A calculation device is configured to calculate a value relating to a combustible target gas containing miscellaneous gas components. The calculation device includes: a heating value calculation unit configured to calculate a heating value of the target gas on the basis of converted heating values obtained from a refractive index and a density of the target gas, respectively; a miscellaneous gas total approximate-concentration calculation unit configured to calculate an approximate concentration of the miscellaneous gas components on the basis of the converted heating values; and a virtual basic-heating-value calculation unit configured to calculate, on the basis of the heating value and the approximate concentration, a virtual basic heating value of the target gas when the miscellaneous gas components are assumed to be removed.