Prediction limits
Race predictors work best between reasonably similar endurance distances. They do not know your training, heat tolerance, fueling, course profile or injury status.
Example
A 50:00 10 km race predicts a half marathon around 1:50 using the Riegel formula, assuming endurance scales normally between the two distances.
Common mistakes
Do not use an old personal best if your current fitness has changed. Short races can also overpredict long-distance performance if endurance is not well developed.
Limitations
This is not coaching or medical advice. Weather, terrain, pacing, fueling, altitude, training history and injury risk can all change race outcomes.
Last reviewed: 2026-05-17