HEALTH · RACE TIME PREDICTOR
Race Time Predictor Calculator
Predict your finish time at any race distance from a recent result using the Riegel formula. Supports 800 m through 100 miles plus custom distances.
Formula: T₂ = T₁ × (D₂ / D₁)1.06 (Riegel, 1981). Assumes similar effort level and course conditions. Accuracy decreases for very different distances or ultra-marathon events.
About This Calculator
How fast could you run a marathon based on your 10K time? The Race Time Predictor uses the Riegel formula — the gold standard among runners for cross-distance time estimates — to project your finish time at any race distance. Enter your recent result and choose a target distance to get your predicted time plus pace.
How It Works
The Riegel formula (published by Peter Riegel in American Scientist, 1981) models the non-linear effect of fatigue over longer distances: T₂ = T₁ × (D₂ / D₁)^1.06. Your reference time (T₁) and distance (D₁) are used to project the finish time (T₂) at the target distance (D₂). The exponent 1.06 means that doubling the distance adds slightly more than double the time — reflecting the additional physiological cost of longer efforts.
The Formula
T₂ = T₁ × (D₂ / D₁)^1.06
- T₁
- Reference finish time (seconds)
- D₁
- Reference race distance (meters)
- D₂
- Target race distance (meters)
- 1.06
- Riegel fatigue exponent
Frequently Asked Questions
- How accurate is the Riegel formula?
- The Riegel formula is reasonably accurate for distances within 2–3x of your reference distance and for runners who train consistently. It tends to over-predict performance for very long distances (50+ miles) where cumulative fatigue and pacing strategy become more significant. It is less reliable for athletes who peak at specific distances.
- Why does the formula use the exponent 1.06?
- The exponent 1.06 was empirically derived by Peter Riegel from analysis of world record performances across many distances. It encodes the observation that running performance degrades non-linearly with distance — you can't simply double your 5K time to predict a 10K. The degradation is slightly worse than linear, captured by the exponent greater than 1.
- Should I use a recent race or a training time?
- Use a recent race result run at close-to-maximum effort for the best predictions. A time trial or tempo run at a controlled pace will underestimate your race performance and lead to overly conservative projections. The formula assumes you raced both distances at maximal sustainable effort.