Providing the correct torsion spring for your application is most important. The guidelines above identify the right garage door spring. Look at the end of the torsion spring to determine the wind direction. If the end of the spring points in the clockwise direction it is a left wound spring. If counter clock wise it is a right wound spring.
Use a tape measure to measure the length of the spring to the nearest quarter of an inch. The wire size is the number of coils and thickness of the wire used to make the spring. Insert the end of a tape measure between two coils or hook the tape on the end of the spring if it does not have a cone on it.Then count 20 coils and pull the tape to the point where the 20th and 21th coils meet. Measure to the nearest 16th of an inch.
Measure 20 coils because it is more accurate, record the 20-coil measurement. If you have one spring that is broken and one that is still wound, you will need to measure 20 coils on the wound spring as well. The spring wire remains the same size after it is wound, so the measurements will not be affected. We measure both springs because some of the garage doors installed have unmatched springs on them.
With the 20-coil measurement you can determine the wire size from the chart above. Note that the measurements are not exact, but rounded to the nearest 16th of an inch. For example, 20 coils of .177 wire will measure to be 3 1/2" to the nearest 16th of an inch, but not exactly 3 1/2". With garage door springs, it's important to measure properly. If you don't measure properly, you can get a spring that will not counterbalance the door properly.