The working standard, and why it is not the whole answer
For a medium or large dog, plan on a fence around six feet high. That figure is not arbitrary: a fit, motivated dog can jump far higher than owners expect, and many do not jump at all — they climb. Chain-link mesh offers toe-holds all the way up, horizontal fence rails make a ladder, and a dog that gets front paws over the top can scramble the rest of the way over. A smooth-faced fence at six feet defeats both the jumper and most climbers, which is why it is the standard recommendation for athletic breeds and young, energetic dogs.
Scale honestly to the dog in front of you. A small, short-legged, elderly, or unathletic dog may be contained safely and kindly by a considerably lower fence, and building fortress walls for a dog who cannot and will not climb wastes money without adding safety. The opposite error is worse: a young herding, sporting, or terrier type with spring and determination should be assumed capable of beating any fence you have merely seen work for a calmer dog.
Launch pads and ladders undo height
Walk your boundary and look at it as a climbing frame. Anything stored against the fence — bins, benches, log piles, a kennel roof, a compost heap, a raised planter — is a step that subtracts its height from your fence. A dog who can stand on a metre-high kennel needs only a modest hop to clear what remains. Move launch points away from the boundary, choose smooth panels over climbable mesh where you can, and if a dog is an established climber, an inward-leaning extension along the top removes the final scramble over the edge.
Corners deserve a look as well: two fences meeting give a dog bracing on both sides, and corner posts are classic escape routes for climbers. Gates are the other weak point — a fence is only as good as its gate latch, and a gate that bounces open, sags, or can be nosed up off its latch will be found by the dog long before you notice it.
The bottom of the fence fails as often as the top
Height is wasted if a dog can go under. Terriers and other digging breeds excavate at the fence line, and soil shrinks away from posts in dry weather, opening gaps. Walk the base looking for daylight, loose boards, and soft digging spots. Fixes include burying mesh below the soil line along the boundary, laying a buried wire skirt angled into the garden, or setting a concrete or paved strip along the base. Check again after storms, groundwork, or a neighbour replacing their side of a shared fence — new gaps appear without anyone deciding to make them.
Containment is also about why he wants out
A bored, under-exercised dog works on a fence all day; a dog whose garden is interesting and whose life outside it is rich treats the fence as scenery. Before spending on extra height, ask whether the escaping is athletic opportunity or motivation: a dog leaving to reach other dogs, chase passing activity, or simply find something to do needs exercise, company, and things to do as much as he needs panels. And keep his ID current as the backstop, because KinsLe's Verified QR ID Pet Profiles give every pet a unique QR code for their collar, so if your dog ever beats the fence, whoever finds him can scan it and reach his profile and you straight away.