Take a flexible measuring tape and place it tightly around your dog's neck, preferably where the collar should sit. Leave 2 fingers space between the neck and the measuring tape.
For slip and half-choke collars, place the measuring tape at the widest point on the head. This is mostly just before the ears and guarantees that the collar will fit over the head later.
If you don't have a measuring tape, take a wide string or shoelace and mark the two points where the beginning and the end meet. Now lay the string lengthwise on a ruler or folding rule and measure from point to point.
Measuring with a well-fitting collar
Open the collar and measure lengthwise, starting at the beginning of the leather to the hole where it was worn.