How is EqLA calculated?
For reference to equations for calculating results from a Blower Door test, you can refer to the ASTM E-779-10 standard.
In engineering (fluid dynamics) terms, Bernoulli's equation is an energy balance equation that assumes incompressible fluid (e.g. air) at steady state through two points (e.g. through a building); The equation can be simplified to an orifice plate equation and applied to a Blower Door test. We can calculate point #1 - the total leakage (EqLA or ELA) of a building, and point #2 - a known area (size of fan hole: Range configuration) by measuring via Blower Door system the airflow rate through these points at a specific induced building pressure (the reference pressure).
Induced building pressure is directly proportional to airflow rate through a building, and EqLA is directly proportional to airflow rate; so that a higher induced building pressure would be a higher EqLA. Hence, there needs to be a reference pressure when referring to a leakage or flow result.
The reference pressure is dictated by the standard. For the ASTM standard, the leakage result is required to be at a reference pressure of 50 Pa, whereas it is 75 Pa for the USACE standard. Reference pressure also applies to permeability (e.g. CFM/ft2) and air change (ACH) results.