To calculate shot size for an injection molding machine, start with the total material used in one cycle: part weight, runner or sprue weight, and cavity count. Then convert that material requirement into volume using the resin density, add enough allowance for a stable cushion, and compare the result to the machine's injection unit rating. That sounds simple, but a lot of buying and processing mistakes happen when shot size is estimated too loosely.
If you are sizing a machine for a new mold, comparing used injection molding equipment, or troubleshooting an existing process, getting shot size right helps you avoid poor fill, inconsistent cushions, resin degradation, and machines that are either too small or unnecessarily oversized.
What shot size means in injection molding
Shot size is the amount of molten plastic the machine can inject in one cycle. In practical terms, you care about two different numbers:
- The required shot for the mold, which is how much material your job needs each cycle
- The machine's available shot capacity, which is how much material the injection unit can reliably meter and inject
Those two numbers need to line up, but they are not always shown in the same units. Many machines are rated in grams or ounces of polystyrene equivalent, while your actual job may run polypropylene, nylon, ABS, polycarbonate, or another resin with a different density.
How to Calculate Shot Size for an Injection Molding Machine
The most reliable way to calculate shot size for an injection molding machine is to work from the mold backward.
- Find the finished part weight
- Add runner, sprue, and cold slug weight if applicable
- Multiply by the number of cavities
- Convert weight to volume using the resin density
- Add enough volume for a consistent cushion and normal process control
- Compare that result to the machine's rated shot capacity
Step 1: Calculate total part weight per cycle
If the mold has more than one cavity, multiply the weight of one part by the number of cavities.
Formula:
Total part weight = Part weight per cavity × Number of cavities
Example: if one part weighs 42 g and the mold has 2 cavities:
Total part weight = 42 g × 2 = 84 g
Step 2: Add runner and sprue weight
If the mold uses a cold runner, the shot must include runner and sprue material. If it is a hot runner system, this number may be minimal or close to zero during steady production.
Formula:
Net cycle weight = Total part weight + Runner/sprue weight
Using the same example, if the runner weighs 18 g:
Net cycle weight = 84 g + 18 g = 102 g
This 102 g is the amount of plastic actually going into the mold each cycle.
Step 3: Convert material weight to shot volume
Machines inject volume, even when the spec sheet shows weight. That is why resin density matters.
Formula:
Required shot volume (cm³) = Net cycle weight (g) ÷ Resin density (g/cm³)
If the material is polypropylene with a density of 0.90 g/cm³:
Required shot volume = 102 ÷ 0.90 = 113.3 cm³
At this point, you know the mold needs about 113.3 cm³ of melt to fill the cavities and runners.
Step 4: Add cushion and process allowance
The machine should not use 100% of its available shot just to fill the mold. A stable process needs a consistent cushion at transfer. That cushion helps keep pressure control repeatable and reduces the risk of short shots or unstable packing.
Many processors also leave some room above the exact fill volume so the machine is not working at the edge of its injection unit on every cycle.
If you estimate a small allowance for cushion and process stability, your metered shot might look like this:
Metered shot volume = Required shot volume + cushion/process allowance
If you add 10% in this example:
Metered shot volume = 113.3 cm³ × 1.10 = 124.6 cm³
That does not mean the extra material goes into the part. It means the machine needs enough injection-unit capacity to fill the mold and maintain a usable cushion.
Step 5: Compare to machine shot capacity
Now compare your required metered shot to the machine's rated capacity.
This is where many buyers make a mistake. Machine data plates and listings often show shot size in polystyrene equivalent. If your resin is not polystyrene, the machine's actual weight capacity for your material will be different.
CalculationFormulaNet cycle weight(Part weight × Cavities) + Runner weightRequired shot volumeNet cycle weight ÷ Resin densityEstimated metered shotRequired shot volume + cushion/process allowanceEquivalent PS shot rating neededMetered shot volume × 1.05 g/cm³Polystyrene density is commonly referenced at about 1.05 g/cm³ for machine rating comparisons. Using the example above:
Equivalent PS shot rating needed = 124.6 × 1.05 = 130.8 g PS
So you would want a machine with a rated shot capacity above roughly 131 g of PS equivalent, and in most cases you would want enough margin that the machine is not operating at the extreme high end of its range.
Quick example: shot size calculation from start to finish
Here is a full example that buyers and process engineers can use as a basic check.
- Part weight: 30 g
- Cavities: 4
- Runner weight: 20 g
- Material: ABS
- ABS density: 1.04 g/cm³
1. Total part weight
30 g × 4 = 120 g
2. Net cycle weight
120 g + 20 g = 140 g
3. Required shot volume
140 ÷ 1.04 = 134.6 cm³
4. Add 10% allowance
134.6 × 1.10 = 148.1 cm³
5. Convert to PS-equivalent rating
148.1 × 1.05 = 155.5 g PS
In this case, the machine should comfortably support a shot size above about 156 g PS equivalent. Then you would still verify screw diameter, injection pressure, recovery rate, and barrel condition before calling it a good match.
Why resin density changes the answer
Two molds with the same volume do not always need the same machine rating in weight units. Denser materials weigh more for the same volume. Less dense materials weigh less.
That is why a machine rated at 200 g shot capacity in polystyrene does not automatically give you 200 g of capacity in every resin.
As a general conversion:
Actual material weight capacity = PS-rated shot capacity × (Your resin density ÷ 1.05)
For example, a 200 g PS-rated machine would have lower weight capacity in polypropylene and slightly higher weight capacity in a denser resin like nylon filled compounds, assuming the same volume limit.
When comparing equipment listings, always ask whether the shot size shown is:
- PS equivalent
- Actual material capacity
- Volume in cm³
- Based on a specific screw diameter
What is a good shot size range for the machine?
Even if a machine can technically inject the required volume, that does not always mean it is the best fit.
A useful rule of thumb is to avoid running too close to either end of the injection unit's capacity. If the shot is too small relative to the barrel, residence time may become excessive and heat history can hurt the material. If the shot is too large relative to capacity, the machine may struggle to maintain cushion and pack consistently.
Many molders prefer the actual process shot to fall somewhere in the middle of the machine's usable range rather than at the extremes. Exact targets depend on the resin, screw design, part sensitivity, and process window, but the key point is simple: do not choose a machine by clamp tonnage alone.
Common shot size calculation mistakes
1. Using only part weight
Ignoring runners, sprues, and startup losses leads to undersized injection units, especially on cold runner molds.
2. Ignoring cavity count
A four-cavity tool changes the shot requirement fast. Always calculate per cycle, not per part.
3. Forgetting resin density
Weight and volume are not interchangeable across materials. This is one of the biggest spec-sheet errors in used equipment buying.
4. Confusing shot size with clamp tonnage
Clamp tonnage controls mold holding force. Shot size controls how much material can be injected. You need both numbers to be right.
5. Leaving no cushion margin
If the machine must use nearly all of its available injection stroke just to fill the mold, process stability usually suffers.
6. Not checking screw diameter and injection pressure
Two machines can have similar rated shot sizes but different screw diameters, injection pressures, and plasticizing behavior. That matters for thin-wall parts, filled materials, and tight process windows.
What to check when comparing injection molding machines
If you are evaluating equipment for purchase, shot size should be one of the first filters, but not the only one. After confirming the calculation, review these machine specs:
- Screw diameter for pressure capability and shot resolution
- Maximum injection pressure for difficult-to-fill molds
- Injection rate for thin-wall or fast-fill applications
- Plasticizing capacity for cycle time support
- Clamp tonnage for projected area and cavity pressure
- Tie bar spacing and platen size for mold fit
- Non-return valve and barrel condition on used machines
- Controller generation and repeatability for process-sensitive jobs
For used machinery in particular, the listed shot size is only the starting point. Screw wear, barrel wear, valve condition, and maintenance history all affect whether the machine can deliver that shot consistently in production.
When the machine is too large
Buyers often focus on avoiding a machine that is too small, but a machine that is too large can create its own problems. If the shot uses only a small fraction of barrel capacity, resin may spend too long in the barrel. That can be a problem for heat-sensitive materials, color changes, and jobs that require tight consistency.
An oversized machine may also cost more to buy, operate, and floor without improving part quality.
Final takeaway
If you need to calculate shot size for an injection molding machine, the core formula is straightforward: determine the total material per cycle, convert it to volume using resin density, add room for cushion and process stability, and then compare that result to the machine's rated injection capacity.
That calculation is one of the fastest ways to narrow down machine options, avoid bad equipment fits, and ask better questions before you buy. If you are reviewing injection molding machines on Machinery Network, use shot size alongside screw diameter, injection pressure, clamp tonnage, and mold dimensions to build a shortlist that makes sense both on paper and on the production floor.