Shot Size Calculator
Calculate barrel capacity needed from part weight, runner volume, number of cavities, and cushion percentage.
About this calculator
Shot size is the total amount of molten plastic a press has to inject in a single cycle, and getting it right starts with adding up everything the screw actually has to push into the mold: the weight of one part times however many cavities the mold produces simultaneously, plus the runner and sprue system that channels plastic to those cavities but ends up as scrap or regrind rather than a usable part. A cushion — a small reserve of extra material left in the barrel at the end of injection — is added on top, since running the screw completely empty at the end of every shot removes the ability to hold consistent packing pressure and leads to shot-to-shot inconsistency. From that final shot weight, the calculator works backward to recommend a range of barrel capacities: a machine whose barrel is too small can't hold enough material to plasticize properly, while one that's too large leaves plastic sitting in the barrel far longer than necessary each cycle, risking thermal degradation from prolonged heat exposure.
The ideal barrel capacity target — running the shot at roughly 50% of the barrel's total capacity — balances these two failure modes, giving enough residence time for the resin to melt evenly without leaving so much extra capacity that material degrades from sitting too long. Runner percentage flags how much of every shot goes toward gates and channels rather than salable parts, which matters directly for material cost and cycle efficiency, especially with a cold runner system that can't be reground and reused as easily as a hot runner setup can.
Inputs
Results
Required Shot Size
170.5 g
≈ 2 sticks of butter
How to Use This Calculator
- Enter part weight (g), number of cavities, and estimated runner/sprue weight (g).
- Set cushion percentage (typically 10–15% of barrel capacity) to maintain pack pressure.
- Review required shot size with cushion, total shot weight, shot volume (cc), and minimum barrel capacity.
- Select an injection molding machine whose barrel capacity is 30–80% of your shot size for optimal plasticizing.
How the result changes with Part Weight
| Part Weight | Required Shot Size |
|---|---|
| 18 | 95.7 g |
| 26 | 130.9 g |
| 53 | 249.7 g |
| 88 | 403.7 g |
What each input means
- Part Weight
- Weight of a single finished part in grams.
- Number of Cavities
- Number of cavities in the mold.
- Runner / Sprue Weight
- Total weight of the cold runner and sprue system (0 for hot runner).
- Cushion
- Cushion as percentage of shot (typically 5-10% for consistent packing).
What each result means
- Min Barrel Capacity
- Barrel capacity where shot is 80% (maximum fill).
- Ideal Barrel Capacity
- Barrel capacity where shot is 50% (optimal residence time).
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersPart Weight = 35, Number of Cavities = 4, Runner / Sprue Weight = 15, Cushion = 10 = 4 input(s) provided
- Calculate Required Shot SizeRequired Shot Size170.5 = 170.5
- Calculate Total Part WeightTotal Part Weight140 = 140
- Calculate Shot WeightShot Weight155 = 155
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator recommend a barrel that's only about half full of shot, rather than one that closely matches the shot size?
Running a shot that's too large a fraction of barrel capacity leaves too little dwell time for the plastic to melt and mix evenly before injection, risking unmelted material reaching the mold, while a shot that's too small a fraction leaves plastic sitting in the hot barrel far longer than needed each cycle, risking thermal degradation. The roughly 50% target, with an 80% maximum and 20% minimum as the outer bounds, balances these two competing failure modes rather than simply matching barrel size to shot size directly.
Why does the barrel utilization figure always show the same percentage regardless of what I enter?
Barrel utilization is calculated against this calculator's own ideal barrel capacity target, and because that ideal capacity is itself defined as exactly double the shot size, comparing the shot against it always produces 50% by definition — it's a built-in reference check confirming the ideal capacity figure was calculated correctly, not a measurement against any specific real machine's actual barrel size. To check utilization on a specific press, compare your shot size directly against that machine's published barrel capacity instead.
Why does runner weight matter so much for overall production cost?
Runner and sprue material becomes part of every single shot's total weight and resin cost but never becomes a sellable part, so a high runner percentage means paying for resin, cycle time, and machine capacity on material that ends up as scrap or, at best, reground and reused with some quality tradeoffs. This is exactly why hot runner systems, which eliminate the runner almost entirely, can pay for their higher upfront tooling cost through material savings on high-volume, long-running production.
Why is a shot cushion necessary instead of just injecting all the material in the barrel?
Injecting the barrel completely empty removes the screw's ability to continue applying pack pressure during the pack/hold phase, since there's no remaining material cushion left to push forward, which leads to inconsistent part dimensions and weight from shot to shot as the exact empty point varies slightly each cycle. A cushion of roughly 5-10% of the shot ensures there's always a controlled buffer of material behind the injection point to maintain consistent packing.
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