🔌 Cable Length Calculator

Enter your horizontal and vertical run distances and a slack allowance to see the cable length you need, rounded up to the nearest standard length.

Horizontal run (ft)

Vertical run (ft)

Slack / service loop (%)

How the length is worked out

The calculator adds your two legs, applies the slack percentage, then rounds up to the first size on the standard retail ladder:

base   = horizontal + vertical
needed = base × (1 + slack / 100)
buy    = smallest standard length ≥ needed

standard ladder (ft): 3, 6, 10, 15, 25, 35, 50, 75, 100

The defaults show it: 10 ft across the room plus a 3 ft drop behind the TV is a 13 ft base, 15% slack makes 14.95 ft, and the ladder rounds that to a 15 ft cable. The legs are added, not squared and rooted, because cable runs along skirting and up a wall. The diagonal would be 10.44 ft, and a 10 ft cable would not reach.

Measuring the run and picking the slack

Walk the path, don't eyeball the room. Detours around door frames, over a joist or into a floor box all add feet.
10-15% slack is normal for a fixed run. Use 25% or more for a swing-arm mount, where the cable must survive the panel being pulled out.
Length changes which cable you need. Passive HDMI gets marginal past about 25 ft at 4K/120 or 8K, where active optical is the fix. Long speaker runs add resistance, so move from 16 AWG to 14 or 12 AWG past 50 ft.

The ladder stops at 100 ft. Anything longer still reports 100 ft, so read that as "beyond this tool" and plan a custom run or an extender.

Frequently asked questions

How much extra cable should I allow for slack?

10-15% of the run for anything fixed in place, which on a 13 ft run is under two extra feet. Raise it to 25% for an articulating mount, and leave a service loop behind the panel so a future swap needs no re-pull.

How long can an HDMI cable be before the picture drops out?

Passive copper is dependable to about 25 ft at 4K/60. Beyond that, or at 4K/120 and 8K, use an active optical HDMI cable - it is directional, so the marked source end must go to the player or receiver.

Should I measure straight-line distance to the TV?

No. Measure the horizontal and vertical legs separately, as this tool does. A diagonal is always shorter than the path the cable really takes.