Runner System Design Calculator

Calculate runner diameter, length, and volume for balanced multi-cavity injection molds.

Inputs

Part Weight

Number of Cavities

Maximum Wall Thickness

Flow Length

Material Viscosity

Tab to move between fields

Results

Main Runner Diameter

5.25 mm

Sub-Runner Diameter4.2 mm

Gate-End Runner2.94 mm

Main Runner Length180 mm

Runner Volume6.39 cm³

Runner Weight6.71 g

Runner-to-Part Ratio5.6%

How to Use This Calculator

Enter part weight (g), number of cavities, and maximum wall thickness (mm).
Set flow length (mm) from sprue to gate and material viscosity class.
Review recommended main runner diameter, sub-runner diameter, and gate-end runner diameter.
Check runner volume relative to shot size — runner waste should typically be below 20% of shot weight.

How the result changes with Maximum Wall Thickness

Maximum Wall Thickness	Main Runner Diameter
2.45	5.15 mm
7.32	15.37 mm
13	27.3 mm
18	37.8 mm

What each input means

Part Weight: Weight of a single finished part.
Number of Cavities: Total number of cavities in the mold.
Maximum Wall Thickness: Thickest wall section of the part.
Flow Length: Distance from the gate to the farthest point in the cavity.
Material Viscosity: Material flow characteristic affects runner sizing.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Part Weight = 30, Number of Cavities = 4, Maximum Wall Thickness = 2.5, Flow Length = 150 = 5 input(s) provided
Calculate Main Runner Diameter
Main Runner Diameter
5.25 = 5.25
Calculate Sub-Runner Diameter
Sub-Runner Diameter
4.2 = 4.2
Calculate Gate-End Runner
Gate-End Runner
2.94 = 2.94

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Injection Molding

Gate Size Calculator

Calculate injection mold gate dimensions from part weight, wall thickness, and material flow characteristics.

Injection Molding

Shot Size Calculator

Calculate barrel capacity needed from part weight, runner volume, number of cavities, and cushion percentage.

Plastics

Injection Molding Clamp Tonnage Calculator

Calculate required clamping force for injection molding from projected part area, cavity pressure, number of cavities, and runner system.

Plastics

Shrinkage Compensation Calculator

Calculate mold cavity dimensions to compensate for plastic shrinkage in flow, cross-flow, and thickness directions. Assesses warpage risk and tolerance feasibility.

Fisheries

Cold Chain Calculator

Ice and refrigeration from catch volume and trip duration.

More in Manufacturing, Industrial & Coatings.