Step 1
Find the capacity of your water heater by looking at the appliance for information about it. Usually, there is a label on the front of the heater that gives pertinent data about the heater, including capacity. This label may be stamped on metal or printed on a sticker, depending on the age of the unit.
Step 2
Assemble water information for the equation. It takes one Btu per pound of water to raise the temperature by one degree Fahrenheit. The standard size of American hot water heaters is 40 gallons, and each gallon of water weighs 8.33 pounds.
Step 3
Calculate the Btu. If you have a 40-gallon water heater, that is 40 multiplied by 8.33, or 333.2 pounds of water. So you need 333.2 Btu to raise the water 1 degree Fahrenheit. However, most areas need more than one degree of heat added to the water to make it warm enough for showering or washing clothes or dishes. Now more information is needed.
Step 4
Get the average temperature of your water. This information is usually available from your water company or municipality. For instance, if your water temperature is 50 degrees Fahrenheit on average, and you want it at 140 degrees in the water heater, subtract 50 from 140. This is how much higher the water needs to be heated. Take that number (90) and multiply it by 8.33, resulting in 749.7. Multiply that by 40, the number of gallons in the tank. This answer is 29,988, which means you need 29,988 Btu.