"Decagram" or "dekagram"

If one wants to describe 10 grams, would the appropriate wording be decagram or dekagram, assuming US English?

Word recognizes both words as valid. Wikipedia lists only decagram. Merriam-Webster knows both. Google recognizes both (although it gives a definition for decagram if you enter dekagram).

There are in fact three spellings in use in English, decagram, dekagram and decagramme.

Now, mostly with such matters we're full of caveats about how there's no official spellings in English, and so we'll most often look to patterns of use with tools like google ngrams to show how accepted or otherwise a given spelling is.

In this case though, we do have standards.

First the SI international standard as currently defined by ISO 80000-1 defines the gram itself, along with the abbreviation g used in all languages (even those that don't otherwise use Latin script), and also the deca- prefix along with its abbreviation da) and as such dag is the formally the only correct formal abbreviation in any language.

Now, that standard also leaves the spellings and forms (e.g. how a plural is formed) in different languages to the authority of the language if it has one (English does not) or to the body responsible for adoption of ISO 80000-1 otherwise.

In the US, that body is NIST. They use the spelling deka. As explained in The International System of Units (SI)—NIST Special Publication 330:

The spelling of English words is in accordance with the United States Government Printing Office Style Manual, which follows Webster's Third New International Dictionary rather than the Oxford Dictionary. Thus the spellings “meter,” “liter,” and “deka” are used rather than “metre,” “litre,” and “deca” as in the original BIPM English text;

However, note also that the SI brochure Le Système International d' Unités (SI), which is in French but with English text following, is the source of this NIST publication, and sometimes used directly by people in the US.

So, in American English you strictly should use dekagram, but some will indeed use decagram.

In Britain, the relevant authoritative text is The Weights and Measures Act 1985, which uses deca- only, but for the spelling of gram notes:

No provision contained in or made under this or any other Act prevents the use of “gram” or “gramme” as alternative ways of spelling that unit, and the same applies for other units in the metric system which are compounds of “gram”.

As such, both decagram and decagramme are officially correct, and we are back to looking at patterns of usage (decagram wins on that score, and increasingly so, as gramme while once a popular spelling has been dying out over the last century).

So, in British English you can use decagram or decagramme, but should probably only use decagram.

Now, all that about officialdom said, even when there is an official standard in how English should be used in a given context, it isn't always followed. Informally, deca- is more popular in the US than the above would suggest. As such, in an informal context you could reasonably choose to use either. This would also have the advantage of being more recognised by users of other variants of English.

However, if you are writing a technical piece in which conforming to NIST standards is important, you should use dekagram.

Or you could just ignore the issue: deca-/deka- is a rarely used prefix in SI, so you could just use 10 grams, which is equally acceptable in SI terms, and more common in terms of English usage. Likewise, for 4 decagrams/dekagrams use 40 grams for 55 decagrams/dekagrams use 550 grams or 0.55 kilograms etc. (You could also use 5.5 hectograms fro the last one, but hectograms aren't commonly used either).

And if you don't need to conform with NIST, then you should definitely do the above to avoid decagram/dekagram/decagramme entirely. They really aren't used much.

Ngram offers some evidence regarding the usage of the two words:

In American English they are both used, whith dekagram being more popular, while the opposite is true in British English.

Dekagram (dĕk′ə-grăm′) n. Abbr. dkg Variant of decagram.