Topup vs top-up

Which noun is correct?

• a topup
• a top-up

Which verb is correct?

• to topup
• to top-up

The correct usage is top-up when used as a noun and top up when used as a verb.

When you want to top up your phone credit, you buy a top-up.

References: Oxford Learner's Dictionaries -- top something up | top-up

Two or more words commonly used together can progress over the years from being spelled “open” (separate words with space between, never the less), to being hyphenated (never-the-less), to being spelled “closed” (as all one word, nevertheless). You can often find things hyphenated in older texts that are spelled closed now, like “to-night” (see n-gram of relative frequencies of hyphenated and closed versions). The process does not always occur: “no one” remains stubbornly open.

In the case at hand, it would appear that the closed version is not yet making much headway, while the open and hyphenated versions are both trending upward. Of course, some of the instances of the open spelling may refer to children’s toys, convertible automobiles, women’s clothing at Mardi Gras in New Orleans, whatever. I presume we are talking here about a person pouring beverages. My inclination would be to use the open spelling always for the verb, especially since the parts are so often separated (“May I top you up?”); but for use as a noun, the hyphenated version (“Care for a top-up?”) might more clearly disambiguate from the child’s-toy and other applications of the term.

The dictionaries I have checked do follow a convention:

• Collins: separate as a verb, hyphenated as a noun or adjective and in top-up card

• Cambridge: separate as a verb, hyphenated as a noun or adjective and in top-up card and top-up fees

• Merriam-Webster: separate as a verb, dashed as a noun

• Oxford Learners: separate as a verb, hyphenated as a noun or adjective and in top-up card

But dictionaries lag general usage, and while MW traces the history of top up as a verb to 1937, modern usage is driven not by fuel or drink but by prepaid mobile phone credits and stored value smartcards. Reviewing the websites of several providers, top-up as a verb form seems to be catching on: the likes of Tesco, Telecom NZ, and even University College Dublin invite you to top-up your card, and various providers in South Asia and the Middle East appear to have gone to topup for the noun form.