Calories (I.T.)
The Calorie (I.T.) or International Table Calorie is defined as the amount of energy required to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere which is approximately 4.1868 joules.
Btu (3.9°C)
The Btu (3.9°C) is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at the temperature where water reaches its maximum density which occurs at approximately 3.9°C (39°F).