There are two ways to measure your dog for a bed:

The first is while your dog is lying down.  Do a measurement from the tip of their nose to their bottom, this can sometimes be split into two measurements (1) Tip of nose to shoulder blades plus (2) Shoulder blades to their bottom.  You will also need to measure from the top of your dog's forehead to the bottom of their paws.  Add on 6 to 12 inches to both measurements to allow for the dog's stretching room.

The second way of measuring your pet can be done while he/she is standing.  Take measurements as above to obtain the correct dog bed size.

For dog beds which give one measurement, this will be the length.  Round beds will be measured by diameter.