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.

Use your scale to measure the weight of the container that your water will be in, then add your water to the container and measure its weight again. Determine the weight of the water by subtracting the weight of the empty container from the weight of the full container. For instance, suppose that a bucket weighs 2 pounds (lbs) while empty and 12 lbs with water in it. The weight of the water is:

Weight of water = 12 lbs – 2 lbs = 10 lbs

Dip the metal bulb of your thermometer into the water to measure its temperature. Let the bulb sit in the water until the thermometer's internal liquid stops moving. Record the reading where is stops moving. For example, suppose that your water has a temperature of 65 degrees Fahrenheit.

Calculate the change in the water temperature that will occur when you add a given number of BTUs of heat to the water. For instance, if you were to add 35 BTUs of heat to your water sample, the calculation would look like this:

Change in water temperature = 1 degree Fahrenheit per BTU per lb of water x 35 BTUs / 10 lbs water = 3.5 degrees Fahrenheit

Add your answer from the previous step to the initial temperature of your water to calculate the new temperature achieved by the addition of the BTUs:

New temperature = 65 degrees Fahrenheit + 3.5 degrees Fahrenheit = 68.5 degrees Fahrenheit