The API 520/521 or EN ISO 4126-6 methodology is applied:
1) Define the relief flow rate (mass or volumetric) according to the scenario (fire, reaction, blocked outlet).
2) Choose the discharge coefficient Kd (0.62 typical for metal discs).
3) Apply the critical flow equation for gases or Bernoulli for liquids.
4) Include the resistance factor Kr and piping losses.
5) Compare the theoretical area with the manufacturer's catalog and select the next larger commercial diameter.
Complete guide: Relief area calculation for a rupture disc according to API 520