Cable length follows door height, with an allowance
A 7 foot door takes a 102 inch cable. An 8 foot door takes 9 foot 7. A 10 foot door takes 11 foot 6, and a 12 foot door takes 13 foot 6. The extra over the bare door height is the wrap on the drum plus the slack you need to set the tension.
Measure the door opening, not the panel stack, and measure it on the inside. If your door sits between two standard heights, order the longer cable: a cable you can trim at the drum is a solved problem, a short one is not.
Always replace both
Cables fail from fatigue, not from accident, and the second cable has done exactly the same number of cycles as the one that broke. Replacing one and leaving the other buys you a few weeks and a door that hangs crooked in the meantime.
That is why our kits ship as a pair. It is also why the pair is the same batch — two cables of the same age stretch at the same rate.
Galvanized strand, and why the finish matters inside a garage
A garage is a damp, unheated box with a concrete floor. Bare steel strand surfaces rust where it leaves the drum, and rust on a cable is not a cosmetic problem: it is an abrasive sitting between strands.
Galvanized aircraft-grade strand resists that, which is the only reason we use it. It is not a performance claim; it is a shelf-life one.
Know when to stop and call somebody
Cables are low-tension. Torsion springs are not. If the job in front of you involves winding bars, a spring under load or a door that will not stay up on its own, that is a trade call and not an afternoon project.
Replacing a pair of cables on a door whose spring is intact and balanced is straightforward. Judging whether the spring is balanced is the part that takes experience, so if you are unsure, stop there.