EPSG Guidance Note #7-2.
This is a parameter-less conversion.
Note: These formulas have been transcribed from EPSG Guidance Note #7-2. Users are encouraged to use that document rather than the text which follows as reference because limitations in the transcription will be avoided. If there is a requirement to either (i) change a height in a source CRS to a depth in a target CRS referenced to the same vertical datum, or (ii) change a depth in the a source CRS to a height in a target CRS referenced to the same vertical datum, then the following generic formula can be used: X2 = –1 * X1