Diesel fuel has a high energy density -- the amount of energy per unit of volume -- that allows it to power engines for a longer period of time than standard gasoline. You can express the energy content of diesel fuel in a number of ways, two of the most common being in Joules (J) or kilowatt hours (kWh). However, converting to the familiar energy unit of the calorie (c) is easy to do. Since 1 c contains 4.184 J, calculating the calorific value of diesel fuel turns out to be a simple process that involves looking up the energy density of diesel fuel and performing one calculation.

Determine the energy density of diesel fuel. According to University of Washington, it is approximately 36.4 MegaJoules per litre (MJ/L). A MegaJoule is 1 million Joules.

Convert the energy density of diesel fuel into Joules by multiplying the energy content in MegaJoules by 1 million. For example, 36.4 MJ/L x 1,000,000 J/MJ = 36,400,000 J/L

Convert the energy density of diesel fuel into calories per litre by dividing the number of Joules per litre by 4.184 -- the number of Joules in a calorie. For example, 36,400,000 J/L / 4.184 J/c = 8,700,000 c/L. You now know that one litre of diesel fuel contains approximately 8.7 million calories.