Is there a way to use the physical constants in calculations and have Mathematica 10 figure out the final unit and numerical magnitude? When I try

pcM = N[Quantity[1, "PlanckConstant"]]

I simply get 1. h without the numerical value of Planck constant. Subsequently, using this in all further steps, keeps the answer in h and does not work out units.

However, if I define the Planck constant by hand and use it in a calculation, everything works as expected. I am curious as to why the internally defined constants do not show up with numerical values.


  • $\begingroup$ what would you like the result to be, i.e. which units should it be with respect to? $\endgroup$
    – chuy
    Commented Aug 27, 2014 at 15:19
  • 5
    $\begingroup$ I learned just a few days ago that one can (often) use UnitConvert for this sort of thing. In[199]:= UnitConvert[Quantity[1, "PlanckConstant"]] Out[199]= Quantity[6.626070*10^-34, ( "Kilograms" ("Meters")^2)/("Seconds")] $\endgroup$ Commented Aug 27, 2014 at 15:20
  • 3
    $\begingroup$ or In[647]:= UnitConvert[Quantity[1, "PlanckConstant"], "eV*s"] Out[647]= Quantity[4.135668*10^-15, "Electronvolts" "Seconds"] $\endgroup$
    – chuy
    Commented Aug 27, 2014 at 15:22

2 Answers 2


In physics, the Planck constant may be used as a natural unit.

If you want to switch to another unit system, use UnitConvert[].

For example, you can switch to standard SI units this way:

UnitConvert[Quantity[1, "PlanckConstant"], "SIBase"]

which will give you:

Quantity[6.626070*10^-34, ("Kilograms" ("Meters")^2)/("Seconds")]

This can be done at the end of calculation.

If you like to get rid of Quantity head, just do:


which outputs:

  • 1
    $\begingroup$ you can also use QuantityMagnitude to get the value out. $\endgroup$
    – chuy
    Commented Aug 27, 2014 at 15:29
  • $\begingroup$ @chuy, indeed, thank you! $\endgroup$ Commented Aug 27, 2014 at 15:31
  • 3
    $\begingroup$ Or just UnitConvert without the second argument, which assumes SI units. $\endgroup$
    – Mark Adler
    Commented Aug 28, 2014 at 3:40
  • $\begingroup$ @MarkAdler: definitely right! Though, of course, using the second argument is a bit more general. For example, in engeneering applications one unit measure can be used and in spectroscopy - something totally different. $\endgroup$ Commented Aug 28, 2014 at 8:04

You should use the Physical Constants Package by using

<< PhysicalConstants`

When you enter now


you directly get the Planck Constant. With

PlanckConstant/(Joule Second)

you get the Planck Constant without units.

  • 1
    $\begingroup$ True, but note that PhysicalConstants package is obsolete and has been biult into Mathematica since version 9. $\endgroup$ Commented Aug 27, 2014 at 15:49

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