Is there a commonly used unit of measure (other than temperature units) that is not absolute? I live in a country where we use Degree-Celsius(°C) to measure the temperature.
Sometimes from one day to the other, the temperature rises from 10°C to 20°C and I hear people say, "Wow! Today is twice as hot as yesterday!".
I try to explain that today is not two times hotter than yesterday because Celsius(°C) is not an absolute unit of measure, that is 0° does not mean the absence of temperature.
If I convert Celsius(°C) to Fahrenheit(°F) or Kelvin(K) it gets clear, but I wish I could provide another example of non-absolute unit of measure to clarify things.
In short, do you know any other unit of measurement (except for temperature ones) where 0 does not mean the absence of the physical phenomenon that is being measured?
 A: *

*Pressure (relative to atmospheric)

*Position (where exactly IS the center of the universe?)

*Time (relative to some epoch) 

*Voltage (relative to ground) -- User58220 already mentioned this

A: Stellar magnitudes are generally measured relative to Vega
A: Fahrenheit is not "absolute" either, by your definition. 0 degrees Fahrenheit is equivalent to -17.8 degrees Celsius; absolute zero is -460 degrees Fahrenheit.
A: Gravitational potential energy is another physical quantity that is measured relative to an arbitrary starting point.
In many derivations, the zero point for the potential energy of a mass is taken to be a point an infinite distance away, and all the potential energy is negative.
A hydro electric engineer would take the bottom of his dam for the zero point of potential energy for the water he has going through his generators, so the higher the water level above the dam, the more energy he has available.
His colleague downstream will take the bottom his second dam as a convenient reference point for the energy available at that dam.
Or the entire system could use sea level as a zero point, and rely on height difference at each dam for their calculations.   It's all relative...
A: Best one I can think of is shoe (or clothing or ring) sizes.  Another common measure might be wire gauge.  
There are many others, but almost all are less common and I doubt they'd be effective.  
