Cable Length
Cable length is measured along the path the cable takes, not across the room. Three segments and a slack allowance produce the figure, and the figure is almost always larger than the room looks.
Three segments
A run behind a screen is almost never one straight line. There is a horizontal stretch along the wall, a vertical stretch up to the panel or down to the shelf, and a stretch that disappears behind furniture and reappears somewhere unexpected.
Measure each one separately with the furniture where it will actually stand. Adding three honest numbers beats estimating one.
Slack, not waste
A cable pulled taut is a cable under load at both connectors. Leave enough that the stand can come forward for cleaning and the panel can tilt without dragging anything.
Slack also covers the bend at each end. A cable that has to turn sharply the moment it leaves a socket is being asked to do something its connector housing does not enjoy.
Rounding up
When the total falls between two listed lengths, take the longer one. A cable that is slightly too long is coiled behind the stand; a cable that is slightly too short is bought twice.
The calculator below picks the shortest length listed on this site that still covers the total, and it says so plainly when nothing here is long enough.
Runs inside a wall
A run that disappears into a wall is a different exercise with different rules, and the length calculation is the smallest part of it. The jacket marking decides whether the cable may be there at all.
Measure the in-wall route connector to connector, including the drop inside the cavity and the slack at each plate, then read the in-wall page before anything is cut.
Length of the run
Inputs
Range 1 to 25
Range 1 to 25
Range 1 to 25
Range 10 to 20
Result
- Measured run
- 11 ft
- Run plus slack
- 12 ft 8 in
- Nearest length listed here
- 15 ft
The three segments added together, before slack.
The shortest length listed here that still covers the run plus slack.
Formula
- Run = along the wall + up or down + behind furniture
- Total = run × (1 + slack ÷ 100)
- Nearest listed length = the shortest length listed here that is at least the total
- Lengths listed on this site: 3 ft, 6 ft, 6.6 ft, 15 ft
Does not account for
- Minimum bend radius at each connector
- The internal width of a cable channel or raceway
- Routes that pass through furniture rather than behind it
- Extra length needed to pull a cable through a wall
- Anything the run has to go around that you did not measure
Common errors
| Error | Consequence | What to do |
|---|---|---|
| Measuring the straight line between the two devices | The cable arrives short by the whole vertical segment and the trip behind the furniture. | Measure the three segments separately, along the path the cable will take. |
| Buying the exact length the tape gave | Both connectors sit under tension and the stand can no longer be pulled out. | Add the slack allowance and then round up to the next listed length. |
| Measuring with the furniture pushed against the wall | The run is measured in a position the furniture only occupies when nobody is cleaning. | Measure with the stand pulled forward, where it spends its awkward moments. |
Before starting
Stop
A cable going inside a wall needs an in-wall rating marked on its jacket. Length has nothing to do with that, and no amount of measuring substitutes for the marking.
Note
The lengths this calculator can pick from are the lengths listed on this site. It is a catalog, not a survey of what exists.