A British thermal unit (BTU) is defined as the amount of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit. To calculate the temperature of a water sample from the BTUs applied to it, you must know the weight of the water and its starting temperature. You can measure the weight of the water using a scale, and the temperature using a Fahrenheit thermometer. Once you have that information, calculating the temperature of the water after applying a known number of BTUs of heat is easy.