How do you calculate runner weight?

Multiply the runner system volume by the resin density and a geometry/sprue factor. An 8.5 cc runner in a 1.04 g/cc resin with a 1.10 geometry factor weighs 8.5 x 1.04 x 1.10 = 9.72 g.

Why add a geometry or sprue factor?

The factor (1.10 here) accounts for the tapered sprue, gate lands, and cold slug wells that a simple runner-volume estimate understates. It scales the basic volume-times-density figure up to a more realistic mass.

Does runner weight count toward shot size?

Yes, on cold-runner molds. The shot must fill the cavities and the entire runner network, so runner weight is added to total part weight to get the full per-cycle shot. Hot-runner molds carry no cold runner to add.

What is a good runner-to-part weight ratio?

Lower is better since runner is waste. Well-designed cold runners often sit around 10%-30% of total shot weight; high ratios on small parts are a strong case for a hot-runner system.

How can I reduce runner weight?

Shorten and slim the runners, reduce sprue length, balance the layout to allow smaller channels, or switch to a hot-runner or insulated-runner system to eliminate cold material entirely.

Runner weight vs part weight - why track both?

Part weight is sellable product; runner weight is recoverable scrap. Tracking both gives the runner-to-part ratio, which tells you how efficient the tool is and whether a hot runner pays off.

Injection Molding calculator

Runner Weight Calculator

Runner weight is the mass of resin in the sprue, runners, and gates that feed the cavities on a cold-runner mold, and it counts as pure overhead because that material is reground or scrapped, not sold. Tooling engineers estimate it from the runner system's CAD volume, resin density, and a geometry factor that captures the sprue taper and fittings the nominal volume misses. The number matters twice: it adds to part weight to set true shot size, and it drives the runner-to-part ratio that determines how much material you waste every cycle. On high-volume jobs, shaving runner weight is one of the clearest paths to lower material cost.

What this calculator does

Estimate cold runner system weight from runner volume, resin density, and a geometry factor for sprue and sub-runners.
Use this when calculating total shot weight for cold runner molds, estimating regrind volume, or comparing cold runner waste to hot runner investment.
It estimates cold-runner mass as runner system volume times resin density times a geometry/sprue factor.

Formula used

Runner weight = Runner volume x Resin density x Geometry factor
Add this to total part weight to get full shot size for cold runner molds

Inputs explained

Runner system volume: Total volume of sprue, main runner, sub-runners, and gates from mold design or physical measurement.
Resin density: Density of the molding resin from the material data sheet (e.g., PP: 0.90, ABS: 1.04, PC: 1.20).
Geometry/sprue factor: Multiplier for extra volume from sprue taper, cold slug wells, and branching. Typical: 1.05 to 1.20.

How to use the result

Use it when sizing the shot for a cold-runner mold and when evaluating runner-to-part waste ratios during tool design.
It uses a single geometry factor to approximate sprue taper and fittings; complex or unbalanced runner layouts may need a direct CAD mass measurement instead.

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 runner weight? Multiply the runner system volume by the resin density and a geometry/sprue factor. An 8.5 cc runner in a 1.04 g/cc resin with a 1.10 geometry factor weighs 8.5 x 1.04 x 1.10 = 9.72 g.
Why add a geometry or sprue factor? The factor (1.10 here) accounts for the tapered sprue, gate lands, and cold slug wells that a simple runner-volume estimate understates. It scales the basic volume-times-density figure up to a more realistic mass.
Does runner weight count toward shot size? Yes, on cold-runner molds. The shot must fill the cavities and the entire runner network, so runner weight is added to total part weight to get the full per-cycle shot. Hot-runner molds carry no cold runner to add.
What is a good runner-to-part weight ratio? Lower is better since runner is waste. Well-designed cold runners often sit around 10%-30% of total shot weight; high ratios on small parts are a strong case for a hot-runner system.
How can I reduce runner weight? Shorten and slim the runners, reduce sprue length, balance the layout to allow smaller channels, or switch to a hot-runner or insulated-runner system to eliminate cold material entirely.

Last reviewed 2026-05-12.