18 Months received generally mixed reviews from music critics. At
Metacritic, which assigns a
normalised rating out of 100 to reviews from mainstream publications, the album received an
average score of 57, based on 17 reviews. Fraser McAlpine of
BBC Music hailed the album as a "collection almost exclusively in the key of triumph", as well as "a portfolio of win for Calvin, an annual report where the graph is almost all peaks and the troughs are so far down they're practically invisible." Arwa Haider of
Metro commented that "
18 Months could be a capsule collection of smash singles, yet it also works brilliantly as an album. That's partly because these are never faceless anthems; its singers [...] are well judged and rise to the songs, while the catchy hooks are lovingly arranged".
AllMusic's Tim Sendra wrote that the album "shows Harris to be a solid producer with an easily identifiable sound." Mikael Wood of the
Los Angeles Times noted that despite the variety of male collaborators, the album "only deepens the impression that Harris is best when linked with a lady; his skills in that area are several times more developed than they are anywhere else."
The Independents Andy Gill was unimpressed by
Ellie Goulding's performance on "I Need Your Love", but complimented Welch on "Sweet Nothing", and cited Harris's collaboration with
Nicky Romero on "Iron" as the album's "killer cut". Emily Mackay of
NME opined that "[t]he best collaborations stand alone, but the rest demands small hours and sweat to animate it", stating the album "feels more like a deserved victory lap than a forward step or a new instalment, but apart from his sole vocal on 'Feel So Close', the victor seems oddly absent." Killian Fox of
The Observer remarked, "Nothing else on
18 Months matches up to the blockbusting collaborations with Kelis, Florence Welch and Rihanna", concluding that "Harris's production has become increasingly homogenised and, despite the array of vocalists, everything here risks sounding the same." At
Entertainment Weekly, Melissa Maerz complimented songs like "We Found Love" and "I Need Your Love", but found that the album does not offer "many surprises". Despite referring to Harris as a "brilliant pop craftsman",
The A.V. Clubs Chris DeVille felt that the album "suffers from
EDM fatigue" and that "almost every track eventually congeals into the same automaton thud." Evan Sawdey of
PopMatters critiqued that "while
18 Months [...] is pretty much the hit-making monster that launched [Harris] in to the world spotlight, the truth of the matter is that it feels like a rather compromised vision of who he is an artist, sacrificing his quirkiness for a brooding new persona that starts to get stale over the course of a complete full-length."
The Guardian critic Rebecca Nicholson expressed that "Harris knows how to make the most of his guests, leading them through a series of euphoric bangers that seem destined for success. But for all the pop divas he has roped in, there's a veneer of cynical, laddy EDM, resulting in the kind of tracks
Skrillex might come up with on an
Ayia Napa booze cruise." ==Commercial performance==