Race Time Predictor Calculator
Predict your race times at other distances from a recent performance using Riegel's formula, with the option to adjust the fatigue exponent for your training background.
🏃 Exercise Science📐 T₂ = T₁ × (D₂ / D₁)^1.06🏥 Health
Known distance
Custom distance (km)
Known time — minutes
Known time — seconds
Fatigue exponent
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Race Time Predictor Calculator | — | T₂ = T₁ × (D₂ / D₁)^1.06 | time |
Step-by-Step Examples
Example 1
From a 5K
5K in 25:00, standard exponent 1.06.
- T₂ = 1500 × (10 / 5)^1.06 for the 10K
- 2^1.06 = 2.085
- 10K ≈ 3128 s = 52:08
- Marathon ≈ 1500 × (42.195/5)^1.06 ≈ 4:01
✓ 10K 52:08, marathon roughly 4:01
Example 2
From a Half Marathon
Half marathon in 1:45:00.
- T₁ = 6300 s at 21.0975 km
- Marathon ratio = 42.195 / 21.0975 = 2.0
- Marathon ≈ 6300 × 2^1.06 = 13,136 s
- Approximately 3:38:56
✓ Marathon roughly 3:39
Example 3
Adjusted Exponent
5K in 20:00, well-trained runner using 1.04.
- Lower exponent means less predicted slowdown at longer distances
- Marathon ≈ 1200 × (42.195/5)^1.04
- Yields a faster marathon prediction than the 1.06 default
✓ Exponent choice materially changes long-distance predictions
Real-World Applications
Goal Setting
Predicted times from a recent race give a realistic starting point for setting targets at a new distance.
Pacing Strategy
Converting predictions into per-kilometre pace helps plan even race execution rather than going out too fast.
Training Progress
Re-running the prediction after a training block shows whether fitness has actually moved.
Exercise Physiology
Riegel's formula is a classic empirical model of how sustainable pace degrades with duration.
Common Mistakes to Avoid
⚠️
Predicting a marathon from a 5K without marathon training
The formula assumes appropriate preparation for the target distance. A fast 5K runner without long runs will not hit the predicted marathon time.
⚠️
Using a stale performance
Predictions are only as current as the input race. A result from six months ago may no longer reflect fitness.
⚠️
Treating the output as a guarantee
Riegel's exponent is an average across runners. Individual endurance profiles vary substantially, and race-day conditions add further variance.
Frequently Asked Questions
What is Riegel's formula? ▾
An empirical model predicting race time at a new distance as the known time multiplied by the distance ratio raised to the power of 1.06.
Why is the exponent 1.06? ▾
It was fitted to observed race performance data, reflecting that sustainable pace slows slightly as distance increases. Values between 1.04 and 1.08 are commonly used depending on training background.
How accurate is race time prediction? ▾
It works well between adjacent distances — 5K to 10K, for example — and becomes progressively less reliable across large jumps, especially into the marathon.
Why is the marathon prediction often optimistic? ▾
Marathon performance depends heavily on fuelling, glycogen depletion, and long-run adaptation, none of which a short-distance result captures.
Should I use a different exponent? ▾
Higher-mileage runners often fit a lower exponent around 1.04, while newer or lower-mileage runners may fit 1.08 or higher. Comparing your own results across distances is the best guide.