The Calculus Affair (French: L'Affaire Tournesol) is the eighteenth volume of The Adventures of Tintin, the comics series by the Belgian cartoonist Hergé.

The story reflected the Cold War tensions that Europe was experiencing during the 1950s, and introduced three recurring characters into the series: Jolyon Wagg, Cutts the Butcher, and Colonel Sponsz.

Hergé continued The Adventures of Tintin with The Red Sea Sharks, and the series as a whole became a defining part of the Franco-Belgian comics tradition.

Tintin surmises that Calculus had invented an ultrasonic device capable of being used as a weapon of mass destruction, which Bordurian intelligence agents are now seeking to obtain.

Suspecting that Bordurian spies have kidnapped Calculus and are holding him hostage in their Rolle embassy, Tintin and Haddock seek to rescue him.

The next morning, Tintin and Haddock learn that Bordurian fighter aircraft forced down the Syldavian plane and recaptured Calculus, who is now being held in Borduria.

When Sponsz comes to visit Castafiore in her dressing room, Tintin is able to steal papers from his overcoat pocket that will secure Calculus' release from the fortress of Bakhine.

At the lost property office, a relieved Calculus finally collects the black umbrella by Snowy alongside Tintin and Haddock.

Reflecting Cold War tensions, The Calculus Affair was published at a time when espionage thrillers were popular in France and Belgium.

[6] In The Calculus Affair Hergé introduced Jolyon Wagg, a Belgian insurance salesman, who appeared in each subsequent adventure with the exception of Tintin in Tibet.

[12] Hergé used his brother, Paul Remi, as the model for Sponsz, although he was also influenced by the image of the Austrian American filmmaker Erich von Stroheim.

[12] A key influence on the plot of The Calculus Affair was an article that Hergé had read in a February 1954 issue of the Belgian weekly La Face à Main, reporting that there had been a number of incidents along the road from Portsmouth to London in southern England in which motorists' car windscreens had spontaneously shattered; the article's author suggested that it may have been caused by experiments undertaken in a nearby secret facility.

[15] Hergé requested that Jean Dupont, the editor of L'Écho illustré — the magazine in which The Adventures of Tintin was serialised in Switzerland — send him documentation on Swiss railways from which he could draw.

[23]A book that Tintin examines in Professor Topolino's house, German Research in World War II by Leslie E. Simon – a retired major general in the United States Army – really existed and was published in 1947.

[27] The Calculus Affair began serialisation in Tintin magazine's Christmas edition on 22 December 1954, and continued to appear in the pages of that publication until 22 February 1956.

[7] For this volume Hergé had designed a front cover; initially, it simply showed Tintin and Haddock hiding Calculus from Bordurian soldiers, but he subsequently added shattered yellow glass around the edges of the image for dramatic effect.

[5] The Lofficiers felt that "the plot seems somewhat shoe-horned into the familiar universe" and "one feels that Hergé's heart was not really much into the action part of the story", ultimately awarding it three stars out of five.

[35] In his psychoanalytical study of The Adventures of Tintin, the literary critic Jean-Marie Apostolidès declared that The Calculus Affair marked the beginning of the third and final period of the series, which he believed could be characterised by Hergé's depiction of a world run by "wheeling and dealing" and in which "detective work takes precedence over any mystical quest".

[38] He viewed the scene at the end of the story in which Calculus burns the plans to his ultrasonic device as "a symbolic castration", allowing the character to become "the Oedipal Father with whom the sons [Tintin and Haddock] can compete", thus stabilising "the family hierarchy" of the series.

[45] In 1991, a collaboration between the French studio Ellipse and the Canadian animation company Nelvana adapted 21 of the stories into a series of episodes, each 22 minutes long.