 1360 kg/m3, equilibrium absorption 1.5 to 1.9%) - picoplast.nl
- BASF, Ultraform N2320 003 LEV AT (POM, density 1410 kg/m3, absorption 0.2% at 50% RH) - download.basf.com
- Borealis, Borstar BB2588 HDPE Product Data Sheet, 05/08/2024 (density 958 kg/m3, melting point 132 °C, Vicat 127 °C) - borealisgroup.com
- Mitsubishi Chemical Group, Nylons as Engineering Plastics (PA6 and PA66, water absorption, wear per ISO 7148-2) - mcam.com
