The calculation of conduit size is based on the method and space factors from Standard AS/NZS 3000  (see below).
To determine the number of cables that can be installed inside a conduit the following data is required:
1. Conduit internal diameter to work out cross-sectional area.
2. Cable diameter to work out cross-sectional area.
3. Number of cables to be installed to determine space factor.
 AS/NZS 3000:2018 Electrical installations (known as the Australian/New Zealand Wiring Rules)