What Ultramarathon Categories and Distances Does IAU Cover?

Updated

The International Association of Ultrarunners (IAU) organizes its work around four discipline labels shown on its site: Trail, 24H, 50K, and 100K. These represent distinct ultradistance race formats rather than a single event type. The IAU describes itself as the international governing body for ultradistance running, with a stated aim to promote and develop long-distance running.

The Four Discipline Labels

Label What it refers to
Trail Ultradistance racing on off-road or trail terrain
24H Time-based racing over 24 hours
50K A fixed distance of 50 kilometers
100K A fixed distance of 100 kilometers

These four labels are the categories the site presents as its discipline areas. They mix two different ways of defining a race: by terrain (Trail), by elapsed time (24H), and by fixed distance (50K and 100K). That mix is normal in ultrarunning, where events are often described by whichever limit — distance or time — defines the finish.

How to Read the Categories

  • Trail is the only label defined by surface rather than by a measured limit. A trail ultramarathon is distinguished by running off paved roads, not by a specific kilometer count in the label itself.
  • 24H is a timed format. Runners aim to cover as much distance as possible within 24 hours, so results are measured by distance completed rather than by finishing order across a fixed course length.
  • 50K and 100K are fixed-distance formats. Every finisher covers the same measured course, which makes them straightforward to compare across events.

If you are looking for a specific event, championship, or record, the site's news and championship announcements are the place to check. For example, a recent site heading announces that Croatia will host the 2027 IAU 100 km European Championships — an illustration of how the 100K label connects to championship-level competition.

What This Means in Practice

If your interest is in a particular format — a timed 24-hour race, a trail ultra, or a fixed 50K or 100K — the IAU's category structure tells you which part of the sport that event belongs to. The labels are organizational groupings, not a full rulebook; specific competition rules, records, and entry conditions are separate matters that the site addresses elsewhere.