How to choose the right size collar for your dog
How to choose the right size collar for your dog, covering measurement techniques, fit checks, and breed-specific risks like bloat in Bloodhounds.
Fit the collar so you can slide exactly two fingers flat between the strap and the neck, measured at the thickest part of the neck where the collar will sit. This rule applies to most breeds, but the exact placement and width depend on the dog's anatomy, especially for deep-chested or thick-necked dogs.
Measure the neck, not the head
The most common error is measuring the head to guess the neck size. A dog's head is wider than its neck, so a collar sized for the head will never fit the neck correctly. Use a flexible tape measure to find the circumference of the neck at its thickest point, usually just below the jawline where the collar naturally rests. For large breeds with thick coats, press the tape firmly against the skin to account for the fur depth, or shave a small patch to measure the actual skin circumference.
The two-finger rule is the baseline
Once you have the measurement, add one to two inches (2.5 to 5 cm) to allow for growth and comfort. The fit is correct when you can slide two fingers flat between the collar and the dog's skin. If you can fit a third finger easily, the collar is too loose and risks slipping over the ears or getting caught on objects. If you cannot fit two fingers, the collar is too tight and may restrict breathing or cause chafing. This rule works for both flat collars and martingales, though martingales have a different sliding mechanism that requires a slightly different check.
Breed anatomy changes the fit
Some breeds require specific adjustments because their neck shapes do not follow the standard cylinder. Bloodhounds, for example, have a massive skeletal structure with very thick bones and a heavy, loose skin fold around the neck. Their neck circumference can be substantial, but the weight is concentrated in the bones rather than muscle. A standard collar might sit too high on a Bloodhound's neck, putting pressure on the trachea rather than the broader, safer area below the jaw. For deep-chested breeds prone to gastric dilatation volvulus (bloat), a collar that sits too low or is too tight can exacerbate internal pressure, so a wider, flat collar that distributes weight evenly is safer than a narrow one.
Collar width matters for pressure distribution
A narrow collar concentrates force on a small area of the neck, which can damage the trachea if the dog pulls. A wider collar, typically 1 to 2 inches (2.5 to 5 cm) for large dogs, spreads the load across more tissue and bone. For a Bloodhound, which can weigh up to 72 kg (160 lbs), a narrow strap is insufficient to support the dog's weight safely during a pull. The thick, hard coat of a Bloodhound also means the collar must be wide enough to prevent it from sinking into the fur and losing its grip on the skin.
Check for growth and seasonal changes
A dog's neck size is not static. Puppies grow rapidly, and adult dogs can gain or lose weight, changing their neck circumference. A collar that fits perfectly in winter may become too tight in summer if the dog loses weight, or too loose if they gain. Check the fit every few weeks, especially after a change in diet or activity level. If the dog is a puppy, you may need to adjust the collar weekly to accommodate growth plates and prevent long-term neck damage.
When to see a vet
If a dog shows signs of coughing, gagging, or difficulty breathing while wearing a collar, the fit is likely incorrect or the collar type is unsuitable. Sudden reluctance to wear a collar can also indicate pain in the neck or throat. For breeds with known respiratory issues, such as brachycephalic dogs or those with tracheal collapse, a harness is often a safer alternative to a collar, as it avoids pressure on the neck entirely.
Summary of fit checks
Ensure the collar sits high on the neck, just below the jawline, and allows two fingers of slack. Adjust for breed-specific anatomy like the Bloodhound's thick bone structure and heavy coat. Regularly re-measure to account for weight fluctuations and growth. A properly fitted collar prevents injury and ensures the dog can be controlled safely without compromising their airway.