The size of the urn you need is primarily determined by your dog’s weight. A general rule of thumb is that you’ll need one cubic inch of urn space for every pound your dog weighed before they passed away. This rule applies to both dogs and other pets and helps ensure that the urn will be large enough to hold all of your pet’s ashes. Formula: 1 pound of body weight = 1 cubic inch of urn space For example, if your dog weighed 50 pounds, you would need an urn with a capacity of at least 50 cubic inches.