Treat the prediction as a scenario with assumptions
A running race-time predictor extends one performance to another distance using a mathematical relationship. It does not observe your durability, course, weather, health, pacing, or fueling. The output is therefore a planning range to test, not a promised finish time.
Decide whether the input race is comparable
| Input check | More useful | Less useful |
|---|---|---|
| Effort | recent, hard, and honestly measured | controlled workout or interrupted run |
| Course | measured and reasonably comparable | steep trail used to predict flat road |
| Conditions | recorded and ordinary enough to interpret | extreme heat, wind, or altitude |
| Distance transfer | nearby distance with relevant endurance | short sprint projected to an ultra |
| Recovery | normal health and function | illness, injury, or major fatigue |
The Riegel model was developed from performance records and assumes that speed declines with distance. The exponent is not a personal physiological constant. Change it only when you understand why, and show several scenarios rather than tuning it until it produces the desired answer.
Build a prediction range
Run the calculator with a conservative, central, and stretch assumption. Compare those results with recent long runs, pacing stability, weekly consistency, and race-specific practice. For a marathon, a strong half marathon cannot substitute for late-run durability or fueling. For a 100K, a road result cannot encode trail surface, climbing, darkness, or aid-station time.
Use the marathon pacing guide, half-marathon pacing plan, and ultra cutoff planner to convert the scenario into event-specific decisions.
Safety boundary
Do not use a predicted time to race through chest pain, fainting, severe breathing difficulty, illness, or pain that changes normal movement. This calculator is educational and not medical advice, readiness clearance, or a guarantee.