How a Race Time Predictor Works
A race time predictor estimates how fast you can run a target distance based on a result you have already achieved at another distance. Enter a recent race time — say a 5K — and the calculator projects your likely finish time for the 10K, half marathon, marathon, and more.
This tool uses Riegel's formula, the most widely used endurance prediction model. It recognizes that runners cannot sustain their short-distance pace over longer distances, so it gradually scales the predicted time upward as the distance grows.
The Riegel Formula
- T2 = T1 × (D2 / D1)^1.06
- T1 is your known finish time, D1 is the distance you ran it at
- T2 is the predicted time for the target distance D2
- The 1.06 fatigue exponent accounts for slowing over longer distances
Getting Accurate Predictions
The predictor is most reliable when the target distance is reasonably close to your known distance and when you have trained appropriately for that target. A marathon prediction based on a 5K time, for example, assumes you have the endurance base to actually cover 42 km.
- Use a race result from the last few weeks for the best accuracy.
- Make sure your known time reflects a genuine all-out effort.
- Real finish times also depend on weather, terrain, and race-day pacing.
- Treat the prediction as a goal-setting guide, not a guarantee.
Frequently Asked Questions
How accurate is the Riegel race predictor?
For distances within a similar range — for example predicting a 10K from a 5K — Riegel's formula is quite accurate for well-trained runners. Predictions become less reliable for big jumps, such as estimating a marathon from a mile time, where endurance and pacing play a larger role.
Why does my predicted pace get slower for longer races?
The 1.06 exponent in the formula models the natural slowdown that occurs as fatigue accumulates over longer distances. No runner can hold their 5K pace for a full marathon, so the predicted per-kilometer pace increases with distance.
Can I use a training run instead of a race?
Yes, but the prediction is only as good as the effort. A hard time trial gives a far better prediction than an easy training run. For best results, use a recent maximal-effort race or time trial at a measured distance.