From my studies i thought so far that:

1 NA * Da = 1 * NA mu = 1 g,

However since 2019 SI says that one dalton is only approximately one gram per mol

1 NA Da ≈ 1 g/mol

This makes sense if I consider the definition of :

1 Da as 1/12 Mass of Carbon-12

while MW of 1 * C 12 is 12,0107 g/mol

However this leads to differences in mass calculation for molecules:

lets say for ssDNA PhiX genome of a length of 5386bp I calculated 1664042 g/mol but 1664051 Da MW leading to a difference of ~ 9 Da

so my question is how should i calculate and report molecular weights from now on to not include errors resulting from the change made by the International System of Units

can anyone explain to me or hint me to a publication or similar on how to correctly do mass calculation ?

  • 2
    $\begingroup$ This is a chemistry question and should be moved to SE Chemistry. $\endgroup$
    – David
    Mar 1, 2022 at 13:47

1 Answer 1


For practical purposes, the answer is that it doesn't matter. The example you give corresponds to a relative difference of approximately 0.00054 %. Depending on your pipetting skills, the volume you are trying to pipette and the pipette you use, your accuracy will be at best within a few percents of the intended volume.

If this helps convince you, try calculating the volume you need to pipette with both molecular weights: the difference will show only after several decimal places, that you would need to round anyway because the pipette only goes to two decimal places (assuming you are using a typical 10 uL pipette).

In addition, you can consult the Gilson Guide to Pipetting, section Tips for Mistake-Free Pipetting (page 23 in the edition I got at the time I wrote this), to learn about common causes of inaccuracy when pipetting and, more interestingly, how much variation they incur. You will see that they all amount to a lot more than the inaccuracy incurred by using an approximate molecular weight in your calculations.

  • $\begingroup$ Yes in actual practical terms this wont make a difference, even in microvolume work >0.1μl. However my question is more aimed at theoretical ideas, and if there is any guideline or strong opinions on how to use Da or g/mol in the future.. $\endgroup$ Mar 2, 2022 at 13:06

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