How do I know what size extension springs I need for my garage door?

To size extension springs correctly, you need three pieces of information: the door height, the door weight, and the existing spring color code. Measure the door height in feet, since springs are sold in lengths matched to specific door heights, most commonly 7-foot or 8-foot doors. Weigh the door by disconnecting the opener, clamping the door fully open with vice grips, then placing a bathroom scale under the bottom edge while you slowly lower it. Record the weight in pounds. Each spring carries half the door weight, so divide by two. Manufacturers color-code extension springs by lift capacity, for example tan for 100 pounds, white for 110, green for 120. Match both the length and the color code, and always replace springs in pairs.