In fish feeding studies, stomach related parameters are mainly measured in order to have biometric relationships. To determine those parameters a direct and straightforward method for measuring real maximum stomach volume in fish based on Archimedes' principle was tested and compared against a traditional method. The new method is based on the measure of the real maximum stomach volume when fish are fed ad libitum. By contrast, the traditional method measures the forced expanded capacity of the stomach under an arbitrarily assigned hydraulic pressure head of a 50 cm column of water. The two methods were assayed in juvenile rainbow trout Oncorhynchus mykiss, and the results displayed significant different stomach volume and related indices. The best relationships for potential regression were obtained using the Archimedes’ principle method, being between fish weight and stomach volume (R2 = 0.68), and stomach content and stomach volume (R2 = 0.95). These results indicate that the measurement of real maximum stomach volume in fish using the Archimedes’ principle method could be a useful method of estimation of how much stomach can expand; this can be a suitable tool to improve fish feeding research by regulating the amount of feed that can be ingested by animals.