The cable is longest when the door is down
This trips people up more often than the arithmetic does. Standing in front of an open door, the cable looks short: most of it is wrapped on the drum. Close the door and it unwinds to its full run, from the anchor on the bottom bracket up to the shaft. That closed length, plus the wraps that never leave the drum, plus a tail at each end, is the whole cable. Travel does not add to it — travel is what moves back and forth between the drum and the run.
At the defaults it comes to 102.3 in a side, or 8.5 ft: a 90 in run, 6.3 in in half a dead wrap, and 6 in of tail. The two together are just over 17 ft.
Half a wrap is not nothing
The dead wraps look like a rounding item and they are not. Half a wrap on a 4 in drum is 6.3 in of cable, because the circumference is 12.57 in and half of that is what stays on. Two dead wraps would be 25 in — two feet of cable that never moves and never appears in any length you can measure with the door in either position.
What one turn of difference actually looks like
A drum turn is a circumference of cable, so on a 4 in drum one turn is 12.57 in. If one drum ends up a turn out from the other, one bottom corner of the door hangs 12.57 in lower than the other. Across a 108 in door that is 6.6 degrees out of level, which is not a subtle thing to look at.
Even a quarter turn is 3.1 in, and there is nothing in the system that averages the two sides out. The cable sets where each corner hangs and that is the end of it. This is why a door that sits visibly crooked is a cable and drum geometry problem rather than a track problem, and why the numbers are worth knowing before anybody starts hunting for a bent track.
Grooves are a counting problem
Each working wrap needs a groove, and so does each dead wrap. The default door asks for 6.68 working plus 0.5 dead, which is 7.18 grooves out of the 12 on the drum. Add a foot of high lift and the working wraps go to 7.64, so 8.14 in total, and the margin narrows by nearly a groove for every foot of extra lift.
When the grooves do run out the cable rides up over the wraps beneath it. That is not just untidy: the winding radius changes, and everything downstream of the radius — the torque on the shaft, the cable paid out per turn — changes with it. The count is cheap to do on paper and the page does it, but whether a given drum suits a given door is a question for the door documentation and not for this page.
Extension spring doors are a different animal
None of this applies to a door counterbalanced by extension springs stretched along the horizontal tracks. Those cables run from the bottom bracket over pulleys and back, there is no drum, no wrap and no groove count, and the length comes off the track layout rather than off a circumference. If there is no shaft across the header with drums on the ends of it, this page is describing something else.
Questions people ask
How long should a garage door cable be?
The run from the bottom bracket anchor up to the shaft with the door closed, plus whatever stays wrapped on the drum, plus a tail for the anchors. On the default figures — a 90 in run, half a dead wrap on a 4 in drum, 6 in of tail — that is 102.3 in, or 8.5 ft a side. Measure the run with the door down, because that is the position where the cable is at full length.
Does the door travel add to the cable length?
No. Travel is the part that winds onto the drum and back off it, so it is already inside the closed length. A door that lifts 84 in does not need 84 in more cable than one that lifts nothing; it needs enough drum grooves to hold 84 in of wrap.
What happens if one drum is a turn out from the other?
One corner of the door hangs a full circumference lower. On a 4 in drum that is 12.57 in, which across a 108 in wide door is 6.6 degrees out of level. A quarter turn is still 3.1 in. Nothing in the system averages the two sides, so the difference goes straight into how the door sits.
How many drum grooves does the lift need?
The working wraps plus the dead wraps. The default lift of 84 in on a 4 in flat drum takes 6.68 working wraps, and half a dead wrap makes 7.18 grooves. A foot of high lift adds about another groove. The page counts them against the number you enter, but it does not tell you whether a drum suits a door — that is what the door documentation is for.
Can I use this for extension springs?
No. Extension spring cables run over pulleys along the horizontal tracks rather than onto a drum, so there is no wrap, no groove count and no circumference in the length. If there is no shaft across the header with a drum on each end, this arithmetic describes a different door.