Recommended widths: XS/Small Breeds- 5/8"  Medium/Large Breeds - 3/4" Large/XL Breeds 1" ^ Please note that these are suggestions, if your pet is well behaved on a leash you can consider using any width. If your pet is a known puller I recommended 3/4" or 1" with an Auto Locking Carabiner Upgrade (See Add Ons)