How do you calculate shot size in injection molding?

Add the part weight to the runner weight for a single shot, multiply by the number of cavities, then multiply by an overfill factor (typically 1.05 to 1.10). With 28 g of part-plus-runner across 4 cavities at a 1.08 factor, the shot is 120.96 g.

What is a good shot size relative to barrel capacity?

Aim for 20% to 80% of the machine's rated shot capacity. A 120.96 g shot is ideal on a press rated around 150-600 g but too small for a 1,000 g barrel, where residence time grows and risks thermal degradation.

Why include an overfill or cushion factor?

The factor (1.08 here) covers the extra material for pack and hold plus the residual cushion left in front of the screw at the end of forward travel, so you never inject to a hard zero and lose pressure control.

Is runner weight part of shot size?

Yes, for cold-runner molds. The shot must fill every cavity and the entire sprue and runner network, so runner mass is added to part mass before multiplying by cavity count. Hot-runner molds exclude the runner because that material stays molten in the manifold.

What happens if my shot size is too large for the barrel?

If the shot exceeds about 80% of barrel capacity you lose recovery time and cushion consistency, and the machine may not build adequate pack pressure. Move to a larger press or reduce cavitation.

Shot size vs shot weight - are they the same?

In practice they are used interchangeably for mass. Shot size can also refer to the volumetric screw stroke; this calculator works in grams, so it gives shot weight, the figure you compare to the machine's rated shot capacity.

Injection Molding calculator

Shot Size Calculator

Shot size is the total mass of molten resin the screw injects per cycle, covering every cavity plus the runner system and a small overfill cushion. Process engineers and tooling buyers calculate it to confirm a job fits a given press, since a healthy shot should land between roughly 20% and 80% of the barrel's rated capacity. Run too small a shot in a big barrel and resin sits and degrades; run too large and you starve the cushion and lose pack control. Getting this number right before a mold trial saves wasted setup time and scrapped first articles.

What this calculator does

Calculate total shot size from part weight, runner weight, number of cavities, and an overfill safety factor.
Use this when sizing a barrel or screw, verifying that the machine shot capacity covers the mold, or comparing actual vs. theoretical shot weight.
It computes total per-cycle shot mass as part-plus-runner weight times the number of cavities times an overfill factor.

Formula used

Total shot size = (Part + runner weight) x Cavities x Overfill factor
Compare to machine barrel capacity (should be 20% to 80% of barrel max)

Inputs explained

Part weight plus runner weight per shot: Total weight of all parts plus runner/sprue for one shot. For hot runner molds, use only total part weight.
Number of cavities: Total number of cavities in the mold. For a family mold, count each unique cavity.
Overfill/cushion factor: Multiplier for barrel cushion and cold slug allowance. Typical: 1.05 to 1.10.

How to use the result

Use it during press selection and mold qualification to confirm the shot falls in the 20%-80% barrel-capacity window before the first trial.
It assumes one resin density across all cavities and a balanced runner; family molds with mixed part weights or hot-runner systems need a per-cavity breakdown.

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 shot size in injection molding? Add the part weight to the runner weight for a single shot, multiply by the number of cavities, then multiply by an overfill factor (typically 1.05 to 1.10). With 28 g of part-plus-runner across 4 cavities at a 1.08 factor, the shot is 120.96 g.
What is a good shot size relative to barrel capacity? Aim for 20% to 80% of the machine's rated shot capacity. A 120.96 g shot is ideal on a press rated around 150-600 g but too small for a 1,000 g barrel, where residence time grows and risks thermal degradation.
Why include an overfill or cushion factor? The factor (1.08 here) covers the extra material for pack and hold plus the residual cushion left in front of the screw at the end of forward travel, so you never inject to a hard zero and lose pressure control.
Is runner weight part of shot size? Yes, for cold-runner molds. The shot must fill every cavity and the entire sprue and runner network, so runner mass is added to part mass before multiplying by cavity count. Hot-runner molds exclude the runner because that material stays molten in the manifold.
What happens if my shot size is too large for the barrel? If the shot exceeds about 80% of barrel capacity you lose recovery time and cushion consistency, and the machine may not build adequate pack pressure. Move to a larger press or reduce cavitation.

Last reviewed 2026-05-12.