You can calculate the output of a radiator, if it is electric, in BTUs by converting it wattage to BTUs. This calculation can be done by hand, but the process is much easier when using an online conversion tool to find the BTUs for your radiator. If you have a standard radiator that uses water, you can determine the BTU output by using a BTU calculator. This will give you the approximate BTU output for your radiator.
Find the wattage of your radiator by looking at the back of the unit. If the wattage is not listed on the unit you may need to refer to the instruction manual.
Go to a website that has a BTU conversion tool, such as the one located at heatershop.com (see Resources).
Enter in the watts for your unit in the "Watts" field.
Click the "Convert" button to see the BTU output of the unit per hour.
Go to a website that has a BTU conversion tool; again, heatershop.com has one (see Resources).
Choose the insulation type from the provided list.
Enter the Fahrenheit temperature increase that is required. If you do not know the Fahrenheit temperature increase you can leave the default set at 10.
Enter the square footage of the room. The first field is for the width and the second field is for the length.
Enter the height of the ceiling and then click the "Calc BTU" button.