There are 2 very simple ways to measure your dog's neck size to determine what size collar you should buy for them. Most people don't have a tailor's measuring tape laying around, and a tape measure is not practical.
1) Use their current collar: Note: do not measure end to end. This is a very common mistake. Measure from the spot indicated here on the 'male' end of the buckle. This is where the end of the female side of the connector meets the male side. If you measure from the end. Your measurement will at least be an inch or two off. If you're near the low or high end of a size range, that can make of break your perfect fit! In this situation, after achieving a good fit on the dog, I would take it off and find that my dog's ideal collar size would be 17 inches.
2) (Gently) Use a phone charger cable. It's easy and most everyone has one. Hold the end of the cable between your finger and thumb and and gently wrap the other side around your dog's neck (thickest area), and pull it snug enough that you can fit two fingers between it and your dog's neck. Let go of the charger end of the cable and mark the spot on the cord with your finger that met the charger end. From there, use a tape measure to get the measurement from your cable measurement. This is far easier than trying to wrap a tape measure around your dog's neck!