Riegel's formula predicts race times based on a recent performance. Adjust the Decay (k) to compare different slowdown assumptions over distance.
| Distance | Predicted Time | Pace |
|---|---|---|
| 1 Mile | 06:01 | 3:44 min/km |
| 5K | 20:00 | 4:00 min/km |
| 10K | 41:42 | 4:10 min/km |
| Half Marathon | 01:32:00 | 4:22 min/km |
| Marathon | 03:11:49 | 4:33 min/km |
| 50K | 03:49:38 | 4:36 min/km |
| 100K | 07:58:46 | 4:47 min/km |
Use the Riegel power-law model to calculate race-time scenarios from a recent performance. Adjusting the exponent changes the assumed slowdown over distance; it does not diagnose physiology or replace event-specific preparation.
The model scales one performance to another with T₂ = T₁ × (D₂ ÷ D₁)ᵏ. The recent result, target distance, and chosen exponent are visible, so you can audit exactly why a prediction changes.
Use a recent, accurately measured effort that reflects current fitness. The model assumes the input is representative; an easy training run, short course, or stale personal best can produce a mathematically correct but unhelpful projection.
The default exponent is 1.06. Lower values produce less modelled slowdown as distance increases, while higher values produce more. Moving the slider explores an assumption; it does not measure whether an athlete is speed- or endurance-oriented.
The equation does not know about terrain, weather, fueling, injury, pacing errors, or event-specific training. Those omissions become more important when projecting far beyond the source distance, especially from a short race to a marathon or ultra.
It is the exponent applied to the distance ratio. With k above 1, predicted time increases slightly faster than distance; 1.06 is a common scenario value, not a personal measurement.
It can produce a transparent mathematical scenario, but the distance gap makes event-specific endurance, fueling, terrain, and pacing especially important. A recent longer race usually provides a closer comparison.
Kilometers and miles represent the same physical distance after conversion. The formula uses the canonical distance ratio, so a display-unit change should leave every predicted time unchanged.