This race time predictor estimates how fast you could race any other distance, from one recent result — using Peter Riegel’s formula, the endurance world’s standard since 1977.
| Distance | Predicted time | Required pace / km |
|---|
Predictions assume equivalent fitness, comparable course and weather, and proper training for the target distance — Riegel projects capability, not a guarantee. Feed the pace column into training with our Pace Calculator.
The Riegel Formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06 — your known time, scaled by the distance ratio raised to a fatigue exponent. The exponent captures a physiological truth: doubling the distance more than doubles the time, because glycogen, heat and muscle damage accumulate. Riegel fitted 1.06 from world-record progressions; it holds remarkably well for trained runners between about 3.5 minutes and several hours of racing. The classic companion tools are our Pace Calculator (which turns these predictions into splits and training zones) and the VO2 Max Calculator. Part of our Sports Calculators collection.
Worked Example (Verified)
You race 10K in exactly 50:00. Riegel (exponent 1.06) predicts:
- Half marathon: 50 × (21.0975/10)^1.06 ≈ 110.3 min → 1:50:18
- Marathon: 50 × (42.195/10)^1.06 ≈ 230.0 min → 3:50:00 — the sub-4 dream, quantified
- 5K: ≈ 24:00 — predictions run both directions, down as well as up
Reading Predictions Honestly
- Exponent choice matters at marathon distance: 1.06 vs 1.08 moves the example above by about ±5 minutes. Less-trained runners fatigue faster — pick 1.07–1.08 if your long runs are new territory; elites can dip toward 1.05.
- Predictions assume you train for the target. A 5K runner’s physiology, projected to a marathon, describes the athlete they would become with marathon training — not what happens if they line up next Sunday.
- Course and conditions bend results: hills, heat, altitude and wind each take their percentage. Compare like with like.
- Predictions stretch best within a reasonable span — 1500m → 10K works; 100m → marathon is astrology, not Riegel.
Related Tools
- Pace Calculator — splits and training paces
- VO2 Max Calculator — the fitness behind the times
- Sports Calculators — the full collection
Frequently Asked Questions
How accurate is the Riegel formula?
For trained runners predicting across similar endurance distances, typically within 1–3% — remarkably good for one exponent. Accuracy degrades for sprint-to-marathon jumps and for runners without distance-specific training.
What marathon time does a 50-minute 10K predict?
About 3:50:00 at the standard 1.06 exponent — try it above. At 1.08 (recreational) it is closer to 3:55.
What is the fatigue exponent?
The power in Riegel’s formula expressing how much you slow as distance grows. 1.06 is the published standard; higher values model faster-fading runners, lower values model elite endurance specialists.
Can I predict a 5K time from a marathon?
Yes — the formula is symmetric. Marathoners often find their predicted 5K is faster than their actual 5K, because marathon training builds endurance more than top-end speed.
Does this work for trail races or ultras?
Roughly, for comparisons on similar terrain — the table includes 50K. Technical trails, big elevation and extreme distances break the assumptions; treat those numbers as optimistic baselines.
Is this race time predictor free?
Yes — free, instant, no sign-up.