Measuring Using a Soft Measuring Tape or string
This method can give you a baseline of where to start with collar sizing and works well for those dogs with uncomplicated, long, sleek, short-coated necks.
Using a soft tailor's tape measure, find the spot where the one end wraps around and touches the rest of the tape measure. Don't fit too loose, but don't choke out your dog, either. If your dog has a chin rest behaviour, this is a great place to use it. If not, some peanut butter, wet food, or other delicious distractor smeared on the fridge or oven door can keep the front end of the dog busy long enough for you to do your measuring. While you are measuring, give your dog an extra smooch or pat for me and tell them they are a wonderful beast.
If you don't have a soft, flexible tape measure, you can use a length of non-stretchy string or cloth or strip of paper. Measure your dog's neck using this MacGyver solution. Hold or mark the spot where the end wraps around and touches. Lie this length of material next to a real measuring device, like a tape measure or ruler, and presto! You have your measurement.
If your dog will not be held for measuring, you can measure using an old collar. Please contact me for information on how to do this.