# Calibration process

- [Manually change the formula](/calibration/calibration-process/manually-change-the-formula.md): An external calibration can be made and then the calculated formula can be entered via the user interface. Here is explained how.
- [Reference method](/calibration/calibration-process/reference-method.md): Reference method is a calibration method that is considered to be very accurate. The density of the current liquid is measured and communicated to the Hydrom. The hydrometer then calculates the calibr