How do you calculate the weight of a plastic part?

Multiply the part's volume from CAD by the resin density (specific gravity) and a packing factor. For an 18 cc part in a 1.04 g/cc resin with a 1.03 packing factor, the weight is 18 x 1.04 x 1.03 = 19.28 g.

What is a packing factor and why use it?

The packing factor (1.03 here) accounts for the extra material forced into the cavity during the pack and hold phase, which raises density slightly above the nominal resin value. It nudges the estimate up by a few percent toward real molded weight.

Where do I find resin density for this calculation?

Use the resin data sheet's specific gravity, in g/cc. Common values are about 0.91 for polypropylene, 1.04 for ABS, and 1.4 for PET. The example uses 1.04 g/cc, typical of ABS.

How accurate is a CAD-based part weight estimate?

Within a few percent for amorphous resins when the packing factor is reasonable. Semi-crystalline materials, gas counter-pressure, or heavy sink can shift actual weight, so always reconcile against a weighed first article.

Why estimate part weight before the mold trial?

It drives shot size, press selection, and resin cost per part. Knowing a part is about 19.28 g lets you size the barrel and quote material cost long before steel exists.

Part weight vs shot weight - what is the difference?

Part weight is the mass of one finished part; shot weight is all parts plus the runner injected per cycle. Multiply part weight by cavity count and add the runner to get shot weight.

Injection Molding calculator

Plastic Part Weight Calculator

Plastic part weight is the mass of a single molded part, estimated from its CAD volume, the resin's density, and a small packing factor that accounts for the material compressed in during hold. Tooling engineers and estimators calculate it before cutting steel to size the shot, price resin per part, and pick the right press. An accurate weight feeds directly into cost-per-part and cycle economics, so a small error compounds across millions of shots. Because it is built from CAD volume and a published specific gravity, you can get a reliable estimate weeks before the first trial.

What this calculator does

Estimate part weight from CAD volume and resin density for shot sizing and cost estimating before mold trials.
Use this early in the quoting or mold design phase when you have a 3D model but no physical parts to weigh. The result feeds shot size, resin cost, and machine selection calculations.
It estimates molded part mass as CAD volume times resin density times a packing factor.

Formula used

Part weight = Volume x Density x Packing factor
Use this estimate for shot size, resin cost, and press selection before mold trial

Inputs explained

Part volume from CAD: Volume in cubic centimeters from your 3D model mass properties. Exclude runner and sprue volume.
Resin density (specific gravity): From the resin technical data sheet. Common values: PP 0.90, ABS 1.04, PC 1.20, Nylon 1.13, POM 1.41.
Packing factor: Multiplier for packing density increase during hold phase. Typical: 1.02 to 1.05 for semi-crystalline resins.

How to use the result

Use it during design and quoting, before a mold trial, to drive shot size, resin cost, and press selection.
It assumes uniform density and a generic packing factor; semi-crystalline resins, foaming, and thick sections can shift real weight by a few percent, so confirm against a weighed first article.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 319.371 (BLS, May 2026), up 19.5% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate the weight of a plastic part? Multiply the part's volume from CAD by the resin density (specific gravity) and a packing factor. For an 18 cc part in a 1.04 g/cc resin with a 1.03 packing factor, the weight is 18 x 1.04 x 1.03 = 19.28 g.
What is a packing factor and why use it? The packing factor (1.03 here) accounts for the extra material forced into the cavity during the pack and hold phase, which raises density slightly above the nominal resin value. It nudges the estimate up by a few percent toward real molded weight.
Where do I find resin density for this calculation? Use the resin data sheet's specific gravity, in g/cc. Common values are about 0.91 for polypropylene, 1.04 for ABS, and 1.4 for PET. The example uses 1.04 g/cc, typical of ABS.
How accurate is a CAD-based part weight estimate? Within a few percent for amorphous resins when the packing factor is reasonable. Semi-crystalline materials, gas counter-pressure, or heavy sink can shift actual weight, so always reconcile against a weighed first article.
Why estimate part weight before the mold trial? It drives shot size, press selection, and resin cost per part. Knowing a part is about 19.28 g lets you size the barrel and quote material cost long before steel exists.

Last reviewed 2026-05-12.