Knowing your foot arch type is extremely important in order to prevent long-term injuries from the wrong footwear.  Low arch (flat feet) can lead to over-pronation, plantar fasciitis, post-tibial tendonitis, heel spurs, medial knee problems, or bunions. Medium arch on the other hand is biomechanically efficient but also susceptible to common foot problems such as heel pain and metatarsalgia from repetitive stress and improper fitting footwear. Lastly, high arch creates excessive pressure to rearfoot and forefoot. Problems stemming from high arch can be plantar fasciitis, heel pain syndrome, arch strain, metatarsalgia, calluses, claw toes

Around one thumb!

Intense, unusual or long-term pain after your runs or during your recovery can indicate wrong footwear just as chronic injuries. If you haven't consulted a professional you need to do so in order to prevent long-term damage and to assist in fitting your running shoes.

Pronation is part of the natural movement of the human body and refers to the way your foot rolls inward for impact distribution upon landing. Understanding your pronation type can help you find a comfortable running shoe. Your pronation type is with high likelihood connected to your foot arch type. Supinators tend to have high arches, people with neutral pronation have medium/normal arch, and over-pronators have low arches/flat feet. 

Your shoes should be tight - but not too tight. Lace your shoes in a way to provide stability and tightness, but at the same time, you should be still able to wiggle your toes while not having your foot slipping out at the heel.

Most people are fine with shoes of normal width. However, if majority of shoes you try are too wide or too tight, you may have narrow or wide feet. Understanding what width type is right for you will prevent injuries and add to comfort to your runs.