Machine sizing · Online calculator · Material quantity

Shot Weight & Shot Volume Calculator

Updated: 2026-05-29 Tool: Online calculator Status: editorially reviewed May 2026 Editorial: KunststoffWissen

Formula · m = shot weight · V_K = cavity volume · n = cavities · ρ = density m [g] = V_K [cm³] × n × (1 + runner% / 100) × ρ [g/cm³] Standard injection-moulding formula. Density = solid density of the polymer; see hint in the calculator for typical values.

Online calculator

Note: Shot weight is based on solid density; real values can vary slightly due to shrinkage, residual moisture and runner geometry. The recommended machine shot size is a PS-equivalent guide value. All information without warranty.

What shot weight is – and why it matters

The shot weight is the total amount of material the machine injects per cycle: all mouldings plus runner and manifold. It is the key figure for machine selection and for material-demand and cost calculation. Too small against the machine and the melt dwells too long in the hot barrel (thermal degradation); too large and there is no metering or cushion reserve.

The calculator above derives both the shot weight and the shot volume from cavity volume, cavity count, runner allowance and material density – and from these a machine recommendation.

The formula and the right density

The relation is m = V_K × n × (1 + runner%/100) × ρ. Cavity volume V_K comes from the part CAD model, the runner allowance covers cold runner and manifold (hot runner ≈ 0). The critical point is the correct density: use the solid density of the polymer at 23 °C, not the bulk density of the pellets. Guide values:

PP 0.905 · PE-HD 0.95 · PS/ABS 1.05 · PA66 1.14 · PC 1.20 · PET 1.38 · POM 1.41 g/cm³.
Glass-fibre reinforcement raises density by about 0.1–0.3 g/cm³ depending on fill level.

Worked example

A 4-cavity mould, cavity volume 25 cm³ per part, 20 % cold-runner allowance, PC with density 1.20 g/cm³:

m = 25 × 4 × 1.20 × 1.20 = 144 g, shot volume 25 × 4 × 1.20 = 120 cm³.

As a rule of thumb the shot weight should be 60–80 % of the machine maximum shot weight (PS-equivalent). At 144 g you therefore pick a machine of about 180–240 g shot capacity. Because machine ratings are PS-equivalent, the shot volume is the more robust comparison figure at deviating density – the calculator outputs both.

Utilisation, residence time and reserve

Too low utilisation (< 20–30 %): the melt dwells too long in the barrel, thermal degradation looms – check with the residence-time calculator.
Too high utilisation (> 80 %): no cushion or metering reserve, the process becomes unstable.
Runner share: a large cold runner drives shot weight and material cost – runner optimisation or a hot runner lowers both.
Density error: using bulk density instead of solid density is the most common mistake and gives clearly too-low values.

Limits and related tools

Shot weight is based on solid density; real values can vary slightly due to shrinkage, residual moisture and runner geometry. The recommended machine shot size is a PS-equivalent guide value. Useful next: the clamping-force calculator (tonnage) and the unit-cost calculator (material and machine cost per part). Material densities are listed in the density table.

FAQ

How do I calculate shot weight?

Shot weight m = cavity volume V_K × number of cavities n × (1 + runner allowance%) × material density ρ. Example: 25 cm³ × 4 × 1.20 × 0.95 g/cm³ = 114 g. Cavity volume comes from the part CAD model; the runner allowance covers the cold runner and manifold.

Which density should I use?

The solid density of the polymer at 23 °C (not the bulk density of the pellets). Guide values: PP 0.905, PE-HD 0.95, PS/ABS 1.05, PA66 1.14, PC 1.20, PET 1.38, POM 1.41 g/cm³. Glass-fibre reinforcement raises density by about 0.1–0.3 g/cm³ depending on fill level.

How large must the machine be?

As a rule of thumb the shot weight should be 60–80 % of the machine maximum shot weight (PS-equivalent). The calculator derives a recommendation. Too low a utilisation lengthens residence time (thermal degradation); too high leaves no cushion/metering reserve. See the residence-time calculator for details.

Do I need to convert for materials other than PS?

Machine shot sizes are quoted in PS equivalent (density ~1.05). For other materials, volume is what counts: a higher-density material gives more weight at the same volume but uses the same screw volume. For machine selection the shot volume is the more robust figure — the calculator outputs both.

Sources & method: Standard shot-weight formula. Material densities from datasheets (solid density at 23 °C).