Indices of condition enable the evaluation of the well-being of fish, with the assumption that heavier fish of a given length are in a better condition. Relative weight (W-r) is one of these indices; it is calculated by comparing the actual weight of a specimen with the ideal weight of a specimen in good physiological condition of the same length from the same species, i.e. standard weight (W-s). In this research, length and weight data over the distribution range for Pursak chub Squalius pursakensis, an endemic species distributed in the Sakarya and Porsuk drainages in Northwest Anatolia (Turkey), were used to compute a W-s equation by means of the empirical percentile (EmP) method. The W-s equation obtained was log(10) W-s = -4.657 + 2.614 log(10) TL + 0.127 (log(10) TL)(2), and the total length range of application was 80-340 mm. Since the EmP W-s equation was not influenced by length variation, the use of this equation to compute the relative weight (W-r) for S. pursakensis throughout its area of distribution is suggested.