Flashing Safety Dog Collar with LED Lights, Multi Function, POM Buckle, Super Bright LED's Glow & Flash, Great Fun & Improved Dog Visibility & Safety provides visibility up to 1,000 Feet in the dark weather resistant, shock-resistant and lightweight Four modes: slow blinking, fast blinking, sustained lighting, and off How to measure your dog's neck size: measure around the neck close to the shoulder and allow room for a comfortable fit.