Running Shoes: Sizing, Fit, and When to Go a Half Size Up

Should I buy running shoes half size bigger?

Generally, yes, for many runners, purchasing running shoes a half size larger than your casual shoes is a widely recommended practice to accommodate foot swelling and natural toe splay during activity, ensuring comfort and preventing common foot ailments.

The General Consensus: Why Bigger Might Be Better (Sometimes)

The advice to size up in running shoes is not a universal mandate but a common and often beneficial guideline rooted in the physiological and biomechanical demands of running. Unlike everyday footwear, running shoes must accommodate dynamic changes in foot volume and shape under repetitive load. The primary reasons for considering a half-size increase are foot swelling and toe splay. During sustained physical activity, blood flow increases to the working muscles, including those in the feet, leading to a natural expansion. Concurrently, with each stride, the toes naturally splay out to provide stability and propulsion. An adequately sized shoe prevents constriction, which can lead to discomfort, blisters, and more serious issues.

Anatomy and Biomechanics of the Running Foot

Understanding the foot's mechanics during running is crucial for proper shoe selection. The foot acts as both a rigid lever for propulsion and a flexible shock absorber during impact.

• Impact Phase: As the foot strikes the ground, it flattens and lengthens, and the arch collapses slightly (pronation) to absorb shock.
• Propulsion Phase: The foot then stiffens, and the toes spread to maximize the push-off, requiring space within the shoe's toe box.
• Foot Volume Changes: The repetitive impact and increased blood flow cause the feet to swell, typically increasing in both length and width by the end of a run, especially longer distances.
• Common Issues from Poor Fit: Shoes that are too small or narrow can lead to:
  • Blisters and Hot Spots: Friction from rubbing against the shoe.
  • Black Toenails: Repeated trauma to the nail bed from toes hitting the front of the shoe.
  • Bunions and Hammertoes: Exacerbation of existing conditions or development due to compression.
  • Neuromas: Nerve irritation caused by pressure.
  • Plantar Fasciitis: While not directly caused, a poor fit can alter foot mechanics and contribute.

Factors Influencing Your Ideal Shoe Size

Several individual and activity-specific factors influence the optimal fit of your running shoes:

• Foot Swelling: As mentioned, feet naturally swell during activity. The longer the run, the more pronounced this effect.
• Foot Shape and Arch Type: Individuals with wider feet, high arches, or flat feet may require specific considerations beyond just length. Some brands offer different widths (e.g., B for narrow, D for standard, 2E/4E for wide).
• Toe Splay: The natural spreading of toes during push-off requires adequate room in the toe box to prevent impingement.
• Running Distance and Terrain:
  • Longer Distances: Marathons and ultramarathons almost always necessitate more room to account for significant swelling over hours of running.
  • Downhill Running: Descending puts increased pressure on the toes as they slide forward, making sufficient toe box space critical.
• Sock Thickness: The type of running socks you wear can significantly impact the fit. Always try on shoes with the socks you typically run in.
• Time of Day: Feet are typically smallest in the morning and swell throughout the day. Trying on shoes in the afternoon or evening can provide a more accurate representation of their size during a run.

How to Properly Size Your Running Shoes (The "Thumb's Width" Rule and Beyond)

Accurate fitting involves more than just selecting a number; it's about how the shoe accommodates your foot's unique contours and dynamics.

• The Thumb's Width Rule: After lacing the shoe, stand up. There should be approximately a thumb's width (about half an inch or 1.25 cm) between the longest toe (which may be the second toe for some individuals, not always the big toe) and the very end of the shoe. This space is crucial for toe splay and swelling.
• Heel Fit: The heel should feel snug and secure, with minimal to no slipping when you walk or jog. Excessive heel slippage can lead to blisters and instability.
• Midfoot Fit: The shoe should feel secure across the top of your foot (instep) without being overly tight or restrictive. You should be able to wiggle your toes freely.
• Width: Ensure the widest part of your foot aligns with the widest part of the shoe. There should be no pinching or bulging on the sides. If your foot feels compressed, you likely need a wider size, not necessarily a longer one.
• Lacing Techniques: Experiment with different lacing patterns to fine-tune the fit, especially if you have a high instep or narrow heel.

When NOT to Go Half a Size Up

While often beneficial, sizing up isn't always the correct approach:

• If your standard size already provides adequate room: If, after trying on your true size, you already have the recommended thumb's width space and a comfortable fit, there's no need to size up further.
• If it leads to excessive slipping or instability: A shoe that is too long can cause your foot to slide around inside, leading to friction, blisters, and an unstable ride, increasing the risk of falls or ankle turns.
• Specific Shoe Types: Some specialized shoes, like race flats designed for minimal weight and a very snug, responsive feel, or certain trail running shoes where precise foot placement and stability on technical terrain are paramount, might be designed for a closer fit. However, even with these, ample toe room is still important.

The Importance of Trying Before Buying

The best way to determine your optimal running shoe size is to visit a specialized running shoe store.

• Professional Assessment: Staff can measure your feet, observe your gait, and recommend shoes based on your foot type, running style, and typical mileage.
• Try Multiple Pairs: Different brands and models have varying lasts (the foot-shaped form around which the shoe is built), meaning a size 9 in one brand might fit differently than a size 9 in another.
• Walk/Jog in the Shoes: Take a short walk or jog around the store to simulate running conditions and assess the fit and feel.

Conclusion: Prioritizing Comfort and Functionality

Ultimately, the decision to buy running shoes a half size bigger is a personal one, guided by the principles of biomechanics and comfort. It's less about strictly adhering to a number and more about achieving a fit that allows your feet to function naturally and comfortably throughout your run. Prioritize adequate toe room, a secure midfoot, and a snug heel. By understanding your foot's needs and following proper fitting guidelines, you can select running shoes that enhance performance, prevent injury, and maximize your enjoyment of the sport.