Vigorous swimming results in the “burning” of 11 food calories per minute. What is amount of energy associated with 30 minutes  swimming expressed in Btu? How many equivalent ounces (mass of gasoline are associated with this energy expenditure? An approximate value for the energy density of gasoline is 20,000 Btu per pound-mass. One food calorie (or large calorie) is equivalent to 4184 joules. One ounce (mass) is 1/16 of a pound-mass.