Decibels

The decibel [dB] is a dimensionless, logarithmic, non-SI metric unit of deviations from some reference level.
  Equal to the decimal logarithm of the ratio of the physical value to the physical value of the same value, taken as the original, multiplied by ten.
  Thus dB = 10 * lg (V / V0); (1)
  where V is the measured value, and V0 is the reference value.

Formula (1) is applicable for transfer in units of decibels of power and energy. But in metrology and automation are widely used units of measurement of voltage and current.
  To translate them into decibels, the following formula applies:
  dB = 20 * lg (V / V0); (2)
  where V is the measured value (voltage or current), and V0 is the reference value (voltage or current).

The application Instrumentation & Automation for Android contains a user interface for calculating decibels. See the figure below:

The user can change both the reference and the physical value. Thus there is a calculation of decibels. The decibel value can also be changed using the keyboard. In this case, the physical value is calculated based on the reference level.

In the upper left part of the window there is a drop-down list for selecting the calculation mode:

• Common. [dB]. Calculated by the formula (1). It is used for general calculations of decibels, for translating decibels «in times», dividing the physical and reference values.
• Power. [dBm]. Calculated by the formula (1). It is used to calculate decibels from power values. The standard reference power level is 0.001 W.
• Voltage. [dBu]. Calculated by the formula (2). It is used to calculate decibels from voltage values. The reference standard voltage level is 0.775 V.
• Current. [dBi]. Calculated by the formula (2). It is used to calculate decibels from current values. The reference standard current level is 0.00129 A.

The reference levels in all cases can be changed for greater functionality of the program.