A dog harness is useful to have under many circumstances, such as for walking your dog, for sledding or for securing your dog in a car for a trip to the vet. Large breeds of dogs, and dogs with heads smaller than their necks, benefit from the use of a harness when you're walking them; a harness helps prevent injuries to the neck and escape from your control. A harness gives you more control over your dog and won't choke him if he pulls against his leash. To properly fit a harness, take careful measurements of your dog's body to ensure the animal's comfort while he's wearing it.
Wrap a tape measure or piece of string around your dog's chest just behind his front legs; place two fingers under the measuring tape so you allow room for a little slack when the dog's wearing the harness. If you're measuring using a string, mark the length on the string and lay it out against a ruler to determine the measurement. Record this measurement -- this is the main dimension that determines the size of a harness.
Measure around your dog's neck to determine the proper positioning of the Y-shaped yoke that will fit around the neck. Stretch the tape measure from the base of the back of the neck, in front of the shoulder blades, around to the front of the dog's breast bone, called the sternum. Some manufacturers use this dimension for neck sizing, while others double the figure to determine sizing.
Determine your dog's length by measuring him from the base of the back of his neck, in front of the shoulder blades, to the base of his tail.
Measure the length between the base of the dog's neck, in front of the shoulder blades, downward diagonally to the base of the ribcage at the bottom of the dog's stomach. Some harness types, especially those for sledding, require this measurement.
Weigh your dog using a pet scale. Place the dog on the scale and have him sit still on it until it registers his weight. Record his weight in pounds. Some harness manufacturers require this to determine sizing for their products.
Purchase a harness based on the measurements and weight you have recorded. Be aware that some harnesses require only one measurement while others require two or more of those you have taken. Follow each manufacturer's sizing, or bring your measuring tape into the store to match up the measurements you have taken to the available harnesses themselves. When in doubt, round each of the measurements up to a whole number to select a larger size rather than a smaller one; this ensures your dog's comfort.
Place the harness on your dog and tighten any buckles to ensure a snug, but not tight, fit. You should be able to insert two fingers under the harness at any point. Some harnesses fit differently than others, there are H-style harnesses, K-style harnesses and X-style harnesses. Follow the manufacturer's directions for attaching the harness around your dog. Some harnesses may require that you lift one or more of your dog's front feet and insert them through the harness, while others consist of buckles that you fasten around your dog's body, neck and legs.
Check your dog's fur and skin while wearing the harness; any signs of skin irritation or fur loss from the harness indicate that the harness should be slightly tighter or looser in those areas. All points of the harness should fall in the proper places when the dog is wearing the harness; for example, in harnesses with a Y-shaped front, the point of the "Y" should fall over the dog's sternum.