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It’s great that Quantity can utilize Wolfram Alpha to interpret unit strings that it doesn’t recognize, but I need my code to work on machines that do not have Internet access.

Is there a complete list of built-in (i.e. canonical) unit strings recognized by Quantity?

Better yet, is there a programmatic way to produce such a list?

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    $\begingroup$ Good question! I hope Wolfram notices this question and intends to make the Quantity capability stand alone (I often use compound units and almost always run into this issue). $\endgroup$
    – Cassini
    Apr 19, 2013 at 21:25

3 Answers 3

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This is certainly the optimal way of obtaining the list you are looking for

Quantity;QuantityUnits`Private`$UnitReplacementRules[[1, All, 1]]

EDIT for v10 (thanks @DavidCreech)

In v10 this undocumented variable format has been changed into an association, whose keys are the units.

Quantity; Keys[QuantityUnits`Private`$UnitReplacementRules]
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    $\begingroup$ Haha, I just came here to post this! Damn you fellow spelunker! $\endgroup$
    – rm -rf ♦
    Jan 26, 2013 at 20:03
  • $\begingroup$ FWIW, this seems to me the optimal way to obtain this list, which incidentally is appropriate. $\endgroup$
    – Oleksandr R.
    Jan 27, 2013 at 0:55
  • $\begingroup$ @OleksandrR. I trust your judgement, editing $\endgroup$
    – Rojo
    Jan 27, 2013 at 1:10
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    $\begingroup$ Haha, nice! The Dispatch object from which this list is extracted is actually built from the downvalues of CalculateUnits`UnitCommonSymbols`KnownUnit0Q, which is the canonical arbiter of whether a unit is valid or not. But since this doesn't change after the QuantityUnits` package is loaded, recovering the list as you do here is surely the most direct approach. $\endgroup$
    – Oleksandr R.
    Jan 27, 2013 at 1:21
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    $\begingroup$ @Rojo Note that in version 10 an association is used, so use Quantity; Keys[QuantityUnitsPrivate$UnitReplacementRules] if you are using 10. $\endgroup$
    – David Creech
    Aug 26, 2014 at 20:25
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As a partial answer the documentation says:

Supported units include all those specified by NIST Special Publication 811.

This is repeated in Unit Discovery. It also states:

Unit interpretation requires internet connectivity, and can entail additional evaluation time. If speed is a concern, it is advisable to use the canonical unit specification, which can be found using InputForm.

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For Mathematica 12.2.

These two are equivalent

Sort@Keys[QuantityUnits`Private`$UnitReplacementRules] == 
 Sort@QuantityUnits`$UnitList
(*True*)

There are also

QuantityUnits`$UnitAlternateStandardNameReplacements

contains something like Celsius which is not contained in $UnitList. Most of these units are readily usable in Quantity except few cases need internet connection for interpretation.

There are also UnitShortName information in QuantityUnits`$UnitTable

shortUnitAssoc = QuantityUnits`$UnitTable // Query[;; , "UnitShortName"] // 
 Select[# =!= None &];

which contains "Milligrams" -> "mg", "Kilograms" -> "kg", "Meters" -> "m" etc. Some of these short unit are readily usable like Quantity[1,"mg"].

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