A prediction converts evidence; it does not create readiness
Use a recent race or controlled time trial from a nearby distance. A 10K usually informs a half-marathon estimate better than an old 5K informs a marathon. Enter the actual result, then treat the output as the centre of a range.
Worked decision: 10K to half marathon
Suppose a runner enters a recent 50-minute 10K. The mathematical prediction is only the first pass. Widen the range when the result is old, the target course is hillier, conditions are warmer, or long-run durability and fueling have not been demonstrated. Narrow it only after repeatable training supports the longer distance.
Create A, B and C race ranges before choosing a pace plan. Use the running pace calculator to convert each range into checkpoints. Do not chase the centre estimate early or use the tool as medical clearance, a qualification guarantee or proof that missed training no longer matters.
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
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.