Following up on the question Units of mass on the atomic scale, Daltons (Da) and unified mass units (u) are equivalent, but which is used seems to depend on context. For example, when discussing single atoms one typically sees the mass expressed as u, but when discussing large biomolecules one typically sees Da or kDa.

Is there a rule about whether to use Da or u? Or, is this simply a matter of convention or left to the writer's choice?


2 Answers 2


It doesn’t matter. Both units are basically identical, much like there is no everyday difference between the units mbar and hPa. There may be situations where one is preferred — e.g. hPa are, in my opinion, rarely used outside weather reports — but they should be understood nonetheless.

If there is anything to be said about the two, it seems using dalton is more popular in biologic contexts. But that wouldn’t be a tendency to base anything on.

Being consistent within one’s lab, within the journal one is publishing in or throughout one’s thesis is far more important.

  • $\begingroup$ Your second paragraph is more along the lines of what I was asking. Roughly, it seems to me that physical chemists tend to use u and biochemists use Da, with everyone else using a mixture. Does that sound right? $\endgroup$ Nov 18, 2016 at 16:12
  • $\begingroup$ @TedCorcovilos Well, semi. As I wrote, Da is rather popular among biologists. But I wouldn’t want to propose any preference of chemists (outside using molar mass and grams per mole). Then again, I don’t have anything to do with physical chemists, maybe I’m biased. $\endgroup$
    – Jan
    Nov 18, 2016 at 16:46

Dalton (Da) is used to characterize polymers. For example, biochemists and molecular biologists specifically to discuss biopolymers such as nucleic acids or proteins.

Usually Da appears in the context "a 60 kDa protein band was observed on SDS-page" or "10 kDa cut off membrane was used for dialysis".

same applies to synthetic polymers.

  • 2
    $\begingroup$ This is mostly incorrect. "Da" appears in a much wider variety of contexts than just in biochemistry and molecular biology. $\endgroup$
    – Curt F.
    Jul 20, 2016 at 15:51
  • $\begingroup$ @Curt F. Why don't you write about it if you know the correct answer? $\endgroup$
    – sixtytrees
    Jul 20, 2016 at 16:24

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