Count one breath each time the chest rises and falls. For the most comparable values, your dog should be lying asleep or resting calmly, not panting after exercise, heat or stress. Count for a full minute, or count for 30 seconds and multiply by two. Write down several measurements on different occasions to get a picture of your dog's normal level.
In a study of healthy adult dogs, the average sleeping breathing rate was low and most individual measurements were below 30 breaths per minute. Similar measurements are also used at home to monitor dogs with heart disease. A single higher value always needs to be interpreted in context, but a clear and repeated increase from your dog's normal sleeping value is worth paying attention to.
Look at how your dog is breathing at the same time. Visible effort from the belly, an outstretched neck, being unable to settle, bluish gums or obvious breathlessness matter more than the number itself and should be assessed as an emergency.