Country Brook Design Heavyduty Nylon Martingale Dog Collar, Purple, Medium: 15 to 21-in neck, 1-in wide

I measured and then bought a size large for an 80 lb pitbull and it was plenty big, I probably could have sized down. The collar is very sturdy and seems durable, but it was too difficult to resize it or remove it compared to other martingales I've used (that also don't have a buckle). We usually need to tighten the collar for proper placement for walks, and then loosen it a little once we get home, or often remove it for cleaning and such. I ended up cutting it off because it got too frustrating and the dog would get irritated and start to freak out while we were trying to adjust it every day. I will switch back to the other brand we used that slides a little better when resizing.

Collar worked great from May until last night, he broke it. It was the adjustable part that broke so it was loose on his neck. Sturdy but didnt last long.

Rather than struggle to size this while trying to slip it over our wiggling dog's head, we gave up and bought another collar with a clip. It's probably fine if you're patient.