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Injection Moulding Shot Size Calculator

Work out injection moulding shot size instantly with clear inputs, formula shown and shareable results.

About Injection Moulding Shot Size Calculator

Shot weight is the weight of all cavities plus the runner system that feeds them. Machine selection then aims for 40-80 percent of rated shot capacity: below that the material sits too long in the barrel and degrades, above it the screw has no cushion and shot-to-shot consistency suffers.

Formulas

Shot weight

Shot = part weight x cavities x (1 + runner allowance)

Utilisation

Utilisation = shot weight / machine shot capacity

Worked example

With the default values below, Injection Moulding Shot Size Calculator returns Shot weight required of 207.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Part weightminimum 0.1 g
45 g
Number of cavitiesminimum 1
4
Runner and sprue allowanceaccepts 0–100 %
15 %
Machine shot capacityminimum 1 g
250 g

Results

Shot weight required
207.00 g
Runner and sprue weight
27.00 g
Machine shot utilisation
82.8% %
Total part weight
180.00 g

Step by step

  1. 1Values used
  2. 2Shot weight
  3. 3Utilisation
  4. 4Shot weight required= 207.00 g
  5. 5Runner and sprue weight= 27.00 g
  6. 6Machine shot utilisation= 82.8 %
  7. 7Total part weight= 180.00 g

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Injection Moulding Shot Size figure worked out?

Shot = part weight x cavities x (1 + runner allowance). Shot weight is the weight of all cavities plus the runner system that feeds them.

Why avoid running near full shot capacity?

The screw needs residual cushion to pack the part and compensate for shrinkage. Without it, dimensions drift and sink marks appear.

What values do I need to enter?

This calculator takes 4 inputs: Part weight, Number of cavities, Runner and sprue allowance, Machine shot capacity. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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