Most runners are wearing the wrong shoe size — typically half a size too small — and don’t know it because running shoes don’t cause the immediate discomfort that dress shoes do when they’re too small. The consequences show up later: black toenails, blisters under the second toe, forefoot pain that’s blamed on the shoe’s foam rather than its fit, and metatarsalgia that resolves the moment the runner gets correctly fitted. Getting measured properly takes five minutes and changes every shoe purchase you make afterward. Here’s exactly how to do it correctly.
Why Running Shoe Sizing Differs From Street Shoes
Running shoes should be half a size to a full size larger than your street shoe size — not because running shoes run small, but because feet expand during running in two distinct ways that static fitting doesn’t capture.
Foot length increases with each footfall as weight-bearing elongates the arch and spreads the toes. Research in the Journal of the American Podiatric Medical Association found that foot length increases by an average of 8-10mm from non-weight-bearing to weight-bearing measurement — roughly half a shoe size. Running amplifies this further as cardiovascular loading increases foot temperature and circulation, causing additional expansion through the forefoot.
The consequence of correctly-sized street shoes worn for running: the toes contact the end of the shoe during the weight-bearing phase, creating the compressive force that produces subungual hematoma (black toenails) on downhills and forefoot blisters during extended runs. A thumb’s width of space between the longest toe and the shoe’s end is the correct standard — it feels like more room than necessary standing still and becomes correct during running.
How to Measure Foot Length
Do this in the afternoon — feet are at their smallest in the morning and reach maximum size by mid-to-late afternoon as daily activity and temperature increase blood flow. Measure both feet; most people have one foot slightly larger than the other, and the larger foot determines your shoe size.
Stand on a piece of paper with your full weight evenly distributed and your heel against a wall. Trace around the foot with a pencil held vertically — not angled inward — to capture the true perimeter. Measure from the heel to the tip of the longest toe in millimeters. Then add 10-12mm to this measurement to find the target internal length of the shoe.
Use the chart from the brand you’re considering — Brooks, ASICS, and New Balance all publish sizing guides in millimeters on their product pages, and Running Warehouse provides detailed fit guidance for each model. The millimeter measurement is more reliable than the US size conversion charts because size standards vary between brands.
Bottom line: Measure both feet in the afternoon, add 10-12mm to the longest measurement, and match to the brand’s millimeter sizing chart — not to US size equivalence tables.
How to Measure Foot Width
Width measurement is where most shoe buyers go wrong by skipping it entirely. A runner can measure foot length accurately and still end up in the wrong shoe if width isn’t assessed. The width of the forefoot — the widest point of the foot across the metatarsal heads — determines whether standard D width (men’s) or B width (women’s) is appropriate, or whether wide or extra-wide sizing is needed.
Stand on the paper after tracing the foot length outline. Mark the widest points on both sides of the foot — typically at the first and fifth metatarsal heads, roughly the ball of the foot. Measure between these marks in millimeters. Compare to the brand’s width chart; most brands publish the forefoot width range for standard, wide (2E), and extra-wide (4E) in their fit documentation.
A practical in-shoe test: put on the shoe and stand with your full weight. Press the upper fabric against the side of your foot at the widest point. If the fabric stretches taut and you can feel resistance from the shoe pushing back, you need a wider width. If the fabric creases loosely on the side, the shoe is too wide. The upper should conform without tension.
Runners with wide forefoot requirements should look specifically at the New Balance 880v14 — the most extensive width program in mainstream running footwear — and the Brooks Ghost 16, which offers verified 2E men’s and women’s width options. The posts on running shoes for wide feet (men) and running shoes for narrow feet cover this in more detail.
Bottom line: Measure forefoot width at the ball of the foot and check against brand width charts — the in-shoe standing test is the most practical verification, with taut upper fabric indicating a width upgrade is needed.
The Heel-to-Toe Drop Consideration
Drop doesn’t affect sizing, but it affects how the shoe fits in use — a distinction worth understanding before buying. Lower-drop shoes (4-6mm) place the foot in a more level position, which can make the heel feel less secure in shoes with heel counters designed for higher-drop geometry. If you’re transitioning from 10-12mm to 4-6mm drop, trying the shoe on with the same socks you’ll run in, at the end of the day, and walking for 5-10 minutes is more informative than a quick standing fit assessment.
The post on how to choose running shoes covers drop selection alongside sizing in more detail for runners making footwear decisions across multiple variables simultaneously.
Getting Fit at a Running Specialty Store
A Brannock device — the metal foot-measuring tool used in shoe stores — provides the most accurate length and width measurement available without specialized equipment. Running specialty stores (as distinct from general sporting goods retailers) staff people trained to use it correctly and to assess fit in the context of running rather than standing.
A proper fitting appointment at a running store includes: Brannock measurement standing and seated (comparing weight-bearing and non-weight-bearing length), gait assessment on treadmill or walking surface, width assessment, and try-on with socks you’ll actually run in. This takes 20-30 minutes and produces better outcomes than any online purchase based on self-measurement alone for runners who haven’t recently been fitted.
The gait assessment component specifically matters for determining whether stability features are appropriate — a measurement no home sizing process can replicate. Runners who haven’t had a gait assessment in the last 2-3 years, or who’ve had significant weight changes, injuries, or biomechanical changes since their last assessment, benefit specifically from the in-store process rather than self-measurement plus online ordering.
Bottom line: Running specialty store fitting with Brannock measurement and gait assessment produces the most accurate fit assessment — especially for first-time running shoe buyers and runners whose feet or gait have changed since their last fitting.
Common Sizing Mistakes and How to Avoid Them
Measuring in the morning. Feet are smallest first thing in the morning — measuring before daily activity produces a size that fits correctly at 8am and too tightly by noon. Always measure after being on your feet for at least a few hours.
Using dress shoe size directly. Many runners report that running shoes “run small” — they’re actually using their dress shoe size, which is typically a half size smaller than the appropriate running shoe size for the same foot. Add half a size to your dress shoe as a starting point, then verify with the thumb-width test.
Ignoring the larger foot. Roughly 60% of people have one foot measurably larger than the other. Fitting to the smaller foot creates a shoe that’s correct for one foot and too small for the other — the tight foot shows up as the one with blisters and black toenails. Always fit to the larger foot.
Skipping the width assessment. Width is as important as length for running shoe comfort and performance. A runner in the correct length but wrong width will experience either forefoot compression (too narrow) causing blistering and metatarsal pain, or heel slippage (too wide) causing Achilles blistering at the back. Both are fit problems, not foam problems.
Trying shoes without running socks. Running socks — particularly thicker or wool options — add meaningful volume inside the shoe. A shoe fitted with thin dress socks or barefoot will feel tight with actual running socks. Bring the specific socks you plan to run in to any fitting.
Frequently Asked Questions
Do feet get bigger with age?
Yes — both in length and width, for most people. Arch ligaments loosen over decades of weight-bearing, which lowers the arch and elongates the foot. Many adults are half a shoe size larger at 50 than they were at 30, though they’ve continued buying the same size. Women may experience additional foot size changes after pregnancy, when relaxin affects ligament laxity throughout the body including the feet.
Why do my toenails turn black when running?
Subungual hematoma — bleeding under the toenail — results from the toenail repeatedly impacting the end or top of the shoe during running. The most common cause is insufficient length between the longest toe and the end of the shoe, which occurs in the weight-bearing running position even when the shoe feels adequate standing still. Half a size up, with the thumb-width length standard applied, resolves most cases. Running downhills in too-short shoes is the highest-risk context for this specific problem.
Should running shoe width match dress shoe width?
Generally yes, with the caveat that running-specific foot expansion during exercise can push previously standard-width feet into wide-width territory for running. Runners who are borderline between standard and wide for daily wear should try wide-width running shoes — the added forefoot room almost never produces problems during running and prevents the issues that arise from standard widths that are too tight under running loads.
How do I know if my running shoes fit correctly?
Five checks when wearing the shoes standing up with full weight: a thumb’s width of space between the longest toe and the end; no stretching or tightness across the forefoot fabric; no heel slippage when you rise onto the ball of the foot; no pressure or pain across the top of the midfoot; and the heel counter holding the heel firmly without rubbing. If all five are true standing, they’ll hold through a run with appropriate socks.
Find Your Perfect Running Shoe
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