Race Predictor Calculator
Riegel-style distance scaling from a recent race to a goal distance. Click any i for detail.
Races
Known race
i
Known
The distance you already raced. |
|
|---|---|
Known distance (km)
i
Custom d1
Used when known race is custom. |
|
Known time (mm:ss or h:mm:ss)
i
Time
Finish time from that race. |
|
Predict race
i
Predict
Distance you want a time estimate for. |
|
Predict distance (km)
i
Custom d2
Used when predict race is custom. |
|
Riegel exponent
i
Exponent
Classic ~1.06; lower if you fade less, higher if you fade more. |
How to use this calculator
- Pick the race you already ran and enter its time.
- Choose the goal distance.
- Optionally tweak the Riegel exponent.
- Treat the result as a training target, not a guarantee.
Results explained
Riegel’s formula estimates t2 = t1 × (d2/d1)^b with b often near 1.06 for road racing. It assumes similar fitness, weather, and course difficulty. Heat, hills, fueling, and under-trained long runs break the model — especially jumping from 5K shape to a marathon.
Quick reference: Riegel
t2 = t1 × (d2 / d1)^exponent.
| Item | Detail |
|---|---|
| Classic exponent | ~1.06 |
| 5K → half | Common planning jump |
| Marathon from 5K | Often optimistic without long-run fitness |
| Custom km | Ultras need different models |
Educational predictor — not a race-day plan.
How the estimate is built
t2 = t1 × (d2/d1)^b with default b = 1.06.
Example scenario
25:00 5K → half marathon ≈ 1:55-ish at b=1.06 (course-dependent).
FAQ
Why is my prediction too fast?
Short-distance fitness over-predicts long races if endurance is lacking. Raise the exponent or train the distance.
Trail races?
Vertical gain and terrain break flat Riegel assumptions.
Should I use chip time?
Yes — use the time that reflects your actual running, not gun time delays.
What format is the time field?
mm:ss or h:mm:ss (for example 25:00 or 1:55:00).