Why are my feet two different sizes?

The right and left sides of our bodies are often thought of as mirror images of each other. However, the same does not hold true for the feet. So why are our feet often two different sizes?

"Nobody's feet are exactly the same," Dr. Corrine Renne, a podiatrist at New York Foot and Ankle, told Live Science. "Our bodies are somewhat symmetrical, but not always symmetrical."

A 1983 study of 4,000 women and 2,800 men across the United States found that none of them had a pair of feet that were exactly alike in size or shape. Supporting that earlier research is a 2018 study from the SATRA Bulletin, a trade publication on footwear and leather. After analyzing 2,890 people in the United Kingdom, the study discovered that up to 19% of people there may have feet that differ in length by more than about one-sixth of an inch (4 millimeters). That is about half a shoe size in the U.S. and the U.K.

"A similar trend is seen in the USA, and in China no less than 24 percent of people have a difference greater than 4 millimeters," the 2018 study reported.

Which foot is usually larger: right or left?

There does not appear to be consensus on whether the left or the right foot is usually the larger of the pair. For instance, the Center for Foot Care in Liberty Township, Ohio, said about 80% of the U.S. population has a larger left foot than their right foot. City Chiropody and Podiatry in London concurred, attributing this difference to how the majority of the world's population is right-handed, which means the left foot gets exercised more and becomes slightly bigger to keep the body balanced.

Related: Why are people left- (or right-) handed?

The 1983 study noted that, although it is generally assumed that the left foot is usually larger than the right, the researchers found there were actually equal chances that a person's larger foot was their left or right one. In contrast, the 2018 study found that the right foot was usually the longer one, with 50.7% of U.K. women, and 54.8% of U.K. men, having their right foot longer than their left.

A certain amount of asymmetry in the human body is due to normal variation. "Our hands aren't completely symmetrical, nor are our eyes," Renne said.

Why our feet are different sizes

There are also other factors that can lead to these size differences. "For instance, women can experience a bit of discrepancy in foot size during or after pregnancy," Renne said. They can experience ligamentous laxity, a condition in which their joints have a wider range of motion than usual, and their feet may end up differing in size if they naturally favor one side of their bodies over the other. "Sometimes their bodies go back; sometimes they don't," she said.

A number of birth conditions can lead to one foot being a significantly different size than the other. For instance, "if you're born with a club foot, that whole extremity is smaller than the opposite side," Dr. Jacob Wynes, an associate professor who is the chief of podiatric services and a podiatric surgeon at the University of Maryland School of Medicine, told Live Science.

Brachymetatarsia and brachymetapody are both conditions in which one or more toes are shorter on a foot, respectively, Wynes noted. For instance, in a condition known as digital hypoplasia, toes may be underdeveloped or incomplete, he said.

Injuries can alter foot size as well, especially during childhood, when the bones are still developing, the Center for Foot Care noted. Wearing a cast on your foot for a prolonged time can also lead its muscles to weaken and shrink, City Chiropody and Podiatry added. Renne added that differences in foot size could result from using one foot more often than another, such as might happen with a ballet dancer — this could increase muscle mass and thicken a foot's structure.

If the difference in foot or limb length is severe enough, surgery may be necessary "if it affects one's ability to perform activities of daily living, or if the pain is unbearable," which might happen due, for instance, to whatever injuries to a foot led to a difference in size, Wynes said. Surgeries such as bone grafts or bone cuts can respectively lengthen or shorten bone. "People can usually tolerate about 2 centimeters [0.8 inches] of change" from surgery, he said.

In most cases, though, having feet of different sizes is just a nuisance. If you simply want to find shoes that fit, there are several options available. One way is to buy a pair of shoes that matches the larger foot and use custom devices from podiatrists or orthotists, such as fillers in the toes or heels "to make shoes fit better and be more symmetrical," Renne said.

Another way is to buy shoes in two different sizes, Wynes said. Renne added that specialty shoe stores can sell just one of each shoe size.