To calculate how much a length of steel will increase, you need to know how much the temperature increases and the original length of the steel. Like most materials, steel expands when the surrounding temperature increases. Each material has a different response to the heat, which is characterized by its thermal expansion coefficient. The thermal expansion coefficient represents the amount that the material expands per each degree increase.
Use a thermometer to measure the change in temperature in degrees Fahrenheit. For example, if the original temperature was 70 degrees Fahrenheit and the final temperature was 75 degrees Fahrenheit, you would have a temperature increase of five degrees.
Multiply the temperature change by 7.2 x 10-6, which is the expansion coefficient for steel. Continuing the example, you would multiply 0.0000072 by 5 to get 0.000036.
Multiply the product of the expansion coefficient and the temperature increase by the original length of the steel. Finishing this example, if the steel rod was originally 100 inches long, you would multiply 100 by 0.000036 to find that the steel would be 0.0036 inches longer when subjected to heat.
TL;DR (Too Long; Didn't Read)
If you are calculating the change in area rather than length, multiply the increase in length by two to find the area increase. If you are calculating the change in volume, multiply the increase in length by three to find the volume increase.