Enter your regular shoe size to get your recommended running shoe size.
Thumb rule: When standing in running shoes, you should feel about 10–15 mm (a thumb-width) between your longest toe and the front of the shoe.
The running shoe size calculator finds your correct running shoe size based on your regular shoe size and the type of running you do. Running shoes should be larger than everyday shoes because feet swell during exercise, your foot slides forward with each stride, and repeated toe-box impact during long runs causes black toenails if there is insufficient room. This tool calculates the right size adjustment for 5K, 10K, half marathon, marathon, and trail running.
Why Running Shoes Should Be Bigger
- Your foot slides forward slightly with every landing, pushing your toes toward the front of the shoe.
- Feet swell during a run — more on long runs and in warm weather.
- Repeated impact compresses the toe box, reducing effective interior space.
Size Adjustment by Running Type
| Distance / Condition | Size Up | Reason |
|---|---|---|
| 5K–10K, cool weather | +0.5 | Standard foot expansion |
| Half marathon | +0.5 | Swelling over distance |
| Marathon / hot weather | +0.5 to +1.0 | Significant swelling |
| Trail running | +0.5 | Thick socks + uneven terrain |
Frequently Asked Questions
Should I size up for running shoes?
Yes — half a size up from your regular shoe size is the most common advice from running shoe specialists. For marathons in warm weather, some runners go a full size up.
Why do my toenails go black from running?
Black toenails are caused by your toes repeatedly hitting the front of the shoe. The usual fix is more toe room — half a size up or a wider toe box — combined with a heel-lock lacing that stops your foot sliding forward.
Do trail running shoes fit differently from road shoes?
Trail running shoes tend to have a more protective, snug feel. Most trail runners still size up half a size in length, but look for a snugger heel and midfoot fit compared to road shoes.
Should I buy running shoes for my morning or evening foot?
Evening — feet swell throughout the day. Buying running shoes in the morning can result in shoes that are too tight by the time you run after work.
Can I use regular shoes for running?
Technically yes for short distances, but running shoes have cushioning, stability, and breathability designed for the repetitive impact of running. Regular shoes compress quickly and offer poor toe room.