There's a trade-off, not a single right number
A longer tie-out cable gives your dog more room to move, sniff, and lie down in a different spot, but it also gives more slack to wrap around a fixed point, a tree, or their own legs, and it lets them build up more speed before hitting the end, which means a harder jolt on the neck or body if they lunge. A short cable tangles less and limits impact force, but it can leave a dog with barely enough room to turn around comfortably. Most everyday setups land somewhere in a moderate middle ground rather than either extreme, sized to the dog and the space rather than to a fixed rule.
Sizing the cable to the space and the dog
Walk the radius the cable will allow before you attach your dog, and check for anything it could catch on, a garden tap, a chair leg, a fence corner, a shrub. If the yard is cluttered, a shorter cable that keeps your dog clear of obstacles is safer than a long one that reaches every corner. A larger, calmer dog can generally handle more length safely than a small, quick, or reactive dog, since a small dog can wind a long cable around itself surprisingly fast.
Preventing tangles regardless of length
Whatever length you choose, a swivel clip at the anchor end and at the harness end stops the cable twisting on itself as the dog moves. Removing loose obstacles from the tie-out area does more to prevent tangling than adjusting the length alone. If you're using a trolley or runner system rather than a single stake, keep the drop line from the overhead cable to the harness reasonably short so it can't hook around the dog's legs.
Watch, don't just set
Even a well-chosen cable length needs checking in use. If you notice your dog circling the same fixed point repeatedly, shorten future sessions or move any obstacle they're winding around. A cable that looks fine unclipped can behave very differently once a dog is moving on it, which is exactly why supervision matters more than any specific length number.
The safety principle behind it
There's no length that removes risk entirely, the goal is a cable long enough for the dog to be comfortable and short enough that a full-speed lunge, and any tangling around nearby objects, stays manageable. Get that balance roughly right, keep the area clear, and stay close enough to intervene if something goes wrong.
The right cable length isn't fixed forever. A growing puppy needs reassessing as it gets bigger and stronger, a dog recovering from injury may need a shorter, gentler setup for a while, and a yard that's had furniture or planting added since you last measured needs a fresh check of the radius. Treat cable length as something you revisit occasionally rather than a setting you choose once and forget.
A cable left coiled loosely on the ground between uses can develop kinks that make it more prone to catching on itself once the dog is moving. Uncoil it fully and lay it out along its expected path before clipping your dog on, rather than trusting it to straighten out on its own, a cable that's already twisted at the start of a session is far more likely to tangle early.
