Injection Molding calculator
Plastic Part Weight Calculator
Predict what one molded part should weigh from its CAD volume and resin density, then check a first-article part against it. You need the CAD volume, the grade's datasheet density and one part off the press.
What this calculator does
- Predicted weight of one molded part, and whether a weighed part points to a short shot, sink or flash.
Formula used
- Predicted part weight (g) = part volume from CAD (cc) × resin density (g/cc)
- Weighed part vs prediction (%) = (weighed part − predicted) ÷ predicted × 100
- Tolerance band (g) = predicted × (1 − tolerance ÷ 100) to predicted × (1 + tolerance ÷ 100)
Inputs explained
- Part Volume From CAD: Mass-properties volume of one part at print size, no runner.
- Resin Density: Solid density of your exact grade, fillers included, from its datasheet.
- Weighed Part: A packed-out part from the press or a sample, runner removed.
- Weight Tolerance (±): Deviation you accept before investigating, e.g. from your control plan.
How to use the result
- Best suited to quoting resin per part from a CAD model, first-article weight check at mold trial, checking a customer's sample part against CAD.
- Semi-crystalline density shifts with mold temperature and wall thickness; allow for it in your tolerance. Weight alone cannot tell a short shot from sink or voids: inspect the part.
Current U.S. benchmarks
- The producer price index for plastic resins and materials stands at 280.569 (BLS, Aug 2026), up 6.6% 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
- Why is there no packing factor? Because datasheet density already describes solid resin. Hold pressure pushes in extra melt to replace the volume lost as the resin cools and crystallizes. A factor of 1.03 would claim the part is 3% denser than its datasheet.
- What density do I enter for a filled or toughened grade? Enter the density on that exact grade's datasheet. DuPont lists 1.42 g/cc for Delrin 500P, 1.34 for toughened 100ST and 1.56 for glass-filled 570, so a base-resin value can miss by nearly 10%.
- Does a light part always mean a short shot? No. Sink marks and internal voids also remove weight, and both usually trace to too little hold pressure or hold time. A short shot is visibly incomplete; voids hide inside thick ribs and bosses.
- Can I check cavity balance with this? Yes. Weigh one part from each cavity of the same shot and enter them in turn. A cavity that runs light against its neighbors is filling or packing short.
Related guides
Last reviewed 2026-10-01.